Figure 1. A Doppler RDF antenna set has vertical elements that are connected to the receiver one at a time, for equal periods, using PIN diodes or other electronic switches. The individual elements should be nondirectional in amplitude and phase.
My wife hates crickets! Their noise keeps her awake and annoyed. If there's one chirping anywhere in the house at bedtime, I'll have to find and dispatch it before anyone will get any sleep.
What does chasing crickets have to do with direction finding? Well, have you ever tried to locate one in a small area with hard walls such as a bathroom? The sound bounces off all of the tile, porcelain, and glass. If the little noisemaker is down in a corner behind the wastebasket, the sound waves can reinforce and cancel such that it appears to come from the shower stall. Sometimes the source appears to change location as I move around the room. And of course as soon as I begin to close in, the little guy detects the heat from my body and stops chirping, only to start again as soon as I leave the room.
The same phenomenon of bouncing signals giving confusing directional indications (which engineers call multipath) causes bearing errors in radio direction finding (RDF) at VHF and UHF frequencies. Minimizing the effects of multipath requires careful antenna system design and installation
It's vital to understand that a Doppler antenna set, whether it has 3 elements, or 16, or any number in between, needs to be a nondirectional array. What, you say? It's giving direction, so isn't it directional? No, it's not in the way that a yagi, quad, rhombic, Sterba curtain or parabolic dish is directional. A Doppler works on phase, but it's not the same as an array of phased verticals that an AM broadcast station or a 160-meter DXer might use.
Figure 1. A Doppler RDF antenna set has vertical elements that are connected to the receiver one at a time, for equal periods, using PIN diodes or other electronic switches. The individual elements should be nondirectional in amplitude and phase.
Simply put, a good Doppler array is completely nondirectional in terms of amplitude. No matter what the azimuth of the incoming signal, the amplitude of the RF signal going into the receiver must be the same, and it must remain constant as the Doppler's switch selects the elements of the array in sequence. The signal phase and apparent frequency will (and must) change with the pseudorotation, but amplitude should not.
At this point, some readers are pounding the table with their fists, saying, "Why does it matter if the amplitude varies? The receiver has an FM detector, which doesn't respond to AM!" Ah, if only that were true.
FM receivers have limiting IF stages, intended to saturate and effectively clip off amplitude changes in the signal. But they aren't perfect. First, they only limit above a fixed signal threshold. You may have observed this yourself on your broadcast FM radio or analog TV audio (which is also frequency modulated). If there is a thunderstorm in the area and your AM broadcast radio is beset by static crashes, you won't hear these crashes on very strong FM or TV audio signals due to the receiver's limiting action. But you will hear them on distant FM/TV signals, because those signals aren't up to the limiting threshold.
Second, the amplitude variations aren't just neatly clipped off in a limiter. They still cause slight changes in the phase of the limited signal presented to the discriminator or ratio detector. This effect is called "AM-to-PM conversion" and it's familiar to radar designers who work with linear-beam tube amplifiers such as klystrons. In saturation (limiting), changes in amplitude at the input cause phase delay changes in the output of up to six degrees per dB of input amplitude change, depending on the gain of the tube. That has an adverse effect on target velocity measurement in a pulse-Doppler aircraft-tracking radar. Similarly, a limiter stage in a receiver will have some AM-to-PM conversion depending on the sharpness of limiting, bandpass versus signal deviation, and so forth.
If a Doppler array is unintentionally directional in amplitude, the AM induced on the signal by pseudorotation will be partially converted to FM/PM in the limiter, causing bearing errors. The more directional the array, the greater the error will be. You may not notice the bearing error when your vehicle is standing still because the error is a fixed amount under these circumstances. You might "calibrate it out" and never realize it. But think what happens when you start moving down a city street, taking bearings on a fixed signal source as you go along.
Like the cricket in the bathroom, the signal you're tracking comes primarily from one direction, but your Doppler also receives momentary reflections from stationary and moving objects between you and the source, especially the close ones. The more amplitude directivity in your Doppler antenna array, the more this multipath will cause deep signal cancellations that will be turned into significant phase changes in the receiver's IF. The result can be wild fluctuations in the LED display on the front panel.
If you make your Doppler antenna less directional in amplitude, you will see less fluctuation in the display as you drive through multipath. You probably already know that a quarter-wavelength vertical antenna requires a horizontal conductive ground plane underneath, and that it works best on a vehicle when it's mounted right in the center of the vehicle's metal rooftop. In that position, a two-meter whip gets more than a quarter wavelength of ground plane in all azimuth directions. And just as importantly, the ground plane is nearly equal in all directions.
Just how important is center-of-rooftop mounting to a Doppler? Antenna analysis software can help provide the answer. Using EZNEC, I simulated a quarter-wavelength vertical monopole (representing one switched element of the Doppler array) on a 4x4 foot SUV roof. Figure 2 below shows the azimuthal pattern of the antenna+radials in free space. It's about as non-directional as you could hope for, with less than 0.1 dB variation in gain. That would be perfect for Doppler performance.
Figure 2. A quarter-wavelength whip centered on a large conductive surface is almost perfectly omnidirectional in azimuth, as this EZNEC polar plot shows.
Next I simulated putting this monopole four inches from a corner of the same roof, an unlikely location but one that makes the point. Figure 3 below shows the results. There is more than 4.3 dB variation in gain around the azimuth circle, enough to degrade Doppler performance in multipath situations due to AM-PM conversion. What's more, antenna elements that are directional in amplitude are also directional in phase.
Figure 3. Mounting a quarter-wavelength whip near a corner of the big ground plane skews the directivity toward the larger plane area. Such directivity is undesirable for elements of a Doppler array.
Unfortunately, EZNEC doesn't compute relative phase versus azimuth or elevation. When I started analyzing Doppler arrays ten years ago with ELNEC for DOS, I asked creator Roy Lewallen W7EL why it doesn't. He told me that until I asked him, no one had requested this feature. Apparently nobody has asked since, because it's not in EZNEC for Windows. However, I know from analysis of vertical AM directional broadcast arrays that 3 dB of gain difference can represent about 45 degrees of relative phase difference.
It's true that most hams are not likely to mount a Doppler array in one corner of the roof, but you might be tempted to put it on the hood or trunk lid. Don't. It will become even more directional because signals are partially blocked from some angles. Signals from opposite angles will be scattered back into the array with unpredictable gain and phase changes. Don't put it on top of a roof rack either, as that would provide an incomplete and non-symmetrical ground plane.
Other antennas on your vehicle might have a surprisingly deleterious effect on your Doppler's performance. Figure 4 below shows the directivity of a two-meter quarter-wavelength whip on a perfect ground plane when a 5/8-wavelength communications antenna for the same band (with 50-ohm simulated load) is two feet to the right, as it would be if mounted on the edge of the roof. Doppler signal pickup toward the side with the 5/8-wave whip is reduced by 3.4 dB, with corresponding phase corruption of the incoming wavefront.
Figure 4. A communications antenna to the right of the Doppler whip makes that whip's pickup quite directional.
Higher gain antennas can affect the wavefront even more. Occasionally I have gone two-meter transmitter hunting with the Doppler antenna centered in the rear half of my 4x8 foot van roof and my RDF quad in front of it. As I rotate the quad to peak up a steady signal, the bearing displayed by the Doppler swings around the LED display, sometimes by up to 90 degrees. The effect is worst for signals coming from the front "through the quad," as you might suspect.
So to recap, the individual elements in switched VHF/UHF Doppler antenna sets must be as nondirectional as possible to achieve the best Doppler performance in multipath environments. Whip antenna sets need a good symmetrical ground plane with minimum obstructions and an absence of other antennas, especially those resonant in the band of interest.
What about interaction among the whips in the array? Inter-whip coupling can cause amplitude directivity effects that are even more severe than those described above. Poor connections to the ground plane can increase this undesirable inter-whip coupling. Some Doppler antenna designers don't consider these issues and then believe it's normal that their displays always go crazy when moving. It doesn't have to be that bad. I'll let you in on the secrets of minimum-directivity antenna arrays in the next installment of this series.
Text and photos © 2003 and 2025 Joseph D. Moell. All rights reserved.
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This page updated 24 April 2025