two dipole doppler project

[N0ZYC]

This year I would like to build a two dipole doppler, for portable use.  Those of you familiar with dopplers will probably immediately be saying "you can'd do that, a doppler requires four antennas".  Those that spend a little more time pondering it will say "well not really, you can get by with three though it's more complicated... but not TWO."

I believe two can be done.  I got my inspiration from the out-of-production Ramsey DF-1 (not DDF-1) that is basically a happy flier with a phase detect on the audio like the DDF-1 and most other dopplers use.  It uses the phase detection to turn on a "left/right" indicator when you are off the null, eliminating the ambiguity of the happy flyer.

I got to thinking about that, and it's only a matter of timing then.  It's phase comparing, but in an absolute way.  Is it before or after the crossover?  There's no reason not to measure the phase variance to obtain a bearing.

I realize with only two antennas, this would produce an ambiguous reading in most cases.  With the two dipoles oriented east and west, with the transmitter to the north, both the north and south lights would be lit at the same time.  But as the transmitter started moving west, the north and south indicators would both start moving toward the west.  When the transmitter was due west of the array, the indicators would have converged and only the west indicator would be lit. (the doppler would realize the signal was coming from an end-fire two element array)

One of the challenges of a portable doppler is managing the antenna array on foot.  Happy fliers are already widely used on foot, and this doppler array would be the same exact physical configuration, but with an interesting difference.  The dipoles would be 1/4y apart, not 1/2y.  Making it eve more compact. (the antenna, there would sill be bulk in the controller etc, but that's what backpacks are for)

And with the benefit of a null, a left/right indicator, and even an actual live bearing.  It would be easy for the hunter to mentally work out which of the two indicators was correct by simply turning the antenna array a little to see which way the indicators swung.  And the sharp null would still be usable.  This should produce a sniffer superior to the happy flier and other dual-dipole tone nulling arrays.

Comments?  Discussion?

[WB6BYU]

If you use a meter to display the left/right indication, as many units do, then
if you calibrate the meter sensitivity for full scale when the signal is off the end
of the array then you can mark the meter face in relative angle.

This probably works better with AM sets such as the L'Per (and I think the Happy
Fliers, though I've never seen anyone using that circuit) then this fairly reliable.
With an FM detector there may be additional phase differences due to modulation
differences, but generally it seems to work reasonably close.

Basically you just put a zero center meter on the analog output of the phase detector
rather than using it to drive a comparator.  (This doesn't work on all designs, such as
those using a NE567 for the detector, since there is only a binary output.)

In practice I've used several such circuits with meters, and it is easiest and fastest
for hand-held use to simply turn the antenna to find the null rather than relying on
the meter calibration (which often changes with signal strength).  But it is easy enough
to try, and the antenna spacing isn't all that critical, either, so you can experiment and
find what works best for your needs.  The same goes for the standard TDOA (Time
Difference of Arrival) sniffers, which are tolerant of a wide range of spacings.)

(Note that the AM receiver versions such as the L'Per and the Happy Fliers are based
on signal strength differences from the switched antenna pattern, while those used
with FM receivers depend on the difference in arrival time/phase between the two
signals.  Neither are based on the principle of a rotating antenna, which would be
the more common use of the term "Doppler".)

[N0ZYC]

The DF-1 does have a meter to indicate phase separation.  It's just difficult to tell what angle you are by looking at the meter.

If you're using a switched capacitance filter locked to the switching frequency, it should simulate a rotating antenna system, even if you're down to two antennas.  The problem is that with only two antennas, it can't tell which direction the system is rotating... clockwise or counterclockwise.  That's what will cause the ambiguity in the display. It will know how far off it is (in degrees), but not in which direction.

The meter on the DF-1 may produce a linear indication of how far off you are, and a simple DAC may be all that's needed to drive a ring of lights.  The problem I foresee with that is the max meter strength is directly related to the signal strength.  That may be difficult to calibrate out.  Being able to see two lights on a ring lit up would be a much more intuitive display.

[N6JSX]

This is a phase difference difference not a true Doppler.  For more interferometer info look for WB2HOL TDOA or K6BMG Super DF'er.

I have had a Super DF'er for +25yrs - it works but is difficult to just pick up and DF with. Since it nulls at the perpendicular (equal phase) angles to the TX ---  is the TX in front Zero-degrees or behind 180-degrees?  It takes practice and more practice to insure you have the DF'er setup the "same" way each time you use it or it will lead you off on frustrating tangents, i.e are you sure your dipoles are placed identically each time (hot element up on both sides) or could one be flipped - guess what that will do to your left-right?

Then there is the TX signal; these TDOA/Phase DF'ers do not work well (if at all) with weak signals (=<S3) and cross-polarity TX signal. Are you sure the TX signal is vertical polarized, how do you determine the TX signal polarity (diagonal or horizontal); what will different phase angles/polarities do to your DF'er? Could you be seeing a multi-path signal, how do you sort out the bounce from the real bearing? Is the signal knife-edging?

I'm not saying that these devices do not or will not RDF a signal. But DO NOT expect to just put the kit together and magically become a super hunter - it is not a magical device! It will take many hunts to become proficient and learn/know/discover the DF'er boundaries/limitations and for you to create methods/techniques in how to use the device effectively (or you're going to walk lots of extra miles becoming more frustrated at each mile)!

Build the kit THEN GET A 2m QRP (<100mW) Hidden T - have a friend/XYL hide it somewhere (in a local park) and do your own mini-training hunts to get proficient (XYL's love to laugh/taunt us when we get frustrated, especially since they already have the answer/location).  I have 100mW 2m TX in a cammy-taped band-aide box with a cammy 1/4wave whip as my practice T --- photos/info can be found on RDF-USA Yahoo User group, that has many good files/info in it, plus many of the top USA Hunters are Group members - they can/will answer intelligent questions. 

The bottom line is I have a TDOA/Super-DF'er but rarely us it, as I do not use it often enough to stay on top of it.  It is much simpler/easier/faster to use my version of the famous WB2HOL tape-measure 3el Sniffer Yagi and an HT (TH-F6a w/S meter). On my mobile I sport my home built/design special 2m 3el T-Hunting Quad and specially modified mobile radios. If/when I mobile RDF 70cm I'll use a 6el Quagi, that can easily double as a portable sniffer antenna.  Mobile RDF 12/10/6m the antennas get big - here is where the use of a Loop antenna is needed.

If you seek a challenge then RDF >70cm, the higher the frequency the more exponentially difficult the RDF due to severe to horrendous TX signal multi-path'ing. Add intermittent TX signals and you'll tear your hair out! 

[N2LXM]

I built a dual dipole RDF unit a few years ago. Housed the dipoles in PVC pipe and mounted a small box to the cross member. The box housed a small circuit board with switched the antennas at a rate of 1000 Hz. So you could turn the antenna for peak or null of the audio tone. Simple but it works. I used it to track down a jammer of a local repeater. Now I used it to give deno's to scouts on Fox Hunting. The switcher is a NE 555 driving a quad switch.

[N0ZYC]

if you're using basic (914 etc) glass diodes to switch, check out ebay.  there's a guy selling motorola pin diodes (big black boxy ones with the flat gold leads, proper pin diodes) that will greatly reduce switching noise.  I remember paying $2/ea for 555 pins when I was building my first doppler... and the guy on ebay is selling them something like 30 for $9.  needless to say I'm set for pin diodes now for awhile