Tracking signals

[zachtheterrible]

What I'm lookin' 2 do is this: I want to be able to track walkie talkie signals. A directional antenna setup would be impractical for me. PLEASE, I've searched google dry and can't find a bloody thing.

Is there any way possibly of building a miniature directional antenna? (I need something that is small)

I know that this is probably straight fiction, but this is basically the kind of thing that I am looking for. This guy put a transmitter on a cat (It was the old "That Darn Cat" movie), and had this little handheld receiver w/ a needle that pointed to where the cat was. As I mentioned before, I have a terrible feeling that this is probably absured . . . but u never know. ANY ideas would be very greatly appreciated.

 

[Nigel Goodwin]

What I'm lookin' 2 do is this: I want to be able to track walkie talkie signals. A directional antenna setup would be impractical for me. PLEASE, I've searched google dry and can't find a bloody thing.

Is there any way possibly of building a miniature directional antenna? (I need something that is small)

I know that this is probably straight fiction, but this is basically the kind of thing that I am looking for. This guy put a transmitter on a cat (It was the old "That Darn Cat" movie), and had this little handheld receiver w/ a needle that pointed to where the cat was. As I mentioned before, I have a terrible feeling that this is probably absured . . . but u never know. ANY ideas would be very greatly appreciated.

That's because it's fiction!. If you use low frequencies, such as medium wave, you can use a ferrite rod aerial which is nice and directional - however, you need two readings to know which direction the signal comes from, as it's got nulls at both ends.

 

[stevez]

I'd have to agree with Nigel with this except to say that with enough money and time you could probably do better than the larger antennas that we use now.

You didn't say if you had control of the walkie talkies. If you did you could employ a GPS based reporting system and possibly automate it. You might then call it an Automated Position Reporting System and use the abbreviation APRS. As might be obvious, the transmitter sends location information when polled or at some interval, possibly when the mic is keyed. Some amateur radio operators employ this stuff. I know little about it at this point.

I am not sure what resources you are using in your research. Keep in mind that the internet, however large and limitless it appears to be, only allows you access to information that someone put there. There are countless publications and papers - some very public and some available on a limited basis - that address this subject. Several libraries within a short drive of my home are full of info on RF systems.

Not a new idea but you may not have thought of this - employ multiple receiving sites that would report information to a central location for processing - and reporting to the miniature handheld device. We use a "manual" version of this to track down sources of interference. Signal strength reports help narrow the search field then multiple directional reports aid in triangulation to a specific point.

Anyway, fun stuff to think about.

 

[zachtheterrible]

Well, looks like I just might have to invent this for myself! I don't have any libraries around me w/ stuff on RF, so that's not an option (and i can't drive to a farther off library either, i'm 15 and dont have license MUST GET LICENSE). GPS isn't really something that I want to do . . . I think i'm kind of set on trying to build this. I've got one idea, sort of along the lines of what stevez said, but different.

Say I have two receivers (but both are part of the circuit). Say that both receivers are the same exact distance from the source, they would both receive the radio wave @ the same time; now if one was a bit farther from the source than the other, they wouldn't receive the wave @ the same time. I would then have to find some way to indicate the direction. Look @ the picture, I think it'll do the explaining better if you don't get it.

Good idea ? Bad idea ?

 

[Dean Huster]

The U.S. Naval Security Group has been doing HF radio direction finding since the cold war began and are now finishing up "deinstallation" of most of their NSGA sites. Each of these sites, costing millions of dollars each, consisted of electronically-steerable antenna arrays that occupied a 1/8-mile circle. Two stations together could triangulate on a signal and determine it's location quite accurately and a third station could verify that location.

The use of spread-spectrum and satellite technology have made most of the purpose of HFDF moot. During it's heyday, it was some really neat, high-technology stuff that went on .... also securityi-classified as high as you can get.

If you're interested, do a Google search under "Wullenweber Antenna" or check out some of the sites you can be directed to by former Communication Technicians. CTs were the naval "spooks" that ran these stations. Pretty interesting stuff.

**broken link removed**
**broken link removed**
http://www.usncva.org/


Dean

 

[Nigel Goodwin]

Say I have two receivers (but both are part of the circuit). Say that both receivers are the same exact distance from the source, they would both receive the radio wave @ the same time; now if one was a bit farther from the source than the other, they wouldn't receive the wave @ the same time. I would then have to find some way to indicate the direction. Look @ the picture, I think it'll do the explaining better if you don't get it.

Good idea ? Bad idea ?

Radio waves travel at the speed of light, so you would have to measure extremely short time intervals - using circuitry which also only works at the speed of light.

So you're talking about extremely complicated designs (assuming it could be made to work at all), which is why it's only done in fiction.

 

[spuffock]

If you have 2 antennae and switch rapidly between them, and feed the switched signal to an fm receiver, then a note will be heard from the receiver's audio output UNLESS the antennae are exactly the same distance from the transmitter. A synchronous detector driven by the switching signal can indicate by the polarity of the signal which of the antennae is nearest the transmitter. The antennae should be spaced around half a wavelength apart.
Yes, I have. And it does. Detects 2 centimetres movement of transmitter at a distance of 30 metres frequency 144Mhz

 

[zachtheterrible]

I haven't gotten to thinking about the actual circuit, but an idea of mine would be to have an LED turn on when the voltage variations in the two antenna are the same. Nigel mentioned that my circuit would have to work at light speed. Would this idea eliminate the need for that?

Spuffock, have u actually built what you are saying? or heard about it? What is the idea behind this also, I don't really get how it works.

 

[spuffock]

I've actually built the thing. As to its operation, first consider one antenna. If you move it towards a fixed transmitter, the frequency seen by it will increase due to the doppler effect. If moved away, the frequency seen will decrease. Now imagine this antenna on the edge of a wheel which rotates. This will put a sinewave frequency modulation on a received carrier, and a tone will be heard from an fm receiver . We don't want to carry a big wheel spinning at a fair rate, so we simply switch between two antennae. If the antennae are at different distances from the transmitter a square wave (in theory) will appear at the audio output of the receiver. A synchronous detector, driven from the same signal used to switch the antennae, will provide a dc level dependent on the phase of the audio, which can be used to light leds to indicate which antenna is nearest. You look for the audio null to indicate direction, (both antennae at he same distance) , and rotate the thing a bit to determine which antenna gets nearest ( is it in front or behind).
You will be listening for the disappearance of a specific tone, so there will be no trouble with speech on the signal. Since it isn't synchronous, speech will average to zero at the detector and will not affect the leds.
For 144Mhz (2 metres) the antennae are dipoles , each made from 2 "rubber ducks", mounted at the ends of a bit of wood about 70 cm long.
For 450Mhz or so, the spacing between the antennae should be 25-30 cm.
Be sure that the cables from the antennae to the switch are EXACTLY the same length, or the thing will squint sideways.
One day I'll figure out how to send circuit diagrams, but until then, the active components are as follows:
2 of dual gate mosfets( antenna switch)
4013 ( oscillator, divider)
4066( synchronous detector)
lm324( amplifier, led driver)
I may still have missed something, it was a few years ago. Maybe I'll have to build another one.

 

[JohnBrown]

You could, of course, do this with a ferrite rod aerial, as I think Nigel mentioned some time back. But you will have the same problem. With only one receiver/aerial you can't tell whether the transmitter is behind you or in front of you. It's the same with sound. You can't tell whether a noise comes from behind or in front of you, unless you move your head about. So, if you wanted to track a pet or an infant, you'd have a 50% chance of guessing the correct direction. Probably better off to stand still and hope the pet/infant finds you.

 

[spuffock]

I've not tried this with ferrites, but I can't see any reason why the principle should not apply. At lower frequencies it may be difficult to obtain sufficient spacing between the antennae and still be able to go round corners :twisted:

 

[JohnBrown]

I've not tried this with ferrites, but I can't see any reason why the principle should not apply. At lower frequencies it may be difficult to obtain sufficient spacing between the antennae and still be able to go round corners :twisted:

Sorry, spuffock, I wasn't replying to you. In fact I was still typing when you posted. I was commenting on the previous post by zachtheterrible.

John

 

[spuffock]

Oh, let's build another one.
I remember now, I had to bugger about with phase shift on the audio to get the leds working at the right point. But there were no PICs at the time.
If I use a 12C508 to generate the antenna switching signal and a phase shifted version to drive the detector , I can be rid of a lot of not-so-stable analogue stuff. ( I know there's better than the '508 !)
In answer to a previous question, the only thing that handles RF is the antenna switch. Everything else is slow old audio.
Can everyone get a PIC programmed if I send the code?

 

[zachtheterrible]

I think for now, I probably won't be building anything yet . . . I'm not advanced enuf yet. I don't have much of a clue about PICs yet. I know that I could probably learn about them, but I want to understand other things first. Is my first idea a good idea? Johnbrown, how small do you think that I could make an aeriel ferrite antenna that's tracking signals @ say 150 MHZ?

 

[Nigel Goodwin]

I think for now, I probably won't be building anything yet . . . I'm not advanced enuf yet. I don't have much of a clue about PICs yet. I know that I could probably learn about them, but I want to understand other things first. Is my first idea a good idea? Johnbrown, how small do you think that I could make an aeriel ferrite antenna that's tracking signals @ say 150 MHZ?

Ferrite rods only work at low frequencies, they are usually used below a couple of Mhz. At 150MHz (presumably you probably mean the 145MHz amateur band?) you can make a reasonably sized dipole.

DF hunting is quite common on the 145MHz band, you should be able to find lots of information about it.