Hand-held Direction Finders

[Bertie Thruster]

I would be grateful for any information on small (handheld) battery operated 121.5/243 direction finders. Has anyone used one, seen one, or know of any commercial suppliers?

[[email protected]]

If they do can they send the information to the SARForce XO as they are probably more accurate than the homers we presently use!

[Ascend Charlie]

Yes, we used one in my previous company. Built by a man in Melbourne, i think, and he doesn't do it any more.

It was simple: the handle looked like it was off a paint roller. It supported a horizontal bar about 2 ft long, and at each end of the bar was a vertical wire whisker antenna, about 3 ft long each (quarter-wave?). Wires connected each whisker to a box 1"x1"x3" and this box had two switches and a rheostat and a socket.

One swith for on / off, one for raw signal or a "click" version, and the rheostat to adjust sensitivity. A socket for some earphones (ear buds worn under the aircraft headset worked best), a 9v battery, and you're in business. In raw signal mode, you adjusted the sensitivity until you heard the signal, then moved the antenna around until the signal was loudest - and if almost overhead, you had to rotate the whiskers horizontal. In click mode, you heard a series of clicks, and the number of clicks increased as the antenna got closer to pointing directly at the signal.

It worked reasonably well - we were often tasked to track down errant beacons in parked aircraft at Bankstown, Camden, or boat beacons in marinas. The best success was finding a beacon buried 3 ft underground at a rubbish dump. A bulldozer had run over it and set it off.

Biggest limitation was its size - needed a Longranger with sliding door to be useful, holding the gizmo outside the machine. Bit of a struggle if we were going more than 50 kt, with the worry of dropping the little bu99er. And the next limitation was frequency bleed from Sydney Ground on 121.7 which would come blasting through every time we were within 10 miles of KSA.

[Huron Topp]

http://www.ltronics.com/portable.htm

[Bertie Thruster]

Thanks Huron. Good kit but the antenna array is too large for my op. In the old days we had a small box for finding F111 beacons. (some special freq) They were about the size of 4 cigarette packs with 2 telescopic antennae about 20cm long. That's about the size I need! Any radio experts out there?!

[Ascend Charlie]

Can't get too small or it won't work. Reasonable efficiency is a quarter wave dipole, so for a UHF beacon on 243 MHz the antennas are 30.8cm long. For the VHF beacons on 121.5 the whiskers are 61.6 cm long.;)

[Bertie Thruster]

Thanks Ascend. The 30 cm size equates roughly to the F111 boxes we had.

Do you think 1/8th length would work? I'm not looking for great distance d/f. Merely to find an downed airman (or lady) near an overland crash site, say 3-4 miles max.

[MBJ]

Must be possible to use 1/8th wave? In the Wasp in the RN the UHF homer was two 9" blade antannae set about 9" apart feeding a standard left/ right deviation bar.

Good luck in the search and I'd like to know if anyone finds one.

[zardoz]

Boring anecdote time...
As a aviation mad teenager in the early sixties I used to cycle to Redhill aerodrome on summer Sundays and was paid ten bob to sit in an ancient tiny caravan all afternoon working the DF.
When one of the club aircraft (or at least one of the very few with radios) wanted a bearing home, he would rattle off a long speech while I turned a 3 foot diameter plywood compass scale round and round until the speech faded away, then read back the bearing (which was offset by 90 degrees). The plywood compass wheel was attached by a scaffold pole that went up through the roof of the caravan (and let rain in) and turned a six foot circular DF aerial.
Some days I might get three requests, usually I got none. But I felt so important!

[leading edge]

Try www.acrelectronics.com/vecta2

See if that's what you need.

LE

[Bertie Thruster]

Thanks LE. It's probably not quite as small as I'd like. Have taken boffin advice and apparently 1/4 or 1/8th wave antennae would work. So it's into the RS catalogue to see if I can build one! Any radio wizards out there?