From: Jeff Liebermann Subject: Re: Doppler Direction Finder (RDF) Newsgroups: rec.radio.scanner, sci.electronics.equipment, rec.radio.amateur.equipment Date: 1997/03/13 View: Complete Thread (9 articles) | Original Format Brian Mileshosky, N5ZGT wrote: : Doppler is good until you start messing with multi-path (Where the : signal is bouncing all over the place). That is when a regular beam : and radio setup might be better. Doppler has a bunch of other problems. This is NOT intended to discourage anyone from using doppler, just to recognize its limitations. Credibility check: I helped design the AN/SRD-22 doppler direction finder for Intech Inc for use by the US Coast Guard in approx 1975. 1. A doppler DF can only display the direction of *ONE* transmitter. If there are any other sources of signal (reflections, other xmitters, adjacent channel junk, computer noise, synthesizer spurious signals, harmonics of clock oscillators, ignition noise, images, etc) the reading will be erronious. Reflections is the real killer. A ground bounce mixed with a direct signal will yield horrible results. (Incidentally, the reason most dopplers have rather large antenna ground planes is to reduce ground bounce). 2. Switched doppler is susceptable to switching noise which acts as modulation sidebands. These sidebands (harmonics of the switching frequency) will cause strong adjacent channel signals to mangle the bearing. The typical "Roanoak" design is highly susceptable to this. Doppler Systems uses DGFet switches which have a much "softer" (less rise time) switching characteristic. Intech used a modified sine wave drive to literally modulate the PIN diodes instead of switching. It totally eliminated the problem, but sensitivity was reduced. PIN diode matching was very critical. 3. The typical vertically polarized doppler system does not work well with horizontally polarized signals. Building a horizontally polarized doppler is no major challenge, but nobody seems to have done it. 4. Doppler is sensitive to fox frequency. The doppler receiver measures the phase shift from the antenna to the FM demodulator. Any change in receiver system phase shift appears as a substantial directional error. The worst error is caused by the crystal IF filter. The phase shift from IF center to filter edge goes through radical phase changes. An AFC (automagic frequency control) is manditory to keep the signal in one place on the crystal filter curve. 5. Doppler is sensitive to rotation frequency and FM demodulator distortion. There two schools of thought here. Doppler Systems uses about 300Hz rotation frequency. This is good for insuring that the signal will remain in the IF bandpass (no AFC) but results in harmonics of the 300Hz in the bandpass. These harmonics cause bearing errors and linearity (approximation of a circle) errors. The typical commutating audio bandpass filter will not remove harmonics. Intech elected to use 4.1KHz. Most of the harmonics are outside the IF bandpass. Combined with the PIN diode modulation technique, this yielded a very low distortion sine wave at the demodulator and very good linearity. However, the group delay (phase shift error) of the IF crystal filter was far worse at the edges (+/-4KHz) than it was near the center (+/-300Hz). AFC became manditory. It turned out that the conventional balanced multiplier FM demodulator was also slightly sensitive to signal level. Since the AN/SRD-22 was both an AM (121.5 aircraft) and FM (Marine VHF band) receiver, an ALC (automagic level control) was used. There error here is not large and can be probably be ignored. 6. Doppler is sensitive to modulation audio. The response time of the doppler DF is totally dependent upon the Q of the doppler tone bandpass filter. The wider the bandwidth, the faster the DF. If you make the filter too narrow, it takes forever to get a bearing. However, too fast and FM audio modulation and fox tones will leak through the filter and mangle the bearing. One favorite trick is to use a 300Hz fox tone to mess up the Doppler Systems DF'ers. 7. Doppler antenna patterns are very sensitive to nearby antennas and reflections. The AN/SRD-22 had to be mounted at the very top of the mast. Unfortunately, almost every other piece of electronics equipment wanted to be in the same place. On the roof of a car, the AM/FM antenna and other radio antennas create horrible errors. Nearby antennas from other hunters at the starting point will mutilate the pattern. This is why the DF book says (several times) that the ONLY way to get a decent bearing is while moving. Moving tends to average out these errors. While the error at a given location may be severe, a few feet away might be perfectly acceptable. The trouble is you have no idea where there is a reflection free location. Moving while DFing is the only way. 73 Magazine carried a series of articles in the HomingIn section based on rotating yagi and quad antenna direction finding. This is basically an AM (signal strength) system. The difference is that you can easily see multiple transmitters, recognize reflections (they tend to blurr), do NOT require an AFC, and have an easily controllable antenna pattern (using boresight calibration). However, driving down the freeway with a spinning 4 element quad is a great way to attract the constabulary. Short transmissions are also difficult to see. Equipment complexity is about the same. Having dealt with doppler professionally and recognized its limitations, I'm partial to a rotating antenna. One place where a rotating antenna is far superior to doppler is tracking a jammer. With a jammer, you have *TWO* signals on a frequency (the person trying to talk and the jammer). A doppler DF will yield a bogus direction because of the two signals. A rotating antenna will show both. I hope this helps. If there are any questions, please email. -- # Jeff Liebermann Liebermann Design 150 Felker St #D Santa Cruz CA 95060 # 408.336.2558 voice wb6ssy@ki6eh.#cca.ca.usa.noam wb6ssy.ampr.org 44.4.18.10 # 408.699.0483 digital_pager 73557,2074 cis [don't] # jeffl@comix.santa-cruz.ca.us http://www.cruzio.com/~jeffl