Direction Finding Application Note - MetricTest

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1 Application Note Direction Finding Part of the Interference Analysis application note series. Introduction You’ve identified an interfering signal, now you need to track it to its source. This is done with various methods of direction finding. The least effective way to locate an emitter is to use a non-directional antenna and just try to go in the direction that gives you the largest signal. In the real world, multi-path propagation makes such a procedure difficult and frustrating. You need a method that shows you the direction of arrival of the signal. Most often this is done with a directional antennas such as Yagi beams, log-periodic antennas, and flat panel directional antennas. It can also be done with an array of non-directional antennas that are diode switched. This procedure is commonly called “Doppler scanning” and quickly yields direction of arrival information. This topic is discussed later in this note. The basic procedure of direction finding is theoretically simple but in practice multi-path propagation often makes the process very complex and fraught with false or confusing direction indications. To do the job effectively you need a map and a directional antenna that works at the frequency of interest. The exact type of map isn’t very important. You could use a topographic map if you have one, but if not a standard road map will do just fine. If you do the work without a map you can still get the job done, but not nearly as effectively since you will need to somehow keep track of direction of arrival information as you go to other locations to take additional directional fixes. For the directional antenna, it is helpful to have the antenna’s radiation pattern. As you will see later in this note, you can employ the side lobe notches if you know where they are in the radiation pattern. Multiple Location DFing By using multiple locations with directional antennas at each location, the approximate location of an interfering signal can be ascertained, assuming the signal stays on the air long enough to turn antennas to determine the direction of arrival. For interferers that are on the air for a short time, this method doesn’t work very well with rotating antennas. The Doppler direction finding method is very helpful for short duration signals, assuming that you are observing the direction of arrival display when the signal is on the air. Single Antenna DFing The assumption here is that you are working on your own or with one other person to find the offending signal. It is wise to make careful and deliberate measurements to accurately determine your present position and your best estimate of the direction to the emitter. This is very difficult to do from a moving vehicle because of multipath enforcement and cancellation. The distraction of aiming the antenna and watching signal strength can be dangerous if you are working alone. To make measurements, stop the vehicle, determine your exact location on the map then rotate the antenna to find direction to the largest signal. Plot that information on the map with an arrow from where you are in the direction of arrival of the signal. Use an arrow rather than just a straight line so you will be sure of the direction of arrival later on. This is especially important if it turns out that you are chasing a multi-path reflection. What you may see if that is the case is that the arrows end up pointing in all sorts of wild directions rather than toward a single point or small area.

Transcript of Direction Finding Application Note - MetricTest

Page 1: Direction Finding Application Note - MetricTest

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Application Note

Direction FindingPart of the Interference Analysis application note series.

IntroductionYou’ve identified an interfering signal, now you need to track it to its source. This is done with various methods of direction finding. The least effective way to locate an emitter is to use a non-directional antenna and just try to go in the direction that gives you the largest signal. In the real world, multi-path propagation makes such a procedure difficult and frustrating. You need a method that shows you the direction of arrival of the signal. Most often this is done with a directional antennas such as Yagi beams, log-periodic antennas, and flat panel directional antennas. It can also be done with an array of non-directional antennas that are diode switched. This procedure is commonly called “Doppler scanning” and quickly yields direction of arrival information. This topic is discussed later in this note.

The basic procedure of direction finding is theoretically simple but in practice multi-path propagation often makes the process very complex and fraught with false or confusing direction indications.

To do the job effectively you need a map and a directional antenna that works at the frequency of interest. The exact type of map isn’t very important. You could use a topographic map if you have one, but if not a standard road map will do just fine. If you do the work without a map you can still get the job done, but not nearly as effectively since you will need to somehow keep track of direction of arrival information as you go to other locations to take additional directional fixes.

For the directional antenna, it is helpful to have the antenna’s radiation pattern. As you will see later in this note, you can employ the side lobe notches if you know where they are in the radiation pattern.

Multiple Location DFingBy using multiple locations with directional antennas at each location, the approximate location of an interfering signal can be ascertained, assuming the signal stays on the air long enough to turn antennas to determine the direction of arrival. For interferers that are on the air for a short time, this method doesn’t work very well with rotating antennas. The Doppler direction finding method is very helpful for short duration signals, assuming that you are observing the direction of arrival display when the signal is on the air.

Single Antenna DFingThe assumption here is that you are working on your own or with one other person to find the offending signal. It is wise to make careful and deliberate measurements to accurately determine your present position and your best estimate of the direction to the emitter. This is very difficult to do from a moving vehicle because of multipath enforcement and cancellation. The distraction of aiming the antenna and watching signal strength can be dangerous if you are working alone. To make measurements, stop the vehicle, determine your exact location on the map then rotate the antenna to find direction to the largest signal. Plot that information on the map with an arrow from where you are in the direction of arrival of the signal. Use an arrow rather than just a straight line so you will be sure of the direction of arrival later on. This is especially important if it turns out that you are chasing a multi-path reflection. What you may see if that is the case is that the arrows end up pointing in all sorts of wild directions rather than toward a single point or small area.

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To see if you are being fooled by multipath signals, move the antenna a quarter wave (see table 1) without rotating to see if the signal strength changes radically. If there is no significant change in signal strength, you probably are aimed properly. If, however, there is a radical shift in power (either up or down) you are experiencing multipath reinforcement or cancellation and the direction of arrival will be skewed. Here is what’s happening. The signal arriving directly from the emitter is coming by the shortest path. Reflected signals take a longer path because they have bounced off of something as shown in Figure 1. By moving _ wavelength, the phase relationship between the direct signal and the bounced signal will change by 1/2 wavelength, so signals are in phase in one position so they add together will be out-of-phase _ wavelength away so they cancel. Pay attention to the signal level as you are making the _ wavelength move since you may go through a peak or null somewhere during the movement.

DFing in CitiesIn urban environments the main propagation paths that the signal follows will probably be multiple bounces off of buildings .since a direct, line-of-site path to the signal source probably does not exist. In this situation don’t bother trying to find the exact direction to the emitter. When you reach an intersection, simply point your antenna down all the roads and follow the strongest signal. The aiming doesn’t have to be very careful, just aim down the street. Usually the strongest signal will be pretty obvious. In figure 2 the direction of arrival would be different if the measurement is taken on the southeast corner than it would be if taken on the southwest corner or the northwest corner. In all these cases, however, the general direction is up the street to the north. If the measurements don’t make sense, you may happen to be at a place where two strong bounces cancel – this could easily happen at the southeast corner. Try moving a short distance and trying again. Continue following the strongest signal until it is very strong.

As you move through a city this way, you may very well come close to other emitters at different frequencies. These may be strong enough to saturate the front-end of the spectrum analyzer. Use a bandpass filter that is tuned to the frequency range in which you are interested to eliminate the potential of having problems with strong out-of-band signals.

Once you have gone past the emitter so that the direction of the strongest signal changes significantly, try triangulating the signal since you may have a direct path to the emitter and give you good results. If you are still receiving the signal via indirect paths it will quickly become apparent because triangulation will yield different answers depending on your exact position and which bounce signal is strongest. If you have been driving up to this point, now may be the time to proceed on foot so you will have more freedom to stop and investigate potential sources. By this time you will very likely be receiving a signal directly from the emitter so more traditional DFing approaches will work.

TransmitAntenna

DirectSignal

ReflectedSignal

Figure 1. Simplified Multi-path propagation

Figure 2. Multipath urban propagation.

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Freq. MHz1/4 wave

Freq. MHz 1/4 wave

Freq. MHz1/4 wave

meters inches meters inches meters inches

1 75.0 2953 250 0.300 11.8 1100 0.068 2.69

5 15.0 591 300 0.250 9.8 1200 0.063 2.46

10 7.50 295 350 0.214 8.4 1300 0.058 2.27

15 5.00 197 400 0.188 7.4 1400 0.054 2.11

20 3.75 148 450 0.167 6.6 1500 0.050 1.97

25 3.00 118 500 0.150 5.9 1600 0.047 1.85

30 2.50 98 550 0.136 5.4 1700 0.044 1.74

40 1.88 74 600 0.125 4.9 1800 0.042 1.64

50 1.50 59 650 0.115 4.5 1900 0.040 1.56

60 1.25 49 700 0.107 4.2 2000 0.038 1.48

70 1.07 42 750 0.100 3.9 2200 0.034 1.34

80 0.94 37 800 0.094 3.7 2400 0.031 1.23

100 0.75 30 850 0.088 3.5 2600 0.029 1.13

150 0.50 20 900 0.083 3.3 2800 0.027 1.06

200 0.38 15 1000 0.075 3.0 3000 0.025 0.98

Table 1. Quarter wavelengths for selected frequencies.

Fixed DFing NetworksWhen you have a need to protect a limited geographic area and be able to rapidly locate signal sources, an array of antennas surrounding the area is the way to go. The antennas can be either directional antennas that are rotated to determine where the signal is coming from or an array of omni-directional antennas coupled with a Doppler direction finding systems. Doppler systems will be discussed in the next section.

A variation of this approach is to have an array of fixed receiving locations within the area to be covered and get a general fix on the location of a signal by seeing with of the receiving locations can detect the signals and how strong. In the case of a cellular network there may be enough receivers that a fairly tight fix of the location could be determined just from analyzing signal strength information from many receivers. By having all the receiver locations plotted on a map in advance, it is feasible to plot signal strength on the map from each location that hears the signal and have a good idea of where to go to start DFing.

In the ideal configuration of a fixed DFing network, you would have receivers situated around the area to be covered and perhaps some within the coverage area so that at least two receivers would be able to receive a signal from the target area at right angles to each other. The worst case would be for the two receivers to receive the signal with opposite bearings – that is one receiver’s antenna is pointing directly at the other receiver’s antenna. The signal could be anywhere along the line between the two stations and no triangulation is possible, as would be the case for antennas 2 and 6 trying to DF a signal that is originating at the X in figure 3. Antennas 1 and 3 on the other hand would give directional lines that would cross somewhere near the X. Other antenna combinations in the example would also yield good cross bearings. The more antennas receiving the signal the better for DFing.

Figure 3. Fixed DFing Network of directional antennas.

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Doppler DFingThis method isn’t really Doppler but is simulated Doppler created by rapidly switching a circular array of antennas one at a time into a receiver. The theoretical underpinning for this type of antenna array is a rapidly rotating antenna that actually creates Doppler frequency shift on arriving signals – the frequency rises when the antenna is moving toward the signal source and drops when the antenna is moving away from the signal source.

This method uses an array of omni-directional antennas mounted _ wavelength apart. The minimum number of antennas is 3. There is no reason for having more than 8 antennas – an array of four antennas does the job just fine. The electrical complexity aside, the more antennas there are in the array, the faster the switching speed needs to be to maintain a reasonably fast simulated rotation speed. Since the feedlines from the antennas to the switching circuit must all be the same length to maintain correct phasing of the array, care is required when building the antenna and feedline system.

The antenna feedlines are routed through a diode switching array. The diodes are turned on and off one at a time in rotation at a relatively high rate such as 1 kHz. At any one moment only one of the antennas is connected to the receiver. FM modulation at a rate (1 kHz in this case) determined by the diode switching rate is imposed on the signal being received. The phase of the FM modulation is used to determine the angle of arrival of the signal relative to the antenna array. A processor controls both the diode switching and receives detected audio from an FM receiver. That is enough information for the processor to determine arrival angle. Regardless of the modulation format of the received signal, this method requires the use of an FM receiver to be able to detect the phase/frequency changes of the signal.

Signal Strength MappingA completely different approach to interference location is signal strength mapping. It only works for signals that are on the air for an extended period of time. Basically you have one or several people drive around the area of interest as thoroughly as possible while capturing signal strength and GPS location information. Then you post process the information, placing measurements on a map and connecting measurements of equal signal strength forming “isobars” such as those on a weather map showing equal barometric pressure. Unless the situation is very unusual, the isobars will form jagged circles around the signal source. This approach is commonly used in spectrum clearing situations where the new licensee of a chunk of spectrum needs to negotiate the removal of all previous licensees of the spectrum plus find and remove any non-licensed users of the spectrum.

Figure 4. A Doppler DF antenna array.Photograph courtesy of Doppler Systems, Inc.

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Using Side Lobe NotchesThe front lobe of most antennas is fairly broad, making it difficult to accurately determine the maximum signal strength. Notice the side lobes notches in the antenna pattern at plus and minus about 60 degrees. The deepest parts of the notches are very narrow, making them useful for direction finding. Here is a way to use them. Find what you think is the largest received signal, then rotate the antenna 60 degrees in either direction to find the deep notch. Knowing the radiation pattern of your particular antenna is important to being able to use this technique since different antennas have their deep notches in different places. Some antennas have multiple notches. The manufacturers of most directional antennas usually can supply radiation patterns of the antennas they make.

One final set of tools for getting to the source of interference are spectrogram, signal strength and Received Signal Strength Indicator (RSSI). Both signal strength and RSSI measurements are made in zero span. They show basically the same information, but in two very different ways. You may end up using all three of these tools while you are tracking down a signal source.

SpectrogramMeasurements using spectrogram can give you valuable insights regarding the on-off pattern of an interfering signal. Notice that the signal at the center frequency is frequently off the air. Knowing the on-off pattern of the signal allows you to be aware that you need to be patient when the signal disappears. Spectrogram measurements are generally used relatively early in the signal identification process since they are ideally suited to give you insights that are hard to get when looking at a series of spectrum analysis sweeps. Once you have those insights.

0

30

60

90

120

150

180

210

240

270

300

330

-3

-10

-20

Figure 5. Antenna pattern of a 9-element Yagi.

Figure 6. Spectrogram showing an intermittent signal.

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Signal StrengthOnce you now the pattern of your target signal, signal strength and RSSI are good tools to employ. When you use which one depends on the circumstances of the search. The signal strength measurement displays an analog meter to show the measured power of the signal. One handy choice in the measurement is an audio beep whose frequency changes with signal strength. If you are working alone this allows you to hear the signal strength without taking your eyes off the road. You can plug-in a cellular headset to hear the beeps in noisy environments while avoiding strange looks from passers-by when you are DFing on foot. This measurement gives you power of the signal at that particular moment.

Figure 7. Signal Strength measurement.

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RSSI shows the signal strength over time. The nice feature of this measurement is that you can easily see the change in signal strength when using a directional antenna. The default time interval between measurements is 70 milliseconds, giving you a very rapid indication of signal strength changes as you rotate the antenna. Don’t use this measurement if you are driving alone since your eyes will be off the road too much.

Ingenuity is the name of the game when it comes to direction finding. You need to gather information quickly and realize as soon as possible when you are dealing with a multipath situation so you don’t waste a lot of time chasing ghosts. The tools built into Anritsu handheld spectrum analyzers are ideally suited to help you be successful in interference mitigation and elimination

Figure 8. RSSI measurement.

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