Friday, January 6, 2012

Canine Radio Direction Finder - Part 2

Its been a long time coming, but my son Matt and I finally finished the radio direction finder project. I described part 1 (the transmitter back in November 2011. This post describes the receiver.

The basic idea is that the transmitter sends beeps and you turn the directional antenna until you get maximum signal and that is the direction of the transmitter.

It doesn't work terribly well. The reason being the antenna is quite sensitive, so even though it is directional, if the transmitter is close then you cant tell which way it is. So it only works in the sweet spot where the transmitter is far enough away for the signal to be smaller when the the antenna is pointing the wrong way but not so far away that you cannot get a signal at all. For us, this range was perhaps 30 to 100 yards.

What is really needed is some means of varying the gain of the antenna. We ran out of enthusiasm before solving this problem.

Anyway, it does work after a fashion and its quite fun and cheap to make.

This is the schematic.

Its just an audio amplifier that amplifies the signal from the RF module. The module is the other half of the transmitter module and something like this. Make sure you get an AM module - that matches the frequency of the transmitter.

If you want to use a speaker, omit R3, but we found headphones better. So in the diagram above the speaker is actually a headphone socket.

The directional antenn is made from a piece of wood, with lengths of retractable steel ruler attached to it. The design was taken from a posting on an Australian forum that I now cannot find. But I'm sure they wouldn't mind my repeating it here.

The second element takes the connection to the receiver circuit and this is made with a short length of TV antenna lead.

You can also see how the strips of ruler are just screwed onto the wood.

As I mentioned the results were a little disappointing. But then the whole project cost about 20 USD and we had some fun. Especially wandering around the neighbourhood wearing headphones and clutching what looks like a home made TV antenna. This is the kind of situation where it is best to just to cultivate a wild look in the eye and hope people don't bother you or call the police.
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1 comment:

Anonymous said...

The antenna is ratshit
Copy a Yagi design
Center rod is metal all elements earthed to it One side of driven element connected with a gamma match or something .
Spacing of all elements should be roughly 20% of wavelength. directors shoorter than driven ,reflector longer but yours are much too close