DF equipment.

[Biggles]

Not sure whether this should go in the info wanted section instead, but here goes. Ages ago I saw a circuit in Radcom which detailed a simple circuit for direction finding on VHF which used two carefully spaced dipoles and two switched dual gate mosfets which fed a receiver of your choice. I think the dipoles were moved about azimuth wise until the background buzzing sound was nulled out, to give the received signal direction. Presumably there was slight phase difference if the dipoles weren't aligned with the signal direction. I was going to build a one a long time ago, but the fets were P type. They proved to be unobtainium so I gave up. I suppose I could have re-engineered the circuit to use N types but never did. The question is, has anyone got a relatively simple DF circuit they could share? as I fancy building one again. Using a separate receiver isn't a problem. Cheers.
Alan.

[Al (astral highway)]

Hey Alan,

Have you seen this ingenious circuit? It uses PIN diodes.

Simples!

[Biggles]

Cheers Al. That looks like it works on the same principle as the one I saw. It's a bit simpler though. I sense a project coming on...
Alan.

[G6Tanuki]

There was a VHF DF system by Datong in the 80s that used *four* antennas in a square [usually magmounted 1/4-wave whips on a car roof] and PIN diodes to provide the commutation.

See https://sites.google.com/site/datongarchive2/dopplerdf for more details. Includes circuit-diagrams . .

Datong no longer sell to the amateur market, but in the 70s and 80s they offered some rather good stuff!

Have a look here too: https://a43.veron.nl/projecten/doppl...on-finder-rdf/

[AC/HL]

Sounds like a modern implementation of the 1919 Adcock antenna

[ex seismic]

Also have a look here: http://www.homingin.com/newdopant.html

Gordon

[Radio Wrangler]

I think the Datong direction finder was a doppler type.

Have an array of antennae in a circle, switch the receiver from one to the next to simulate a single antenna moving in a circle at say 1000 revs per second.

Use an FM receiver.

Antenna 'motion' and the doppler effect puts FM on the incoming signal. The FM receiver demodulates this to give an audio tone. Compare the phase of the 1kHz audio with the phase of the antenna 'rotation' and you get the bearing of the incoming signal. Simples!

There was a good construction article in the American 'Ham Radio' magazine in the '80s. He had 8 whips in a circle on a roof rack of a car. One memorable photo in the article had the caption that this was the array on the roof of the Ford Mustang he wrote-off by paying too much attention to the direction finder display :-(

David

[KeithsTV]

I remember that article as I was contemplating building it back in the early 80s but never got round to it.

I've attached a copy of the article which is a scanned copy of a photocopy. It includes a few of my notes written on the circuit diagram.

I suspect this type of system is used by the "Tracker" used by the police to locate stolen cars. The Tracker system appears to use 4 quarter wave aerials on the roof of the police car.

Keith

[Biggles]

Some good articles there. If I can source the old chips I may have a go at building a full blown one. In the meantime I could play around with the front end switching using PIN diodes as a starting point to see if I can get any usable results. Thanks for the input.
Alan.

[Tyso_Bl]

The setup you mention sounds like the "happy fliers" unit described in Transmitter Hunting: Radio Direction Finding Simplified by Joseph D Moell, an old book now but a good read.

I built the Roanoke doppler unit from the circuit in the book soon after it was published.

[Radio Wrangler]

It was a useful tool for squeaky hunting. You could get a fix from even a short blip. The difficulty came with multipath and concrete canyons.

David