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Author Topic: Are doppler shift direction finders effective?  (Read 46634 times)

Posts: 172

« on: February 16, 2012, 06:48:56 PM »

Or just a vaste of money?

Posts: 17052

« Reply #1 on: February 16, 2012, 09:50:38 PM »

It depends what you expect them to do.

First, I presume you mean a true doppler system, which uses an array of antennas that is
electrically rotated.  Some people use the term for the two-antenna switched antenna arrays,
but they really don't use the doppler shift.  A better term for the two-antenna variants is
TDOA (Time Difference of Arrival).

Dopper DF units aren't as sensitive as a beam with a good receiver because the commutation
noise can mask weaker signals.  They don't give you as much information, either:  you still
get just one point on the display if you have a reflection, while a beam will show two peaks
to alert you that there is a reflection.  For fast mobile hunting in relative open terrain they
can work well.  In hilly terrain, or in a city area where buildings are more than two stories,
they tend to have more problems with reflections.  For very long range work - where you have
to take bearings from hilltops to triangulate the signal, a good beam or Long Baseline
Interferometer can give a more accurate bearing.

It's another tool to have available.  In the flat plains of the central US, or in the Los Angeles
basin, they are quite popular.  I think I have a board around here somewhere, but have never
seen that one would be very useful in among the local hills.

And if your unit has a 100Hz switching frequency and the hider modulates the signal with
a 101 Hz tone, you'll just sit there and watch the indicator turn circles around the display.
because conditions are suitable.

Posts: 14332

« Reply #2 on: February 17, 2012, 11:17:59 AM »

If you are talking about a $25K+ professional unit then yes, they are pretty effective. If you are talking about one of the "add on" units that plug into your HT then probably not so much.

It also depends on your operational limitations. For example, if you are driving around in a vehicle and the target is also moving then its pretty difficult to use a directional antenna. If on the other hand, you are walking and the target is fixed then a hand-held directional antenna can work quite well.

Also, many of the mobile automatic units have moved away from doppler shift to TDOA.

Bob  AA4PB
Garrisonville, VA

Posts: 255

« Reply #3 on: February 18, 2012, 08:25:54 AM »

If I knew T-Hunters were using Dopplers I'd put my hidden T antenna horizontal negating the effectiveness of heir Doppler use as vehicle Doppler RX antennas are vertical.   Tongue

Doppler mobile/vehicle use requires a strong signal >S6 for direction and relegated to forward/aft-port/starboard of the vehicle while the vehicle is moving to allow visual averaging of reflections. Dopplers are of little to no use on weak signals.  Huh

I did use a Doppler, for a short time when going after repeater problems when speed is of the essence in finding these problems. Since I was the driver/hunter I found I could not study the Doppler while driving safely in Southern CA traffic.   Shocked

I found it much easier to become proficient at using my 2m 3element special T hunt Quad. One of the keys to my success was using my design/built external noise/S meter. I placed it directly in front of me on the dash but the real key was incorporating an audible tone speaker that parroted the meter response, stronger the signal the higher the audible tone. The other key was my two step built in attenuation built around controlling the B+ of the radio's 1st RF mixer; one step removed the B+ to the base & the other step 10K variable pot to control the collector B+. This was needed to keep the external meter displays centered and not pegging/saturating the meter/circuit.  Grin 

Sometimes I could psych myself into thinking I was close to the Fox/T ; to keep me in perspective I placed a HT on my dash removing the rubber-duckie (helical dummy load), set the HT to just barely squelch out full volume. If this HT started to bark at me I knew I was within 1/4mi or closer and usually I was already switching in first step of radio attenuation.  Cry

I've been thinking about trying a Doppler again, but the issue I always found was the cumbersome mediocre antenna system was less than adequate. For my next Doppler I'm going to make each of the four antennas 1/2wave dipole (like Adcock Sys). Now the trick will be how high off the vehicle does it need to be to negate vehicle metal/field affects?   Cool 
The techno fun of T hunting!   Wink


Posts: 3302

« Reply #4 on: March 14, 2012, 08:00:30 AM »

In my foolish youth, when I was a repeater operator and trustee, I'd use a doppler to track down
clowns who were jamming our local repeaters. More than once I'd be out at midnight, chasing drunks
and other social misfits with two meter rigs.

I found the doppler to be absolutely essential. It always brought me right to the driveway of
the miscreant. The problem with the cheaper units is the use of digital swtching to simulate a
rotating array, that causes a lot of receiver noise and reciprical (sp?) mixing problems. And you
have to be moving to average out the reflections to see the true heading. You can usually
"hear" the reflections in the speaker as it garbles the tone.

Fortunately these days I've moved on to a small repeater with close knit group, and the
lids have given up jamming since the large audiences are no longer on FM.


Posts: 22

« Reply #5 on: May 25, 2013, 10:06:25 AM »

If I knew T-Hunters were using Dopplers I'd put my hidden T antenna horizontal negating the effectiveness of heir Doppler use as vehicle Doppler RX antennas are vertical.   Tongue

Not surprisingly, "no one tool is best for all jobs".  I use a combination of equipment here.  4 element 2m beam extends up from my truck out the rear passenger window, gets it about 7' above the truck.  Polarized horizontally (but easily changed) works very well to get accurate bearings at distance. (doesn't interfere with the doppler either... which was surprising)

Once the meter comes up, the doppler takes over and bee-lines me right into the neighborhood.  A switchable 0-70db (10db steps) attenuator on the omni provides an excellent "you're getting warmer" indicator to complement the doppler. (and can hold its own as the solo indicator)  These two bring the vehicle as close as physically possible to the transmitter.

From there, it's on foot with a DF-1 (non ambiguous tone nuller) and a variety of other gear I'm currently testing out such as offset attenuators, dedicated sniffers, loops, and even a man-portable / bicycle-mounted doppler.

Experience with your equipment trumps ALL.  The more hours you put on whatever you're using, the more effective you will become.  The original group I played with had outright banned directional equipment because of our skill level, yet typical hunts lasted less than 30 minutes.  One exceptionally good player didn't even have an s-meter on his radio, he played entirely by ear.  An experienced player that has a good understanding of how his gear works will smoke anyone that walks onto the field even with the most sophisticated and expensive kit available if they don't have experience and skill in its use.

If you have several different kinds of gear to use, I recommend trying to do your next few hunts while selecting a single piece of gear to use the entire hunt.  This helps you to focus on the subtleties of its behavior, learn how to identify, understand, and overcome its weaknesses, and how to capitalize on its strengths.  Don't bounce back and forth quickly between different gear until you thoroughly understand each one and can adjust your gameplay immediately to suit your newly selected equipment.

Posts: 10

« Reply #6 on: November 18, 2016, 11:43:40 PM »

in 1992 or so, I made 4 and 8 element dopplar directional finder. 
it is simple, except I use PIN diode for switching one of 4 or 8 antenna elements.

I feed demodulated FM directly from discriminator.
and feed to oscillo CH 2 of scope. 
Clock generator to run antenna sequence selection to CH1 of scope.

so I can see phase difference.

Doppler DF worked fine, as long as there is NO multipath.
when there are multipath signal, you see 2 wave super imposing to each other. 

you see sort of 2 signal with different phase and amplitude , amplitude is determined by signal strength of  main signal or multipath.

today use of DSP, convolution, can detect strength and phase difference of each Multipath signal.   


Posts: 7

« Reply #7 on: November 19, 2016, 06:44:41 PM »

Several great comments, WB6BYU and N0ZYL are right on the money.

The doppler is one more tool. I have used mine to find jammers. As others have said, you need a stronger signal. But once you do, it is almost a simple job to drive right up to the source.

That said, with our local hunts, if anyone shows up for the hunt with a doppler, they are in a different category. They may be first, or the lowest mileage, but we encourage everyone to learn to use a beam, lean multipath, learn your equipment, much like what the others have said.

I've done over 200 hunts, and I have seen brand new folks show up and beat me. It's usually because they adhere to the guidelines that I taught them at a club meeting, and me? I start guessing and ignoring my equipment. Silly me.

Please, try a doppler if you want. Use a Byonics microtransmitter or the Pircon controller and send a family member out. Take your time to learn, switch over to a beam and attenuator. But have fun.

Posts: 22

« Reply #8 on: November 21, 2016, 05:06:45 AM »

>>and me? I start guessing and ignoring my equipment. Silly me.

I know I've been guilty of that a few times!  And I'd call that the #1 mistake of the noob.  I've lost count of the number of times I've seen a new player or new team quit paying attention to their equipment (or forget the trends of the past) and start looking around, expecting to spot the transmitter.  Or shotguns wanting to get out of the car to go check out that spot over there, when we're clearly still a mile or more out.  And I've seen several hunts end and have to wait on a group to get back to their vehicle, who all set out on foot to find the T that was over a mile away from them.  Trust your equipment!

(an then there was the hunt that kept pointing straight ahead until we crossed the bridge and I noticed it was now solidly behind me...  "it's on the bridge, go get it."   "really?  it is?  you sure?"   "oh, yes.")

Posts: 130


« Reply #9 on: October 30, 2017, 01:42:31 PM »

I have a lot of experience with all kinds of DF gear  (MF HF VHF UHF) from my many years in commercial fishing. Dopplers (actually pseudo Doppler (phase based) ADFs) work great on the high seas, not so well in areas where mountains or high terrain can reflect signals. The same Doppler VHF gear that gave absolutely reliable bearings offshore of SF in the Gulf of the Farallons fishing salmon would give all sorts of erratic and often false bearings when used in herring fishing inside SF Bay near Sausalito. The problem inshore was due to steep terrain reflecting signals. If you know the antenna scan rate of the Doppler ADF that is trying to get a bearing on you, you can add an audio signal that will confuse the ADF and give unreliable bearings. When Regency came out with the wildly popular and inexpensive Polaris 7200 VHF transceiver with built in ADF, some enterprising marine radio techs made and sold  black boxes that mixed in an audio tone that would confuse the Regency ADFs. The tone that worked for Regency ADFs did not confuse other ADFs (Taiyo, Simrad. Furuno) that used a different antenna scan (sequential switching through PIN didoes) rate. Spy vs Spy. You'd be amazed at all the ECM/EW techniques that fishermen used. You wanted to know where others catching fish were but you didn't want others knowing where you were when you were catching.

Posts: 22

« Reply #10 on: October 30, 2017, 02:50:51 PM »

oh yes since the antenna combiner is creating a frequency modulation whose phase (relative to the scan source in the control box) is the bearing, as produced on the audio output of the radio, adding the specific frequency of the scan source to the fox really messes with a doppler.  And that's why my boxes can adjust the scan rate Wink   Of course your pass and reject filters need to be able to compensate, but the switched capacitor filter that does most of the heavy lifting just tracks right along by design.  Despite my being aware of that trick, the huntmasters around here haven't caught onto that idea yet.... I've been able to keep ahead of the curve on the technology war so far.  Last hunt he started placing the foxes in areas totally inaccessible by vehicle to combat my doppler.   "I'll see your off-road, and raise you a *bicycle*!"

Oh I've also noticed that just a blurt of rapidly changing tones (as some fox boxes can do as one of their modes) can mess with a doppler somewhat without knowing the frequency of their scan.  As for multipath, it's important to have a speaker attached to your doppler receiver so you can listen to the filtered signal it's trying to process.  Multipath causes a very noticeable octave shift in the sound, and lets you know when "not to trust the bearing" so much vs when you have a reliable bearing.  It's pretty rare to have the reflections be a lot stronger than the main signal when your finder is sweeping 360 degrees, so you're likely to still get a piece of the true bearing in there somewhere and that'll make even the strongest reflection sound really raspy.
« Last Edit: October 30, 2017, 03:02:39 PM by N0ZYC » Logged
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