Direction finding antenna technology

[George]

I posted an inquiry (below) regarding my search for a very small DFing
antenna. I received some much appreciated replies, but not quite what I was
looking for.

Here's a link to a commercial product:
http://store.yahoo.com/fulfillmentadventures/baavtr.html

This product displays bearing and distance to a transmitter over a very
short range, operating below 500 kHz. This is the physical size I had in
mind.

I'd like to be able to build a similar unit in the 144 MHz band for
direction finding over short distances. In other words, for use on the
final leg of a DF contest where the transmitter is buried in brush in a
remote area. A parasitic array or even a single loop wouldn't be as rugged
or hand-holdable as I'd like.

Does anyone know what antenna technology is used in this product to be able
to develop directionality at such a low frequency with such a small size?

I suppose I could ask the manufacturer, but they might not want to disclose
their design info. Or, I could buy one for big $$$ and try to figure it
out, but that's not a sure thing either ... :o)

Thanks for any help.

George, K6GW

[Richard Clark]

On Fri, 12 Mar 2004 02:59:50 GMT, "George" <look@signature_to.reply>
wrote:

>Does anyone know what antenna technology is used in this product to be able
>to develop directionality at such a low frequency with such a small size?

Hi George,

It's called a loopstick, been around for 50 or 60 years now. You can
find one in any pocket transistor radio. They probably gussied it up
with a pair of them for discrimination (driving the left/right
indicator).

If you are in a DF contest, you already have to carry the array, no?
So why carry more stuff? You might want to consider that with a
smaller antenna, your body becomes a bigger source of confusion
insofar as reflecting the signal (something the huge wavelength of
457KHz is immune to).

73's
Richard Clark, KB7QHC

[Tom Bruhns]

If you _want_ to be found, presumably you use completely different
technology than if you're making a hide-and-seek game of it.

When you get really close to a hidden 146MHz transmitter, you can do a
lot of good simply with signal strength. You need a receiver (or
signal strength meter) which does not overload even for large signals,
however. If you insist on getting wavefront direction information,
and you have plenty of signal, it's possible to do it switching
rapidly between two antennas. They can be small and not spaced very
far apart and still work. I think there's a larger version known as a
"handyfinder." That close in, I never felt a need for directional
info. Note that you can also search on the third harmonic when close;
the hidden transmitter has to go to extremes to filter the third to a
low enough value you wouldn't be able to use it close in.

Cheers,
Tom

"George" <look@signature_to.reply> wrote in message news:<GI94c.16185$%06.1...@newsread2.news.pas.earthlink.net>...

[Crazy George]

I am puzzled as to your actual requirement. Explanations and descriptions
of the techniques and hardware to do what I *believe* you are asking have
been published for years in innumerable journals and magazines. One
specific application which comes to mind is the location of VHF/UHF ELT
transmitters inside a hangar where they have been accidentally activated.
Those are described in various flying publications. Real challenge inside a
reflecting metal hangar where the transmitter might be in any one of a half
dozen aircraft, and the signal appears to be stronger out in the middle of
the room than near any of them. There are articles about amateur radio fox
hunting (HF, VHF, MF) hardware published in all the English speaking
countries I am aware of, and I see references to articles in many other
languages as well. Just out of curiosity, I did a Yahoo search (based on
your Yahoo reference) and found commercial products, references to many
literature articles, construction plans, and other indeterminate citations.
I used VHF, radio and "direction finders" as the search string. Maybe one
of the problems here is that looking for a "DF antenna" is a fruitless
search. It has to be a complete DF system to perform any useful function.
Almost any antenna can be used as a DF antenna with appropriate support
electronics. The big elephant cage DF antennas (FLR-9, FRD-10) use simple
vertical monopoles as the actual antenna elements, but the system is much
more complicated than that.

--
Crazy George
Remove N O and S P A M imbedded in return address

[Yuri Blanarovich]

>
>> I'd like to be able to build a similar unit in the 144 MHz band for
>> direction finding over short distances. In other words, for use on the
>> final leg of a DF contest where the transmitter is buried in brush in a
>> remote area. A parasitic array or even a single loop wouldn't be as rugged
>> or hand-holdable as I'd like.
>>

Do a search for Fox Hunt. There is considerable activity in US now in that
sport. 73 mag had plenty of articles, that technology is well developed.

For 2m hunt antenna, the smallest one would be 2 el Moxon or ZL special, or if
you need to shorten the elements further, you can load them with coils. This is
another case where current in loading coils distribution is important :-)

You can make elements out of springy wires, so they can bend and return to
shape while going through the bushes (why?).

Yuri, K3BU