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Is it possible to create a circuit to hear (through earphones) or show (with an LED) the direction of a radio source? What does a really simple circuit look like and how does it work? How does the frequency of the radio source affect this circuit and antenna? Is it possible to locate WiFi, Bluetooth or ZigBee/XBee with this approach?

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Is it possible to create a circuit to hear (through earphones) or show (with an LED) the direction of a radio source?

Yes. There are basically two different types of direction finding techniques:

  • You can simply change the orientation of your antenna and observe whether this makes the received signal weaker or stronger. This has the advantage of working with any radio receiver, as long as either

    • the receiver displays some indication of the received signal strength, or
    • your antenna is sufficiently directional (e.g. a Yagi or loop antenna) that it makes the signal strength drop enough to make the signal vanish under the noise. (In fact, when using a loop antenna, it is normal to look for that point, the null, because it is easier to hear the absence of the signal than when it is at maximum strength.)

    This method is commonly used by amateur radio operators who do it for fun, on foot (this goes under a number of names: foxhunting, T-hunting, radiosport); their equipment typically consists of a handheld transceiver, a Yagi or loop antenna, and a variable attenuator (to avoid overload when too close).

  • You can use a direction-finding receiver; this requires at least two antennas (or an antenna with multiple outputs, depending on how you look at it), connected to a receiver with multiple inputs, which compares the signal amplitude and phase to derive the direction information.

    Amateur radio operators who do direction-finding in their vehicles (either as a sport or to locate interferers) often use the so-called “Doppler direction finder” design, which can give an indication of direction on LEDs, uses an array of four antennas, and passes through to an ordinary FM receiver to also listen to the signal.

Aircraft used to have an automatic direction finder (ADF) as standard equipment. These may work on either of the above principles. They are not typically used for navigation today, having been replaced by VOR and GPS.

What does a really simple circuit look like and how does it work?

In general, any design for a radio receiver which has either an audio (if the signal is continuous, anyway) or signal-strength output can be used together with a directional antenna.

Here's another question on this site which happens to have a schematic of one two-antenna design for a specialized receiver, and my answer discusses how it works and includes results of a simulation.

How does the frequency of the radio source affect this circuit and antenna?

Very loosely speaking, radio works the same at all frequencies. In practice,

  • Lower frequencies are more likely to propagate long distances, pass through walls, refract around obstacles, and otherwise complicate direction finding. Higher frequencies act more like visible light, making it more straightforward to locate them if you can receive them.

  • Lower frequencies require larger antennas, making moving antennas less practical to use.

Is it possible to locate WiFi, Bluetooth or ZigBee/XBee with this approach?

Yes, but because these are packet protocols you will practically need receivers for the purpose, which can recognize the quick bursts and distinguish them from noise. You will also need the receiver to report the signal strength (RSSI); I understand this is commonly available but I haven't worked with this sort of packet radio hardware so I can't give any details.

In principle, you should be able to connect a standard module to a tiny directional antenna, write software to ask it for the RSSI of every packet it receives, and be ready to go. The trickiest part will be fabricating the correctly sized and correctly matched antenna (more or less depending on what test gear you have available).

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Is it possible to create a circuit to hear(earphones) or show(with an LED) the direction of a radio source? How does a really simple circuit looks like and how does it work?

Virtually any receiver, connected to any antenna, will work. With some knowledge of the antenna pattern, and a receiver that can indicate the received signal strength, then one can infer how the antenna is oriented relative to the source.

How does the frequency of the radio source affects this circuit and antenna?

In a lot of ways. The size of an antenna is proportional to wavelength, so while you might be able to use a hand-held parabolic dish at 5 GHz, the same wouldn't be possible at 5 MHz since the frequency is 1000 times lower, and consequently the wavelength and antenna 1000 times larger.

Additionally, the frequency will dictate the kinds of components you can use. It's quite easy to make a radio that will work at low frequencies like 20 MHz, and this can be accomplished with ordinary, discrete components and a soldering iron. As frequency increases this becomes increasingly impracticable and you'll need to use fabricated components like MMICs.

Is it possible to locate wifi, bluetooth or zigbee/xbee with this aproach?

Yes, provided you have a receiver that can operate at those frequencies.

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