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I would like to build a system in which the emitter emits omnidirectional RF signals, and the receiver gets a stronger signal (it does not have to be accurate) with a) closer distance from the emitter b) better pointing with the emitter (directional receiver).

Kind of like a "you're getting warm/cold" game.

What's the simplest technology and type of circuit that would allow me to do this?

I have thought of a signal strength circuit with a directional antenna, is that the simplest way? i.e. If I wire "data in" of a 315MHz transmitter such as this one https://www.sparkfun.com/products/10535 to Vcc, and if I build a tuned (adjustable) tank circuit + OA81 diode + ADC at the receiver, could it work?

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    $\begingroup$ I've seen exactly this sort of thing used on TV for animal tracking. The suggested approach of directional antenna + signal strength meter sounds reasonable; you'll get slightly different behaviour depending on which band you use, which will probably have to be one of the ISM ones. (And let's not be quite so harsh on new users with typos!) $\endgroup$
    – pjc50
    Commented Nov 2, 2016 at 11:03
  • $\begingroup$ @pjc50, thanks. I was thinking that perhaps a simple diode + resonant circuit feeding an ADC was enough, as long as I have a way to direct the receiver (a simple can [grounded] without the top?). I wonder if I should get an oscillator wired to the 433MHz transmitter or if the transmitter is enough. $\endgroup$
    – user42875
    Commented Nov 2, 2016 at 13:31
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    $\begingroup$ You should think slightly more about this if you don't have radio experience; yes, you should modulate the transmitter somehow, and a simple resonant circuit may or may not have the reciever selectivity you need (how are you going to tune it, old-style variable capacitor?) Note that normally the reciever modules of those paired 433Mhz sets have automatic gain control, which is exactly what you don't want. $\endgroup$
    – pjc50
    Commented Nov 2, 2016 at 13:46
  • $\begingroup$ @pjc50: If I wire "data in" of a 315MHz transmitter such as this one sparkfun.com/products/10535 to Vcc, and if I build a tuned (adjustable) tank circuit + OA81 diode + ADC at the receiver, could it work? $\endgroup$
    – user42875
    Commented Nov 3, 2016 at 9:21
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    $\begingroup$ Ham radio enthusiasts call this kind of game "Fox Hunting". A transmitter is hidden out in the countryside somewhere and everyone else has to race to find it using directional antenna receivers. The Ham radio SE will likely have some tips for you. $\endgroup$
    – Wossname
    Commented Nov 3, 2016 at 10:02

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First the answer: When I worked for the forest service I built single-transistor transmitters where one of the leads of the transistor was the antenna. It doesn't get any more simple than that.

They used hearing aid batteries. Then we could enclose them in beeswax so we could stuff them down the throats of animals, even snakes. The batteries would last about three days. EPA said we were supposed to recover the batteries, but some of them apparently got lost on the way back. The receivers were about the size of a canteen, using D cells and hung around the neck so one operator could handle it. We used 3-element Yagi antennas to follow the animals.

Now a story:

We played 2-meter "rabbit hunting" every Saturday for years in Fairbanks.

The receiver wasn't the key to success. It was the antenna and the skill to use it.

It turned out that instead of using Yagi antennas to find the source, the best way to get a fix was to use a tuned loop antenna and utilize the nulls in the donut-shaped pattern of the antenna.

Nulls are much sharper than the broader beamwidth of a Yagi or a quad.

Newer DF systems, (like the 4 antennas on police cars to find transmitter packs inserted in stolen money), use time-of-arrival, and are best used when they are close to the source. Plus they generally can only give you 8 LEDs to indicate the first antenna (and in between two) where the signal arrives.

We didn't use computers like that. About the only portable computer we had back then was the KIM-1, anyway.

Our loops could go for miles.

For the record, one of our rules was to never work in teams other than having two people in the vehicle. It was up to each pair of us to establish our own bearings. The second rule was that the rabbit couldn't move. But there was no rule as to how often the rabbit must transmit.

Then as we got closer, we would detune the loops (with a capacitor at the top of the loop) to avoid desense in the receiver.

We had very modest prizes, like first plates of food at the inevitable BBQ.

Those were really fun days.

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  • $\begingroup$ re modulation, we used regular FM simplex - the Rabbit in our game would transmit a "QRZ from KL7xx" and maybe chat a bit. Often we would talk back, trying to goad the rabbit to transmit more - but the rabbit would just listen until the mood struck them to transmit another QRZ. At the beginning of the game the transmissions would be longer, but if we said "We've got you' they would get shorter, naturally. Good times had by all. My driver was very experienced and taught me all about how to use the nulls and such. When you get closer there is a lot less spinning of the loop. $\endgroup$
    – SDsolar
    Commented Feb 24, 2017 at 0:13
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Well, as @Wossname indicates this is a 'game' and it can be 'played' in any frequency domain and with any modulation method you have legal permission to use. The proper name is Radio Direction Finding(RDF). It could be done with IR pulses and an IR receiver since light is just a different frequency in the RF band. The Warm/Cold feature you are looking for is a function of the Inverse Square fall off of an omni-directional signal in open space.

Most of these are done with a simple transmitter unit which is creating a transmission either continuously (for the very low power units), or more likely for a foxhunt, intermittently as a set time for a set time -- every 5 minutes for a minute for instance. The amount of power is a function of the frequency/wavelength and the area you need to cover.

During the signal on time you need to spin your receiving antenna around looking for a variation in signal strength at the receiver. Then you go someplace else and make the same measurement. Amateur receivers have a signal bar or S-Meter that indicates a decibel reading of the received signal. If you have a sensitive receiver, as you approach the signal source, you may need to attenuate the received signal so that it doesn't read full scale. Even without a directional antenna, you can use this method alone to track a signal to its source. Your body mass of water and meat is a good attenuator for many frequencies and could be used to shield your antenna (lowest signal strength and the signal would be behind you)

This excellent answer by Kevin Reid provides more technical details on the receiver side of the solution.

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