How is the location of a radio transmitter found in order to stop unauthorized radio broadcasting?

741 views

I know that if someone were to be interfering with critical radio communications (such as police and fire communications, air traffic control communication, etc.), the FCC (or another country’s equivalent) has equipment to trace down the source.

How does the equipment to find the location work? And also, how does it tell how many transmitters are currently broadcasting at that frequency?

In: Technology

It generally starts with a report – someone using licensed channels having problems, thinks there might be interference, so they ask the FCC to take a look.

So an agent goes out with a directional antenna. This is an antenna that detects signals where it is pointed, but not anywhere else. They connect it to a tuner set to the channel the interference seems to be on, and drive around/point it around until they pick up a signal that is not coming from the licensed transmitter. Then they go investigate.

Generally, they find a malfunctioning device that is transmitting noise, and they tell that person to turn it off and get it fixed. If they don’t then they get legal, but normally this is as far as it goes. But if the person is purposefully running a transmitter, especially if for a commercial purpose. the fines and other penalties can be serious.

Mostly through triangulation with directional antennas. When you take two directional readings from two different points, the lines drawn by the readings will intersect at a point. That’s where your transmitter is, in general. Once you get into a neighborhood, you can use raw signal strength and your directional antenna to know if you are getting closer or further away.

Ham radio folks like to do this for fun in a game they call “fox hunting”. One guy sits somewhere and transmits occasionally and everyone else has to track him down. I’ve done it many times, it’s a blast.

You use a spectrum analyzer. They are super expensive (like 20,000 usd) but the screen shows frequency and amplitude. By scanning the electromagnetic spectrum you’ll notice “peaks” or little mountains. The bigger the peak the stronger the signal. With directional antennas (think old school tv antennas on your roof) you point it around, if the peak gets bigger then that’s the direction the signal is coming from.

There is automated antennas and systems that Use antenna arrays link together that will triangulate on a signal, so you will either drive around or walk and it will throw lines towards the source. Where these lines cross is high probability that the source is.

Multiple [directional antenna](https://en.wikipedia.org/wiki/Directional_antenna) (usually 3) provide you with a direction in the way that your ears do for sound, the strength of the signal gives you a rough distance. Moving around with this setup provides you with more data, the more you move around the more data you have. The more data you have, the less places the signal could come from.

Imagine three [directional antenna](https://en.wikipedia.org/wiki/Directional_antenna) (A,B and C) which are set in a Y formation.
– Standing still 50 meters from the signal and turning until one antenna (say A) has the strongest signal it can have. The direction it faces is the direction of the signal.
– Move towards the signal, it should get stronger on all antenna but especially A.
– If you move past the signal, the signal for A will get weaker while stronger for B and C.
– Repeat until the distance is 0 and all antenna have equal signal, while moving in any direction makes the signal share unequal.

One way to interfere with this is to have signal repeaters meaning that there are multiple sources but the above method can still work. You just follow the strongest one till you get to either the source or a repeater, if it’s a repeater you take it offline then repeat till you find the source.