Basic Microwave Tutorial (FM Modulation)

Microwave System Basics

Video Signal

The Video Transmitter interfaces to the video source via a short cable with connectors on both ends. The Video and audio signals modulate the transmitter RF carrier in one of several manners, the most common of which is “wide band FM,” an analog system involving transmission of the signal over a fairly broad channel in a particular frequency range. Most manufacturers specify a channel bandwidth of 16MHz for full motion color video and audio.

Input amplitude of the video signal will generally be 1 Volt, peak-to-peak, and applied to the transmitter with a 75ohm coaxial cable. This is an important figure since it typically has a direct influence on the amount of deviation (commonly called “modulation”) the transmitter generates. The amount of deviation will have a direct correlation to the quality of the video signal after it is demodulated in the receiver.

Many transmitters are calibrated to provide the correct amount of deviation at only a single frequency. The amount of deviation varies with the operating frequency because of a property known as “modulation sensitivity” or “mode sense” associated with the oscillator employed in the transmitter. Some equipment may have a video level adjusted to compensate for these variations in sensitivity so the operator can optimize the video signal for each frequency.

 

Video Levels and Modulation

Great care and attention should be paid when adjusting modulation gain controls on a transmitter. Prior to making any adjustments, the video source signal level should be verified. Insure that the input level of the video signal is at, or very near, one volt peak-to-peak. This is vital since if the signal is below standard level and the transmitter is adjusted to compensate for this decreased level, when a signal of proper level is applied to the transmitter an “over-deviation” condition will result. This over-deviation will result in distortion of the received video signal and likely cause interference to adjacent RF channels.

The best way to compensate for modulation sensitivity is to adjust the gain of the source input signal. If required, adjust the video output level of the camera system for the proper indication on the link receiver. Be sure the receiver is at the calibrated output level setting. Following this procedure will mitigate the likelihood of future interferences or over-modulation problems.

 

The Audio Signal

Most microwave video transmitters include the provision for the transmission of at least on high quality audio signal. Typically, the audio channels are referred to as “subcarriers” since they are actually additional low-power RF carriers superimposed on the main RF video carrier. Up to two independent audio subcarriers are ordinarily used for sound transmission and are generally referred to by their frequency. The most popular subcarrier frequencies are 6.2MHz and 6.8MHz. The subcarriers are actually FM transmissions on 6.2 and 6.8 MHz that are summed with the video signal. It is the highest frequency of the audio subcarrier that ultimately defines the “occupied bandwidth” of the transmitted signal.

One of the most important characteristic of the transmitter audio subcarrier is the input level. There are commonly two choices; microphone or line. Microphone inputs are configured to accept a nominal level of around -50dBm. This input level is compatible with a wide range of dynamic, electrets, and condenser microphones. Many transmitters include “phantom power” or “bias” to supply operating voltage to condenser type microphones.

Microphones have a very low output, and consequently a large amount of amplification is required inside the transmitter to increase the signal from the microphone. This is important since the amplifiers not only increase the desired signal from the microphone but also the undesired noise level. It is critical to always use the best quality shielded cables and keep microphone audio runs as short as possible to avoid picking up and amplifying unwanted noise and interferences.

The other common input configuration for audio subcarriers is known as “line level.” This is the voltage commonly associated with the outputs of tape recorders, televisions, or other devices with an RCA or XLR type connector. The nominal input level associated with line level sources is typically between 0 - +4dBm. Generally, the specified input level is approximately 10dB below the clipping level. For this reason it is important to adjust audio levels to minimize distortion in the receiver.

 

Frequency of Operation

Most microwave transmitters operate on a given number of pre-defined channels with a given band. Frequency bands (L, S, or C) are typically “channelized” since that is the most efficient manner to maximize the scarce resources of spectrum. Most channel plans take into account the transmitter occupied bandwidth and a sufficient “guard band” to facilitate some rejection of adjacent channels in the receiver.

 

Frequency Selection

Many methods of frequency selection are employed in transmitters today. One of the simplest methods is the “single channel” or “fixed channel” transmitter. When configured in this manner the transmitter frequency cannot be changed by the operator. Single channel transmitters do not exhibit the problem of variations in modulation sensitivity discussed above since the transmitter can be optimized by the manufacturer on any given frequency.

A prime disadvantage of the signal channel transmitter is that is has no inherent back-up or redundancy properties. If a single channel transmitter fails, it must be replaced with another transmitter operating on the same channel. When a multi-channel transmitter fails, it can be replaced by selecting the correct channel on another multi-channel transmitter. Single channel transmitters typically require more shelf stock to maintain the same level of back-up capacity as a fleet of multi-channel radios provides.

 

Power Output

One of the most distinguishing characteristic of any microwave transmitter is the RF power output. Most transmitters intended for professional applications range in power output from 0.1W (100mW) up to 5.0W. External power amplifiers are available to increase the power up to 12W or more. Understanding RF power is an important part of successful deployment of microwave systems.

 

Heat

From a designer’s standpoint, RF output is a balance between link budget requirements, power consumption, and size. Generally, the greater the link distance, the more power required. Higher power requirements dictate greater DC power consumption and larger physical size. If low power consumption is the design goal, then RF output power will be reduced. It is important to remember, however, that RF output power is only one of the factors that impact link performance. One of the most critical elements in any microwave link is the antennas.

More power is not always better. The best microwave engineers design systems to operate with the minimum amount of RF power required. While it sounds counter intuitive to assume that more is not better, it is easy to prove in practice. Microwave amplifier devices are not very efficient and require significant input power to generate moderate amounts of output power. For example, a typical 2W transmitter operating in L-Band requires 12W of dc power for 2W of output power. This equates to an efficiency of approximately 17%! Where does the remaining 83% (10W) of the input energy go? It is converted into heat. Another important consideration with microwave transmitters is heat.

A heat sink may often be required if sufficient surface area is not available on the housing itself or on the surface the transmitter is attached to as part of the installment. Remember that the heat sink transfers the heat from the transmitter to the air. If the air is enclosed in housing, it may be necessary to use a small fan to circulate the air to the outside.

 

Spread Spectrum

Another modulation scheme is spread spectrum, where the analog video signal is first digitalized, compressed using industry standard algorithms, and then transmitted. Two primary spread spectrum techniques are in use: Frequency hopping and direct sequence.

In frequency hopped systems, transmission occurs at several frequencies which change over time. Noise or interference on one channel will not impact the overall transmission, since the signal can move to another channel where the interferences are not present. In direct sequence spread spectrum, information is transmitted in a particular pattern or sequence. Data is transmitted at different frequencies simultaneously. The loss of a small number of bits doesn’t impact overall system performance, since the data can still be recreated at the receiver.

 

Receivers

The part of communications system responsible for extracting information from radio signals is the receiver. The receiver functions to select, amplify, detect and demodulate the transmitted signal. These processes result in a received video signal that is a replication of the signal input to the transmitter.

 

Receiver Characteristics

Two of the most important characteristic of a radio receiver are selectivity and sensitivity. Selectivity is a measurement of the ability of the receiver to reject adjacent and out-of-the band signals that are potential sources of interference. Sensitivity is a measurement of the receiver to adequate recover a weak signal.

 

Selectivity

Design engineers must make decisions about the selectivity of a receiver while considering the environment the receiver will operate in. While a receiver that is very selective (high rejection of unwanted signals) will deliver maximum performance on a given channel, such a design does not lead itself to receivers that must operate over a number of channels within a band of frequencies. A receiver with limited selectivity will perform consistently over a broad range of frequencies providing there are no strong unwanted signals present. Strong adjacent or unwanted signals result in de-sensitation (or de-sense) of the receiver and a reduction of performance.

The typical approach to this problem is to design the receiver with adequate selectivity to protect it from interference from out-of-band signals while insuring consistent performance across the desired band operation. When a condition arises where strong adjacent or in-band signals are causing interferences, external filters must be employed to improve receiver selectivity.

 

Filters

These filters are commonly called single channel filters and are inserted between the receiver input and antenna. These single channel filters are highly selective and allow only the desired energy at the desired frequency to pass through to the receiver. Since they are so selective, they must be carefully chosen for the specific frequency used on the link. Single channel filters are good “problem solvers” and should be part of any RF toolbox. Another filter, known as a band pass filter, is also a useful tool for combating interference by enhancing receiver selectivity.

A band pass filter is similar to a single channel filter in its ability to reject signals outside of its center frequency. However, the important distinction between single channel and band pass filters is the acceptance bandwidth of the filter. A band pass filter is designed to reject signals outside of a broader range of frequencies than a single channel filter. Often times the band pass filter is the first step in identifying and eliminating receiver interference.

A band pass filter designed for the S-band channels can provide a significant level of additional receiver selectivity and can all but eliminate signals present in the L-band. This fact is important to remember since many receivers are capable of reception in both L-band and S-bands. A multi-band receiver set to receive at S-band with an S-band band pass filter will not receive signals on the L-band channels and vice-versa. There are two solutions to this problem. One solution is to replace the S-band band pass filter with an L-band band pass filter. The second is to operate the receiver without any band pass filter. The best solution will depend on the environment the receiver is operated in. While band pass filters and signal channel filters reduce energy from the signals outside the desired band or channel, they also reduce some of the desired energy. This is known as insertion loss. Insertion loss is a measurement of loss at the filter pass frequency. Generally, the narrower the filter the more insertion loss it exhibits. This is important when considering the link budget. Often, selectivity must be traded for sensitivity.

 

Sensitivity

Receiver sensitivity defines the receivers’ ability to recover signals with reference to the noise floor. A very sensitive receiver will have the ability to demodulate weak signals. The sensitivity of a receiver is an important factor in determining microwave link budgets. Typically, the more sensitive the receiver the greater the distance the link will perform. When examining a link from the standpoint of required RF power, a more sensitive receiver will require less transmitted power for a given performance criteria.

Many factors contribute to the effective sensitivity of a microwave video receiver. It is important to remember, however that a high performance receiver will deliver a useable signal with an input signal around -78dBm. This figure can be used to compare receivers from the manufacturer’s specification sheets. This will also provide a useful number when using the spectrum analyzer as a trouble-shooting tool.

 

Low Noise Amplifier

When receiver performance is inadequate for the desired link, it may be necessary to provide additional amplification of RF signals to increase the apparent sensitivity of the receiver. Such an amplifier is known as a pre-amp or low noise amplifier (LNA).

An LNA is best deployed when receiver system losses conspired to attenuate the signal between the antenna and receiver. Long cable runs, antenna splitters, and band pass filters introduce losses resulting in a reduction of signal delivered to the receiver. Placement of the LNA following the antenna will act to cancel out the losses incurred in the feed line or splitter. In this case, the LNA does not really increase the sensitivity of the receiver rather it compensates for system losses between the antenna and receiver input.

If a receiver has poor sensitivity due to deficient design characteristics, the LNA will typically increase the sensitivity of the receiver. Only high quality LNA’s should be used or they may create more problems than they solve. Occasionally the opposite condition will exist. In environments where high RF energy levels are present, such as mountain top and rooftop communication sites, it may be necessary to install an attenuator or pad prior to the receiver input to reduce the amount of energy delivered to the receiver. While this may seem counter-intuitive at first, a simple understanding of amplifier characteristics will make the concept clear.

Amplifiers deliver gain over a certain input range. When this input range is exceeded, the amplifier is in what is known as compression. When this occurs the amplifier no longer provides gain. This condition actually reduces the sensitivity of the receiver.

There are two common fixes to alleviate overload problems. First, the use of a filter as noted above may result in sufficient reduction of energy to place the amplifier back in its desired operating range. Second, the use of a pad will reduce the energy that is seen by the receiver. While this will also reduce the desired signal, it is often the best method to keep the receiver LNA out of compression. In some cases, a combined filtering and padding may be required. Remember, the receiver is useless if the LNA is in compression.

Receiver Hardware

Radio signals arrive at the antenna connector and are amplified by the internal low noise amplifier. This is the first stage of gain encountered in the receiver. The amplified signals are next mixed with another fixed frequency to produce an intermediate frequency (IF). The mix signal is generated by the local oscillator (LO). By verifying the frequency of the LO, the receiver is able to convert the desired signal to the intermediate frequency in the converter. When channels are selected on the receiver, the operator is actually changing the frequency of the LO. The LO is really a very low power transmitter, and as such, emits RF energy. Some of this energy escapes the receiver housing through the antenna connector and may result in interference to co-located receivers on adjacent channels.

It is possible for a receiver tuned to an S-band channel to interfere with a receiver tuned to an L-band channel due to LO radiation. Typically, the LO operates on a frequency around 400MHz below the desired signal (low side injection). An S-band receiver tuned to 2110MHz will have a local oscillator operating on approximately 1719MHz. An L-band receiver in close proximity tuned to a frequency around 1710MHz might receive interference from the S-band receiver. Since the energy level of the radiated LO signal is so low, such problems are quickly solved by adequate separation between receivers.

The resultant mix form the LO and desired signal produces the IF signal. This signal is filtered and amplified. The process is repeated using the second LO to achieve a new IF signal. Receivers that mix and filter the RF signal twice are known as dual conversion receivers. The first mix results to the first IF and the second mix results to the second IF. The gain and filtering that are part of the conversions contribute to the receivers’ selectivity and sensitivity.

The second IF is filtered and demodulated to provide video and audio signals. The audio subcarriers are removed from the composite video signals and converted from RF signals to audio signals by the audio demodulator. The video demodulator converts the remaining RF signals to a baseband video signal. This signal is again filtered and amplified prior to application to the chassis mounted video output connector. In some situations the IF signal is not demodulated. Since the demodulation process results to none-linearity being attributed to the signal from amplifier and filter characteristics, it is often desirable to use the non-demodulated IF signal in repeaters. The IF signal is mixed or up-converted in a similar manner to the desired transmit frequency. This process is known as an IF or heterodyne repeater and is the best way to accomplish video and audio relay.

 

Receiver Care

A few items are worthy of mention with respect to precautions that should be taken when using microwave receivers. As with most electronic devices, receivers generate heat. Heat is the enemy modern solid state electronics, and adequate heat sinking should always be a part of any installation. In addition, a stable and reliable DC power source should always be used. Since radio signals are present at such low levels, the input circuitry of a receiver is very sensitive to the misapplication of energy at the input terminal. Never connect the output of the transmitter directly to the receiver to bench test a system! In the absence of the signal generator, sufficient RF pads must be inserted between the transmitter and receiver to attenuate the transmitter energy to a range suitable to the receiver input. If in doubt, simply connect an antenna to the transmitter, and another to the receiver to perform a link on the bench prior to deployment.

In dry climates static electricity presents a potential problem. A static discharge from the fingertip to the antenna input terminal could damage the receiver LNA, dramatically reducing the sensitivity of the receiver. Adequate grounding and liberal use of wrist straps can mitigate this failure mode.

 

Radio Waves

Just as light and sound represents energy that we are able to detect with sensors attach to our bodies (eyes and ears), radio waves consist of electromagnetic fields we are able to detect with man-made sensors we call receivers. Sound energy has a very long wavelength and a unique set of propagation characteristics associated with it. Light energy has a much shorter wavelength with its set of unique propagation characteristics. Electromagnetic energy (radio waves) exists between the frequency bands associated with sound and light.

Since microwave signals have a very short wavelength relative to the radio spectrum, the propagation characteristics in the microwave bands are similar to those found with light energy. The concept of line-of-sight propagation is intuitive with light transmission. Microwave signals behave very much in the same manner.

 

The Microwave Bands

The frequencies typically used for microwave video transmission extend from approximately 1700MHz through 8500MHz. these frequencies are much higher than the 160MHz and 450MHz frequencies used for two-way communications. The microwave frequencies are divided in bands and labeled as follows:

Frequency vs. Wavelength

One of the fundamental characteristics of radio energy is the relationship between frequency and wavelength. As frequency increases, wavelength decreases. Wavelength is the distance between the crest of one cycle energy to the next. At 100MHz (in FM Broadcast Band), one wavelength is equal to 3 meters. At 1300MHz (in the military Satellite Communications band) one wave length is equal to 1 meter. In the microwave band at 1300MHz, one wavelength is equal to 0.1 meter. Wavelength in meters is calculated by dividing 300,000,000 by the frequency in megahertz. Antenna length typically represents some part of a wavelength at the desired operating frequency. An antenna may be ¼ or ½ the wavelength for most applications. Since wavelength decreases as frequency increases, microwave antennas are much smaller for a given amount of gain than antennas designed for use in the two-way bands. This makes them ideal for covert usage.

 

Cable Loss

Another important relationship to remember is – as frequency increases so do the losses associated with the coaxial transmission cables. At higher frequencies, dielectric losses are greater and cable radiation increases. For these reasons, only the highest quality cables should be used for microwave applications. At a minimum, only double shielded cables should be used to prevent the signal from radiating from the coax prior to arrival at the antenna or receiver input. Double shielded cabled provide between 98% - 99% shield coverage while semi-rigid cables provide 100% shield coverage. It is important to always keep RF cable runs as short as possible since cable attenuation is cumulative. A cable exhibits 3dB loss at 10 feet (half power) will exhibit 6dB loss at 20feet (one fourth power). Excessive cable losses can make it impossible to design a working microwave link.

 

Microwave Propagation

One of the phenomenon’s associated with microwave frequencies is the line of sight propagation model. In general, a microwave link will work given sufficient power and antenna gain if the receive antenna can be seen by the transmit antenna. If an obstacle is present in the path, some of the energy will be absorbed and some energy will be reflected. Generally, surfaces are more reflective to energy at higher frequencies. Unlike radio waves at lower frequencies, microwaves signals do not propagate well through obstacles. In fact, the radio energy is often reflected off the intervening surface and redirected in another direction. When reflected waves arrive at the receive antenna they arrive delayed in time respect to the direct wave. This time shift is due to the time required to travel the extra distance from the emitter to the reflective surface and finally to the receiver antenna. This is known as multipath and the result is typically an unstable picture.

 

Propagation Fundamentals

Radio waves are said to propagate as they move through the atmosphere or other mediums from the transmitter to the receiver. As the wave is propagated, its energy level is constantly decreasing. This is known as propagation loss. The amount of propagation loss increases with frequency. That is why microwave links in the L and S band will perform better over long distances with fixed system gains than a link at C and X-bands.

With this knowledge, one can plan multi-hop microwave systems to take advantage of higher frequency equipment for the “first-mile” links and the lower frequency gear for the “back-haul” requirements. In general, the energy in a radio wave is dissipated along the route of propagation according to the inverse square law. This law states that each time the distance is doubled the resulting energy is ¼ that of the beginning level. If the path length is increase by four times, only 1/16 of the energy will be available. From this we can see how important is to minimize system losses and maximize system gains.

 

Fade Margin

Microwave signals rarely are propagated through a medium that exhibits stable characteristics. Signal levels are constantly varying over the path due to the effects of atmospheric conditions as well as absorption and reflection of the energy along the path. Propagation conditions change over time as a result of moisture, temperature and others factors. As a result, a path may fade slowly or rapidly over time. The flicker of starlight due to variation in the earth’s atmosphere provides an analogy to this phenomenon. When designing a microwave link, it is desirable to include a fade margin. The fade margin is factored in to allow for the variations in signal propagation noted above. A fade margin of between 10 – 20 dB is considered desired. A link may perform adequately without a fade margin, however, any variation in the signal level due to propagation losses will results in link failure.

The primary factors that determine the performance of a link are: transmit power, transmitter system losses, transmit antenna gain, propagation loss, receive antenna gain, receive system losses, and receiver gain.

 

Fresnel Zones

The requirement for line-of-sight for microwave transmission is only one of the important issues concerning microwave propagation. Another important factor affecting link performance is Fresnel zone clearance. This clearance is required around obstacles along the path. Remember just as light is dispersed from a flashlight, radio waves are dispersed from the antenna. The energy does not continue in a pencil like beam, but rather is dispersed in a concentric manner depending on the antenna design. While line-of-sight conditions may exist from the transmitter to receiver antennas over a narrow path, nearby obstacles which intrude into the Fresnel zone will degrade the link performance. Under some conditions, the curvature of the earth itself could violate Fresnel zone clearance!

Imagine a path from a hotel window to a receive location in a building across town. Clear line of sight exists between the transmitter and receiver and the antennas can be bore sighted for alignment. However, there are numerous buildings in the urban environment that flank the microwave path. Since the energy from the transmit antenna is dispersed off the bore sight to some extent, there will be blockage due to the buildings surrounding the path. These buildings are protruding into the Fresnel zone. Microwave engineers strive to design toward establishing a minimum 1st Fresnel zone clearance of 1.5 over obstacle for a highly reliable link. This is not always practice for short haul investigative links. The issue of Fresnel clearance is brought up to rise for consideration when examining the performance characteristics of a link. Awareness of the effects of nearby objects, off the bore sight, on microwave propagation is an important tool for the microwave technician.

 

Signal-to-Noise Ratio

When calculating system gains and losses we must establish a target figure that is considered acceptable to yield good results for the link. A signal arriving at the receiver at this level will result in a demodulated video signal with a signal-to-noise ratio (S/N or SNR) of 20dB. This is a rather noisy signal and is generally not considered a good quality picture. Remember, the receiver input level can be read directly off a spectrum analyzer. Being able to relate signal level to picture quality can be useful when using a spectrum analyzer to set up or troubleshoot a path.

In general terms, the larger the number representing the signal-to-noise-ratio the better the picture quality is. As changes in propagation effect the amount of signal arriving at the receiver input the picture quality will vary. This points out why it is important to maintain an acceptable fade margin when designing the link budget. It is the desired S/N that will ultimately dictate the factors that make up the entire link system.

Path Loss

Another factor that impacts link design is the attenuation of the signal as it travels from the transmitter to the receiver through the atmosphere. This is known as “free space” loss or path loss. Much greater reductions or losses are experienced when microwave signals are propagated through other mediums (i.e. walls). In some cases, the signal is reflected off a surface and no energy is propagated through. Path loss is a combination of attenuation of the RF signal due to frequency of operation and the length of the path. Path loss increases with frequency and distance. It is greater at C-band (5000MHz) than it is at L-band (1700MHz). The formula 20 log 4πd / λ where d and λ are in the same units will determine path loss. Over a one mile path at L-band, the path loss is equal to 102 dB and at C-band the path loss is equal to 110dB.

The difference in path loss between L-band and C-band is 8dB. This difference can be compensated for in antenna gain! Remember that as frequency increases wavelength decreases. This means that the same size dish antenna will deliver more gain at C-band than L-band. For example, a 14” dish results in about 16dB of gain at L-band but nearly 22dB of gain at C-band! More gain for the same size antenna is not the entire picture, however. It is generally more difficult to generate the same RF power level at C-band as L-band transmitters tend to run less power for the equivalent package size when compare to L or S-band transmitters. (Recall that a reduction of 3dB is equal to reducing the power to half.)

Also, increase antenna gain means decrease beam width making antenna alignment more time consuming and critical. These additional factors must be contemplated when considering link design.

 

Multi Path

Multipath interference is a common occurrence with microwave deployed in investigative capacities. Multipath is a result of the RF energy emitted from the transmitter taking more than one path (the direct path) on its way to the receiver. These multiple paths are usually the result of the energy being reflected off of surfaces (walls, buildings, or even the earth itself) in the nearby environment.

Since the energy has taken a path other than the direct route, the amount of time it takes for the wave front to travel from the transmitter to the receiver has increased. This increase in path length, or time, results in a phase shift since the reflected wave has been “delayed.” Radio waves add and subtract just like sound waves. If you place yourself in a room with a lot of reflective flat surfaces and put a high frequency tone (8 kHz will do) through a loudspeaker you can create audio multipath. Walk around the room and turn your head around while carefully noting the intensity of the audio signal. When the signal gets louder, you are either in the field of the direct wave form the loudspeaker, or in a zone where the reflections are additive. When the intensity of the sound is decreased, you are in zone where the reflections are subtractive. This is exactly what happens with radio waves as well.

Multipath can create a number of aberrations in the video signal. The most notable example is the case that results in Chroma shift. Color information in a television signal is dependent upon phase. When the phase of the signal is corrupted by multipath, the integrity of the color information is compromised. In more extreme cases, the signal will cancel out altogether and no recoverable picture will be present.

To visualize how multipath is generated in the environment, imagine a candle in the middle of a room. The candle emits light energy in all directions and the light is reflected off walls and others surfaces to the light room. If we place a reflector behind the candle the light is directed in one direction at the expense of another. However, since the light is reflecting off the walls, the area behind the candle is not perfectly dark. Light arrives in the area behind the candle because of multipath. If we now replace the candle with a microwave transmitter and an Omni directional antenna, we can duplicate the experiment in the RF domain. Energy from the antenna is reflected off the walls (one or more times even) to create an environment rich in multipath. A receiver with an Omni directional antenna would experience interferences from the reflecting signals.

 

Reducing Multipath

One of the best ways to mitigate the effects of multipath is to employ directional antennas on each end of the link. On the transmit end, a directional antenna focuses the energy in one particular direction and reduces the possibilities for the energy to reflect off surfaces that are not within the beam pattern of the antenna. The potential still exists, however, for multipath to be created by reflections off surfaces in the beam of the antenna. That’s why having antennas in the clear, away from obstacles has a profound effect on link performance.

Extending this concept on the receive side, a directional antenna provides a high level of discrimination from unwanted signals (in addition to gain in the desired direction). Signals arriving at the antenna off the main beam will be greatly attenuated. Since the reflected energy is low in level as result of the directional antenna, its effect is greatly minimized. Directional transmit antenna act to reduce the opportunity for multipath generation while directional receive antennas act to further reduce the effects of multipath by attenuating the reflected signals not in the main beam of the antenna. Understanding antenna patterns is important to successfully designing a link. Antenna with different directional characteristics and sizes should be evaluated for their suitability for each installation and the likelihood that multipath will be a problem.

 

Polarization

Another characteristic of antenna can be used to combat the effect of multipath. When a signal is reflected from a surface its polarity is changed. Consider again the analogy to light energy. When light is reflected from a surface such as a road, the polarity of the light is changed. This is why polarized sunglasses are so popular and effective. By filtering the light with a polarized lens, the reflected (or interfering) light can be greatly reduced, improving visibility.

 

Linear Antennas

Most of us are familiar with antennas that are linear polarized. A Yagi directional antenna is a good example of this. When the antenna is oriented in the vertical plane it is said to be vertically polarized. When it is held in the horizontal plane it is horizontally polarized. It is important to match the polarity of the transmitting and receiving antennas in a link since as much as 15dB of energy can be lost through cross polarizing.

Circular Antennas

Antennas that radiate energy in an ever changing polarity are known as circular antennas. Circular polarity is either right hand of left hand. This designation defines whether the energy field is rotating in a clockwise or counter-clockwise direction. When a right hand circular wave is reflected it becomes a left hand circular wave. If a right hand circular wave is used on the receive end it will reject the out of polarity energy by as much as 25dB! This is a significant reduction in energy arriving at the antenna. This is why circular antennas are the best choice in a multipath environment. While circular antennas provide excellent rejection of out of polarity signals, they are generally larger and more expensive than linear antennas. Not all reflected signals cause harm to the received picture quality. Indeed, if this were the case, very few microwave links would work at all. Because of the properties of FM receivers, signals that are greater than 30dB weaker (-30dB) than the desired signal tend to have little or no impact on the desired signal. This is known as the capture ratio. If a reflection form the surface is 35dB weaker than the direct wave we may never know it is there even though it represents multipath. Since most surfaces are not perfect reflectors, a substantial amount of energy is also absorbed by the surface. Because of this, many reflections pose no threat to link performance.

 

Fade Margin

Since the ratio of the direct signal to the reflected signal may not always be constant over a given path due to changes in propagation, it is best to always strive for the greatest fade margin achievable. If a path results in a difference in direct a reflected wave energy level of only 32dB, then a degradation of the desired signal by only -2dB will cause problems. In many cases, in the absence of interfering reflected waves, the -2dB fade would only eat away at the fade margin and do not produce noticeable results in the picture quality.

Microwave System Basics

Pads

There is another tool use to combat effects of multipath. Consider a direct wave arriving at the receiver input terminal with a signal level of -60dBm and a reflected wave with a signal level of -70dBm. Since the difference in these signal levels is not greater than 30dB, we are likely to see interference from the reflected signal. Remember that the receiver threshold is -78dBm. If we insert a pad, or attenuator in the receive coax line to reduce the signals from the antenna by -10dB, we now have a desired signal level of -70dBm and an undesired signal level of -80dBm. The undesired signal is now below the threshold of the receiver and no longer passes any threat! A set of RF pads should be included in every toolbox.

It can be concluded that the worst case link configuration employs an Omni directional antenna on both the transmitter and receiver in a reflective environment. Recognizing that it is not always practical to deploy a directional antenna in a covert transmitter installations because of space limitations, it is useful to understand multipath and how to minimize its effects on the receive side of the link. Since it is difficult to predict all the possible reflections resulting in multipath, often experimentations with the receiver antenna location and RF attenuators can yield configuration where the interference is not objectionable.

 

Transmission Bandwidth

Spectrum is generally divided up into 25MHz channels for FM video transmission. Each channel is defined by the center frequency. For example, a channel frequency of 1735MHz occupies the spectrum from 1722.5MHz to 1747.5MHz. A typical video signal containing two audio subcarriers occupies approximately 16MHz of the 25MHz channel. The additional 9 MHz is divided equally on each side of the channel to provide a “guard band.” Therefore, a total guard band of 9 MHz exists from each center channel. This guard band serves to provide sufficient separation of signals for multiple channel operation.

When external RF filters are employed to help solve interferences issues, they must be wide enough to pass the full 16 MHz signal without degradation while reducing the energy outside of this band. Any energy within the 16 MHz wide channel will result in interference. This is why it is important to comply with a channel plan and not operate on self-assigned frequencies. A quick glance at a spectrum analyzer can indicate if interfering signals are within the bandwidth of the desired signal. A graph of the spectrum of a video/audio transmitter reveals the video signal occupies the spectrum from 0 - 4.5MHz with subcarriers located at 6.2 and 6.8 MHz. Color signals require more bandwidth than monochrome signals and signals with audio subcarriers require more spectrum than those without audio subcarriers.

Audio subcarriers are additional signals that are transmitted on the main carrier in addition to the baseband modulating signal. The subcarriers are typically sent at a level of -28dBc or 28dB bellow the main carrier. A number of subcarriers may be added to the RF carrier if required as long as the highest modulating frequency is 6.8 MHz. Practical concerns for interferences potential between subcarriers dictate that up to four subcarriers can be used at one time. The use of additional subcarriers requires a receiver filter designed to remove the carriers from the video band to eliminate artifacts created by the presence of the additional carriers.

As noted above, propagation loss increases with frequency. All things being equal, longer ranges can be achieved at lower frequencies. Also, surfaces become more reflective and absorptive at higher frequencies. An L-band signal will experience less loss through a typical gypsum wall than will a C-band signal. These factors all contribute to frequency band selection for a particular operation.

 

Antennas

Probably the single most important factor which will determine the performance of a microwave system is the antenna. The antenna system is responsible for transferring the RF energy from the transmitter to the receiver. The gain and efficiency of the antenna system plays a critical role. Antennas are application specific in investigative microwave. This means that there is no one solution that works best under all circumstances. Each operation should be examined individually to arrive at the best antenna solution. Success in the design and implementation of an investigative microwave system lies in a good understanding of antenna characteristics and suitability for the requirement. A well-stocked tool box of antennas is important for microwave system design.

Antennas are classified by a number of different attributes. Primary among these are radiation pattern and polarization. The radiation pattern defines the way in which the energy is dissipated from the antenna. An Omni-directional antenna distributes energy in equal amounts with respect to the azimuth plane. A uni-direction antenna distributes energy in primarily single direction. Some special application antennas have radiation patterns distribute energy in two or more specific directions. Such antennas are uncommon in microwave applications.

 

Isotropic Radiator

Omni- directional antennas are simple in design and usually occupy least amount of physical space. The prefect Omni-directional antenna is the isotropic radiator. This antenna radiates energy in equal amounts in all planes. Consider an antenna at the center of a basketball. If the outer skin of the basketball represented the RF energy from the antenna in the center, you can imagine an isotropic radiator.

Most Omni-directional antennas, however, are not perfect radiators and have a radiation pattern more like that of a doughnut. Picture the same antenna located at the center of a doughnut and perpendicular to the horizontal plane. This is the radiation pattern of the typical dipole type Omni-directional antenna. Notice that the energy that we had above and below in the prefect isotropic radiator is now missing. This hole in the pattern of the dipole Omni-directional antenna is significant for a number of reasons.

First, directly above and below a vertically oriented Omni-directional dipole antenna there is a minimum amount of energy. If the antenna were placed in the horizontal plane, the reduction of energy would be off the ends of the antenna. It is important to understand the pattern of an Omni antenna so as not to expect it to perform well off the antenna ends in the respective plane. Second, the energy did not disappear without a trace when compared to the isotropic radiator. Rather, the energy was concentrated in a different plane, away from the ends of the vertical radiator and toward the horizon. This phenomenon is known as gain.

In fact, 2.1dB more energy exists on the horizon from a dipole antenna than an isotropic radiator. In most cases, energy on the horizon is much more useful than energy emitted into space. The difference in the distribution of energy between these two antennas types forms the basis for the reference point of gain measurements. Antenna gain referenced to an isotropic radiator is noted as dBi and antenna gain reference to a dipole radiator is noted as dBi. The difference is noteworthy.

 

Gain

For example, if a manufacturer claims an antenna gain of 5dB without providing a reference, the actual antenna gain could be 2.9dBd. Antenna gain is expressed as gain over the reference. So gain of 5dB over an isotropic radiator is only 2.9dB over a dipole antenna. The difference are important when calculating path budgets and evaluating antenna performance specifications. Most gain figures are given as dBd since the isotropic radiator is considered a model antenna while the dipole is accepted as a practical real-world antenna. Antenna gain is very important in microwave systems since it is difficult to generate high power at microwave frequencies. An antenna gain of only 3dB is equivalent to doubling the RF power. For example, a system with a 2W transmitter using an antenna with unity gain will radiate 2W of power. This radiated power is expressed as effective radiated power or ERP. The same 2W transmitter feeding an antenna with 3dB of antenna gain will have a system ERP of 4W. The same transmitter with a 6dB antenna will have an ERP of 8W! This represents a substantial gain in system performance without the power supply penalty associated with a power amplifier.

It’s important not to give up antenna gain in feed line losses. Coaxial cable exhibits substantial loss at microwave frequencies. Only the best quality cable in the shortest run practical should be used.

 

Polarization

Polarization of antennas was discovered in the section on multipath. Linear polarization is exhibited by most Omni-directional and Yagi type antennas. Linear antennas can be either vertically or horizontally polarized depending upon their orientation. Circular polarized antennas are characterized by the direction of the rotating energy field. Right hand (RHCP) and left hand (LHCP) are the two polarizations associated with circular antennas. Circular antennas may be used in systems with linear antennas with a penalty in the gain factor of the circular antenna. For example, a 16dB gain circular antenna receiving a signal from a vertically polarized linear signal will behave like a 13dB gain vertically polarized linear antenna.

Polarization can be used to the link designers’ advantage by the deliberate implementation of cross-polarization channels. If, for example two adjacent channels were used along the same path to send two independent video signals, the potential exists for interference at the receive end for the respective adjacent signal. Through the use of cross-polarization antennas, vertical on the first and horizontal on the second, for example, up to 15dB of isolation between the signals can be achieved.

 

Omni-directional Antennas

Omni-directional antennas typically transmit and receive equally poorly in all directions. Normally, when an Omni is referenced it is assumed that the antenna is oriented vertically and the Omni-directional pattern of the antenna is defined by the radiation pattern in the azimuth axis. Omni-directional antennas can exhibit gain focusing the radiated energy toward the horizon. Gain is achieved by stacking a number of antennas on top of each other so that the radiated energy adds in-phase in azimuth. Therefore, for a given frequency band, the longer an Omni-directional antenna is, the more radiating elements it contains, and the more gain it exhibits. Gain can be visualized as the compression of the doughnut with more energy radiated along the x-axis as opposed to the y-axis.

Omni-directional antennas are typically avoided in fixed installations because of their propensity to encourage a multipath environment. In mobile and body-worn applications the deployment of an Omni-directional may be a requirement since it would be impossible to maintain the orientation of a directional antenna toward the receiving station while the transmitter is in motion. Additionally, Omni-directional antennas are physically much smaller than directional antennas. In such a case, a directional antenna should be employed on the receive end. As is the case with any antenna system, maintenance of the orientation of the radiated energy is important with Omni-directional antennas. Remember that up to 15dB of loss can be exhibited by cross-polarization antennas. If an Omni is used in a concealment for example, the antenna on the receive end of the link should be oriented in the same polarity. A cross-polarized receive antenna could produce the effect of having a 250mW concealment transmitter behaving like a 15mW transmitter!

Directional Antennas

There are two primary directional antennas used in investigative microwave applications. The Yagi antenna delivers good performance in a compact package. The dish antenna better results in a larger package. The Yagi antenna is a linear polarized antenna while dish antennas are commonly used as linear or circular antennas. Directional antennas achieve their directionality, and hence gain by redirecting energy in a single direction. A directional antenna with a gain of 3dB has the effect of doubling the transmitter power in a given direction at the expense of forfeiting the energy in another direction. Uniline power amplifiers, there is no increase in RF power in terms of Watts, rather the increase in performance is due to focusing the energy where it will do the most good.

Beam width

The greater the gain of an antenna, the larger its size and the narrower its beam width. Yagi antenna gain increase as the antenna is lengthened, adding additional elements to the antenna boom. Dish antenna gain increase with the diameter of the reflector. In both cases, more energy is focused tightly on a single point as the gain goes up. This increased focus results in a narrowing of the antenna beam width, or pattern. The narrower the beam width of an antenna, the more difficult it is to align the antenna system. The very narrow beam of a high gain antenna requires careful attention and precision during alignment. A solid mount with considerable mechanical integrity is also required to maintain the point.

The beam width of a directional antenna is defined by the angular distance from the center line where the energy field drops off by one-half, or -3dB. For example if a dish antenna has a beam width of 40°, the half-power points are +-20° from the center of the antenna.

 

Dish Antenna

A dish antenna is very much like a flashlight. The feed can be likened to the light bulb and the dish to the polished reflector. Indeed, the dish part of the antenna is called the reflector since it reflects the RF energy and focuses it on the feed. The feed is located at the focal point of the antenna. Energy arriving off axis to the dish antenna is not reflected back to the feed and therefore, not picked up by the antenna. Energy off the back of the antenna does not arrive at the feed because it is blocked by the reflector.

 

Yagi Antenna

Yagi antennas are directional because of the parasitic properties of the elements in the near field of the antenna. The part of the Yagi antenna that RF power is applied to is known as the driven element. Generally, there is a single reflector element located behind the driven element and a number of director elements spaced out in front of the driven element. The more elements a Yagi antenna has the greater it’s’ gain. The addition of director elements has a larger impact on the gain characteristics of the antenna than does the addition of reflector elements. The size and spacing of the director and reflector elements are related to the operating frequency of the antenna. The directors and reflectors are known as parasitic elements because no RF energy is applied to them directly to arrive at the directional pattern.

 

Front-to-Back Ratio

Directional antennas not only are useful because they direct energy in a single direction but also because they discriminate against energy in the opposite direction. The directional properties of an antenna can be used to null the effects of an interfering signal by orienting the antenna so that the interfering signal is off the back of the antenna. This is an effective technique providing a sufficient amount of energy from the desired source is available from the desired source in the beam width of the antenna. The directionality of an antenna is often expressed as the front-to-back ratio. This is the measure of the amount of gain in the forward direction and rejection in the rearward direction.

 

The Link Budget

Often, a covert microwave installation does not fit the model of the ideal microwave system. This is the case since line-of-sight conditions cannot be met with concealments buried in hotel rooms and target offices. However, point-to-point backhaul links fit the model well and the understanding of the factors involved in constructing a reliable link is useful to apply to all installations. The link budget is a mathematical representation of the system gains and losses that comprise the path from the transmitter to the receiver. By working the budget analysis prior to installing the link, deficiencies can be discovered and rectified saving valuable troubleshooting time in the field.

 

Quality and Reliability

Before doing the arithmetic, the desired quality and reliability of the link must first be established. The quality of the link is defined by the desired signal-to-noise ratio of the demodulated video signal. The signal-to-noise ratio defines how clear and snow-free the picture is. A typical target number for surveillance applications is a signal-to-noise ratio of 50dB (un-weighted). The reliability of the link refers to its immunity to conditions that will reduce the signal level at the receiver resulting in a change of the desired signal-to-noise ratio. In the link budget this is known as the fade margin. A typical target number for surveillance application is a fade margin of 10dB.

The first parameter to consider is the desired signal-to-noise ratio. Once that is set, the amount of fade margin can be assigned. The fade margin describes the amount of signal loss that can be tolerated by the link before there is an impact on the signal-to-noise ratio performance. For example, a fade margin of 10dB allows a reasonable amount of system headroom before the picture quality suffers. Factors that influence the performance of a link path includes, attenuation due to foliage, rain, fog, snow, variations in temperature over different earth surfaces, or intervening obstacles such as cars or trucks. It is the amount of change in path conditions, changes that can vary on an hourly, daily, or seasonal basis, which are accounted for in the fade margin. In some cases, a generous fade margin may be a luxury and in other cases in may be a necessity. The terrain path traverses and other local conditions will have a profound impact on the amount of fade margin assigned to the link.

Losses in a microwave system can sometimes be subtle. Corroded coaxial connectors or old weathered cables can introduce additional losses. Coaxial adapters and RF band pass and single channel filters all contribute additional loss. Coax cables that are deformed or kinked can also be a source of unwanted loss. Remember, loss is easier to come by than gain. Examine system components carefully to insure they are suited for the task.

 

The Path Equation

The path equation is an arithmetic calculation that takes into account the gains and losses on the transmit side, the loss of the transmission medium and finally the gains and losses associated with the receive end of the link.

The essential components of the path equation are:

1. Transmitter power
2. Transmission line losses
3. Transmit antenna gain
4. Free space path loss
5. Obstacle attenuation
6. Receive antenna gain
7. Transmission line loss
8. Receiver level necessary to achieve desired S/N
9. Fade margin

The first three items are associated with the transmitter side of the link. Items four and five relate to the transmission medium. Items six through eight are associated with the receive side of the link. Finally, the fade margin determines how much headroom the system has before the desired signal-to-noise ratio is compromised.

In order to complete a path equation, the units of measure must be normalized. For example, transmitter power, commonly referred in Watts, must be converted to dBm. Required receiver sensitivity must be known in terms of dBm. Attenuation in dB must be determined for the particular RF cable used in the system. Antenna gains must be specified in dBi (remember to add 2.1dB to published antenna gains in dBd).

 

Practice Problem

Assume a path at 2300 MHz over a distance of 2 miles with a Transmitter set to high power (2W). The transmit antenna is a Yagi antenna with a gain of 15 dBd. There is 10 feet of RG-9/U low loss cable between the transmitter and receiver with published loss of 8.2dB per 100 feet. The receive antenna and cable configuration is identical to the transmit complement. We would like to achieve a picture quality of 60dB signal-to-noise and provide for a 10dB fade margin.

 

Transmit Calculations

Starting with the transmitter power output level, we must convert 2W to dBm. Referring to the conversion chart, it is determined that 2W equals +33dBm, next, the loss attributed to the RF interconnected cable must be calculated. The manufacturer’s data indicates a loss of 8.2dB per 100ft at 1000MHz. assuming that the loss function is linear, we obtain a loss of 18.9dB per 100 ft. at 2300MHz by multiplying the loss at 1000MHz by 2.3. Next, dividing by 10 to convert the 100ft. calculation to 10ft. yields a loss of -1.89dB. The antenna gain is converted from dBd to dBi by adding 2.1. The resultant antenna gain is +17.1dB.

 

Path Calculations

Free space attenuation at 2300MHz over the specified path is -110dB. An easy rule of the thumb for determining path loss at L and S-bands is to assume a figure of -104dB over a one mile path. For each doubling of the distance, there is an additional -6dB of loss. For example, the path loss at four miles is -116dB and the path loss at 8 miles is -122dB. There are a few trees acting as obstacles over the path contributing -3dB.

 

Receiver Calculations

The receiver gain and coaxial cable loss is the same as the transmit side with +17.1dB and -1.89dB respectively. The signal level required by a receiver to deliver a picture with a 60dB signal-to-noise ratio is determined from the chart to be -56dBm.

Adding the components of the link together gives the following result:

 

Transmitter power                     +33

Coaxial cable                             -1.89

Antenna gain                             +17.1

Path loss                                    -110

Obstacle                                     -3

Antenna gain                             +17.1

Coaxial cable                             -1.89

Energy delivered to receiver = -49.58dB

 

Since a signal level of -56dBm is required to obtain desired picture quality, it is determined that the link will perform well. In addition, the excess signal available is +6.42dB. The excess signal represents the fade margin and is fails to meet the desired 10dB figure. We could accept a reduced fade margin or replace one or more of the antennas with a higher gain antenna.

Assuming the same equipment is available for a required path of 8 miles, we can solve the equation to determine the amount of energy delivered to the receiver. From this figure, we can ascertain the picture quality from the calibration chart. Substituting the path loss of 8miles (-122 dB) the following results are obtained: Transmitter power +33 Coaxial cable -1.89 Obstacle -3 Antenna gain +17.1 Path loss -122 Antenna gain +17.1 Coaxial cable -1.89 Energy delivered to receiver = -60.58dB

A cross-check to the calibration chart shows that the resultant signal level will deliver a signal-to-noise of 58dB. This is very good picture, however we have not allowed for any fade margin. From the calibration chart it can be determined that if the link experiences a fade of -10dB, the picture quality will degrade to 50dB S/N. This is still a good picture and this link budget should yield excellent results.

The above examples illustrate the interactions of the different elements that make up a microwave system. It is possible to fix any one of the factors and manipulate others to determine minimum requirements. For example, it is acceptable to achieve a 50dB S/N with no fade margin, the system can tolerate an additional loss of 10dB. It is possible to reduce the transmitter power by -10dB or the antenna gain by -10dB and still maintain system requirements. It may be desirable in some circumstances to solve for the lowest value of transmitter RF power due to battery power concerns and in other cases to solve for minimum antenna gain due to concerns over mounting or hiding an antenna.

Reducing the transmitter power by -10dB results in a power output of +23dBm or 200mW. This can translate into a substantial impact on the power requirements. Many factors influence the performance of the link. The primary concern in link budget design is the application for the system. The application will drive the manner in which the link is constructed.

 

Antenna Beam width and Gain

Directional antennas are generally defined by the amount of gain they deliver over a reference antenna. This gain is the result of forming the RF energy into a beam that is pointed in the direction of the receiving or transmitting station depending on which end of the link you consider. Generally speaking, the more gain associated with a particular antenna, the narrower the beam width of the transmitted or received signal. This is why it is harder to align very high gain antennas when compared to moderate gain antennas. When placing antennas it is important to remember that the energy from the antenna diverges at a given angle (or cone) from the end of the antenna and does not propagate from the antenna in a pencil thin straight line. A typical 10dB gain antenna will have a beam width of 53° while a 16dB gain antenna will have a beam width of 36°. Note how beam width decreases as gain increases.

Remember that path calculations are based on line-of-sight conditions and any obstacles in the path (i.e., buildings, trees, hills) will conspire to reduce the effective range of the link by blocking a portion of the energy emitted from the transmitter!

 

Effective Isotropic Radiated Power

Effective Isotropic Radiated Power (EIRP or ERP) is a measure of the effective power radiated from an antenna when the antenna gain factor is applied to the value of RF power exciting the antenna. Consider that an antenna can provide gain to a system without having to increase the transmitter output power and suffering the subsequent increase in dc supply requirements and heat dissipation! Of course, nothing comes for free, but antenna gain is very good bargain.

Increase antenna gain is associated with increased antenna size and decreased beam width. If Omni-directional coverage form an antenna was required, a directional antenna would not suffice and an increase in transmitter power might be the only acceptable solution. However, if the receiving station is fixed in one direction from the transmitter, a directional gain antenna is the solution.

 

Troubleshooting

Careful attention to proper planning of a link must eventually move out of the office and into the field for the installation. Preceding under the assumption that if anything can go wrong it will. If things don’t work out as they did in the shop, it’s time to troubleshoot the link. This is where the preparation work really pays off. Instead of attempting to engineer a non-working link on the fly, your attention can be focused on identifying the problem through isolating subsections of the link.

 

Step-by-Step

A methodical approach to troubleshooting will yield the best results in the shortest period of time. This technique, coupled with the proper tools and test equipment, can quickly turn a scapegoat into a hero. Isolation of the sections of the link where the problem is likely to exist is the first step. The obvious things are the first place to start. Let’s consider the link system we configured for the link budget analysis.

Assuming that the link budget indicated adequate signal will be present to deliver the required signal-to-noise ratio and fade margin, we might investigate obstacles in the link path to begin with. A site survey is usually one of the first actions that take place prior to installing a link. However, things sometimes change from the survey date to the installation date. It’s not uncommon for the spring time foliage to render a link inoperable that appeared clear only week earlier. Visual re-verification of the link path is a good first step in troubleshooting.

The next step is to isolate either transmit or receive sides of the link. Experience will give clues to which end to start on. Let’s begin on the transmit side.

 

Transmit Troubleshooting

Verify that all connections are tight and cables are in good condition. Check that the power supply connection polarity is correct. If the positive and negative leads are reversed, no damage will result to the transmitter since the input is protected with a diode. Make sure the channel selector is on the correct frequency and the proper power level is selected. If a directional antenna is employed, it must be pointed in the correct orientation.

A digital voltmeter can provide a quick check of the dc power system for the transmitter. The nominal input voltage for the transmitter is 12VDC. Check the power supply output voltage to confirm that it is between 10 -14Vdc. A voltage less than 10VDC will not operate the transmitter. Long DC power cords between the power supply and radio can be the source of a voltage drop that can have the effect of reducing an otherwise acceptable voltage level to one below the threshold. Small diameter wire is particularly troublesome as a source of voltage drop.

Another voltmeter check at the transmitter end requires measuring the current flowing from the power source to the transmitter. This procedure involves interrupting the positive side of the supply and inserting the meter in series. Proper operation of the transmitter on high power should show a reading of approximately 1.0A and low power should read about 300mA. Readings outside these specifications could indicate a transmitter problem.

The next step is to verify the transmitter output power. A thru line wattmeter and properly sized 50 ohms dummy load is required to make the measurement. Select the proper plug-in element for the power level and frequency being measured. Connect the transmitter output to one side of the meter and the dummy load to the other. With DC applied to the transmitter, the RF output level should be indicated on the wattmeter. An incorrect reading could indicate a transmitter problem.

Next, remove the DC power from the transmitter and remove the dummy load from the wattmeter. Connect the antenna feed line to the port on the wattmeter where the dummy load was connected. With the arrow on the plug-in element pointed toward the antenna, reapply the DC power. The reading will indicate the energy flowing from the transmitter toward the antenna. Any power reflected back to the transmitter can be measured by turning the plug-in element so that the arrow points toward the transmitter.

Significant energy flowing in the reverse direction point to a cable or antenna system problem, the reverse power indicates that the energy is not being absorbed and radiated by the antenna system. The next step is to isolate the problem to the remaining sections of the link. Moving the wattmeter to the end of the cable prior to the antenna connector can isolate the feed line. Place the dummy load back on one side of the wattmeter. Rotate the arrow on the plug-in to point away from the transmitter and toward the dummy load. Reapply DC and make a RF power measurement. Rotate the plug-in element so that the arrow points toward the transmitter. Significant energy flowing in the reverse direction points to a problem with the cable.

Next, remove the dummy load and attach the antenna to the wattmeter. With the arrow pointing toward the antenna make another RF power measurement. Again, rotate the arrow on the plug-in toward the transmitter. Significant energy flowing in the reverse direction points to a problem with the antenna.

Finally, a spectrum analyzer or frequency counter may be used to verify that the transmitter is operating on the correct frequency. An off- frequency condition indicates a failure in the transmitter.

 

Receive Troubleshooting

A similar step-by-step procedure for troubleshooting is followed at the receive end. Verify the all connections are tight and cables are in good condition. Check that the power supply connection polarity is correct. If the positive and negative leads are reversed, no damage will result to the receiver since the input is protected with a diode. Make sure the correct frequency is selected. If a directional antenna is employed, it must be pointed in the correct orientation.

It’s usually simpler to begin the process at the receiver end rather than at the receive antenna. First, check that there are no inappropriate RF band pass filters installed at the receiver input. A band pass filter for L-band will not pass energy in the S-band and vice versa. A single channel filter will only pass energy on the channel it is designed for. The filter must be removed for the receiver to operate properly.

If no picture is seen on the monitor but a signal level is indicated on the signal strength indicator, check to see that the video output level is not turned down all the way. Reset the video level to reading of 80 and check again for a picture. A reading of 80 should result in an output level of around 1V p/p. Remove the antenna input from the receiver and verify that the indicated signal level drops away. If the signal level does go away, there is likely a failure in the receiver demodulator.

Attach the feed line to the input of a spectrum analyzer tuned to the correct operating frequency. The carrier from the transmitter should be visible on the display in addition to modulation. The carrier level can be read directly on the analyzer and should correspond with the level calculated during the link budget analysis. A lack of signal, or a significantly lower signal than calculated could indicate a problem with the feed line or antenna system.

Use the spectrum analyzer to identify interference that may be causing the link failure. The occupied bandwidth of the video signal is 16MHz. Therefore, check up to 8MHz either side of the center carrier for the other carriers. Signals present in this area will cause the receiver demodulator to be unable to differentiate the video signal. If the source of the interference cannot be determined, and the interference terminated, another clear frequency must be selected. The spectrum analyzer can be used to locate a clear channel.

If strong carriers are noted very near the 16MHz channel bandwidth, a single channel filter may solve the problem. If strong carriers are noted a few hundred megahertz away from the desired channel, a band pass filter may solve the problem. Inserting the proper filter between the feed line and analyzer input will give an immediate visual indication of the result of the filter. If the nearby carriers are reduced, reconnect the antenna to the receiver and verify proper operation.

If few or no signals are indicated on the analyzer, there may be a problem with the feed line or antenna. The next step is to move the analyzer to the antenna output. If the signal levels increase, there is likely a problem with the feed line. If there is still no indication of signals, there may be a problem with the antenna.

Isolating each part of the link, step-by-step is the best method of troubleshooting. If a wattmeter and spectrum analyzer are not available at the site but replacement equipment is available, then the “shotgun” approach must be used. Never violate the law of replacing only one component at a time. While that may get the system running, it will not identify exactly which component is defective leaving open the possibility that the component will mistakenly be installed again with the same results. While time is always at a premium during the troubleshooting process, it is always worth the few seconds it takes to identify in writing the defective component for later identification and repair.

 

Summary

While there is no substitute for real-world experience in the discipline of technical investigation, a solid background in the fundamentals of microwave equipment, theory and practice is important part of the skill set of the effective technical investigator. Investigative microwave is both a science and an art. Covert video violated most of the tenants recognized by trained microwave engineers! Why would anyone place an antenna where it is prevented from achieving line-of-sight? The answer to this question is obvious to a technical investigator. The technical investigator is largely responsible for developing effective techniques that are specific to covert investigations. Don’t be afraid to experiment and pass along you experiences.

MICROWAVE SYSTEM BASICS

MICROWAVE SYSTEM BASICS

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