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Author Topic: Are doppler shift direction finders effective?  (Read 17291 times)
LB5KE
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Posts: 141




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

Or just a vaste of money?
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WB6BYU
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Posts: 13281




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« 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.
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AA4PB
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Posts: 12854




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« 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.
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N6JSX
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Posts: 217




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« 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
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K1ZJH
Member

Posts: 1003




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« 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.


Pete
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N0ZYC
Member

Posts: 20




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« 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.
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