Directional Antenna for Cobra Walkie-Talkie

[uscra112]

OK, so last Saturday my #1 motorglider succumbed to (I think) interference, and dove straight into a heavily wooded area from 500 feet, about 1/2 mile from my field. Heavily wooded in Michigan means undergrowth you often have to hack through with a machete, and thru which you can't see more than 10 to 20 feet most of the time.

The airplane had a Picolario aboard, (instead of my Walston Retrieval Systems beacon, :( ), but lacking a directional antenna for the little walkie-talkie that the Picolario talks to, I had NO way of using it as a locator. Eventually I had to listen to it die as the battery voltage dropped toward zero. :mad:

Surely it's possible to make a directional antenna for one of these things, but I, being a mere greasy-fingernail MECHANICAL penguineer, have no idea how to go about it.

Anybody care to help?

BTW this plane had a Berg RX in it, and was my first BARF glider, having a CF/resin fuselage. Although the antenna was trailing out and down from just abaft the wing, I was having an AWFUL lot of "attention" notices from the Picolario, and was actually in the process of bringing it home for that reason. Same exact setup in my small (54") woody goes to limits of my vision, (2000+ feet up or 1/2 mile away at 500 feet, with no trouble at all. No more CF fuselages for me, until I find out why the Berg had such a problem.

[AndyOne]

Uscra,

Do you know what frequency the retreval system uses, as this determines what kind of antenna would be practical. Also does the wakie-talkie you are using for a receiver have a coaxial socket where you can put the RF signal in, if not, what does it look like if you remove the existing antenna.

Andy.

[Miami Mike]

The FRS-GMRS frequencies that Picolarios use range from about 462.5 to 462.7 MHz. Here's a more specific table: http://www.ba-marc.org/writeups/gmrs-frs-freq.htm

Ham radio operators have direction-finding contests called "foxhunts", and have designed extremely effective antennas (and techniques) especially for that purpose. Some of those foxhunts use the UHF 70 cm. Amateur Radio band, which ranges from 420 to 450 Mhz, so the plans for a 70 cm. foxhunt antenna could easily be scaled for GMRS-FRS use by decreasing the overall size by the appropriate ratio.

To get more details, I suggest a Google search using various keywords taken from what I just wrote above. I'll bet you'll find just what you need.

[uscra112]

Thanks, guys !

The existing antenna in a Cobra walkie-talkie is a stiff wire coiled like a compression spring, and soldered to a pad on the edge of the board. I have not found a ground, (yet). I did try using my Walston antenna, with a short bit of wire from the pad to the center of the coax. This didn't work worth a darn, as any unshielded wire from 1/4" length on up is an adeqate antenna for the waklie-talkie. So I wrapped the thing up in aluminum foil to shield the whole device, with just the Walston cable protruding, but the Walston antenna still was in no way directional.

I was using Channel 1, which according to that chart mentioned by Miami Mike is 462.5875.

I'm happy to cut up one of the walkie-talkies to add a coax connector.

Will doo the search.

[uscra112]

Boy, there's a wealth of info in those sites - I had no idea that sport even existed.

Thanks !

[AndyOne]

Uscra,

You will need a proper RF connector to your receiver preferably connected to a point of defined impedance. This will probalbly be about 50Ω at the antenna base if it was originally designed to feed a 1/4 wavelength. You will also need to pick up on a good ground point close-by to make the shield connection effective.

Having done a number of fox hunts and even won some when I was an active radio ham there are a number of points to note...

Most passive directional antennas aren't very effective at less than a few hunderd yards.
Passive antennas are better at producing a null than showing you where the maximum signal is so using it sideways, pointing at right angles to the signal works better to see the direction of the signal. Using an attenuator box in the line from the antenna will help to get closer to the transmitter but eventually signal will leak directly into the case of the receiver if it isn't completely shielded, this will render any directionality of the antenna useless.

At UHF frequencies there will be a host of local reflections in the vicinity of the receiving antenna which can indicate completely the wrong direction. Averaging the direction of reception over a distance will give more accurate results.

A switched dipole antenna is the best type for DFing as it will work right down to zero range but it is necesarily more complaicated.

Andy.

[uscra112]

Andy, I'm trying to soak up what you say. Fitting it into a semi-ossified 60-year-old brain is a slow go, however.

I have power lines right over my house, so I moved a good 100 yards away.

My Walston antenna is an Arrow, an aluminum bar about 24 inches long with three cross-bars of aluminum tube, spaced apart about 10 inches. Some kind of bar on the middle crosspiece, connected to the lead to the RX box. I take this to be the actual antenna.

I may well have been within 200 yards of the airplane, so your comment about loss of sensitivity at short range may bear out. I did search for nulls in the way you suggested, but the auto squelch in that cheap walkie-talkie made that hard to be sure of.

What's a "switched dipole" antenna look like? That's what I'd need, since this kind of search often needs sensitivity down to 15 or 20 feet.

Fabrication of metal parts is no problem for me - I have a lathe and a mill, and some other gadgets, right at the house.

[AndyOne]

[QUOTE=uscra112]What's a "switched dipole" antenna look like? That's what I'd need, since this kind of search often needs sensitivity down to 15 or 20 feet.QUOTE]

Uscra,

I designed a switched DF antenna back in the late 1970s, the design was improved by a fellow radio ham who was an RF professional, I'll see if I can dig out any notes I made. There are commercially available vehicle mounted versions that use 4 elements but for portability I used 2 mounted on a cross bar with a centre handle. There was centre zero meter mounted on the box of circuitry which indicated the direction of the signal.
The system needs a connection to the receiver's antenna socket and another to the audio. It switches between the two antennas at audio frequency and if there is a difference in distance to the transmitter, the switching imposes a phase shift on the received audio so it needs an FM receiver to work. The audio is put through a synchronous detector which feeds the meter. When the two antennas are the same distance to the Tx there is no imposed tone and the meter reads centre. When you turn, the meter turns in the opposite direction if you are facing the Tx, should you have your back to the Tx when you turn the meter turnes in the same direction, easy. I even used this device to determine which of two car mounted antennas was transmitting at about 25Watts so it really will work down to zero. One point worth noting with this system was that the audio filtering was fairly crude so any modulation on the signal tended to upset the reading a bit so it is best used with plain unmodulated RF.

Andy.

[AndyOne]

Pics of Switched DF antenna.

[uscra112]

Hmmm. I get the picture - not too much unlike the laser straightness interferometers we use for testing the accuracy of machine tools.

Probably beyond me to build the RX, since I'm an electronic illiterate, but the concept is neat! That is the exact tool that is needed.

Unfortunately the RF from the Picolario is modulated (at audio frequencies) much of the time, but it is possible to tell it to shut up. Whether it then continues to send the CW unmodulated, I do not know.

[AndyOne]

You don't need to design a new receiver just use the one you have, the device is only a kind of active antenna with audio feed for to get a readout. You should be able to filter the audio if it is a a fixed frequency.

Andy.

[AndyOne]

Uscra,

here's a scan of the circuits I drew about 30 years ago, they are a little short on detail in some places but I can clarify if necessary.

Andy.