Detecting radio transmitters

I have a neat little gizmo from Optoelectronics called “The Scout”. It’s a frequency counter. If your near a transmitter the Scout tells you what frequency the transmitter is broadcasting at. There are many companies that make frequency counters, but Optos’ Scout is the best one I’ve ever seen!

The down fall of the Scout is that it can only display a frequency if the transmitter is actually broadcasting.
Is it scientifically possible to make a device that could detect a transmitter even when it wasn’t transmitting. I’m thinking the military might have something like this. Any radio buffs out there?

Total WAG here, but… If the detector could transmit a frequency sympathetic to the frequency of that being scanned, wouldn’t the object being scanned react in a telling way? So, if you broadcast a weak, but wide frequency transmission, you could detect the frequency at which the object responded. This would not work with radios.


pkbites will translate your input between various European languages. Would you expect it to translate text that you did not input, or to translate when it was switched off? Of course not. Why would you expect a frequency detector to detect frequencies that are not there?

I guess another downfall of the Scout is that it will only work if the Scout is actually switched on. Perhaps if the Scout is switched off, it can detect transmitters that are also switched off?


If you measure the reluctance of the a specific volume at a specific frequency you could probably establish the existence of a circuit designed to resonate at that frequency even when that circuit was not energized. That might sound like a yes, but it is only a theoretical yes.

The volume would be small, and have to be very close to the device doing the measuring. (feet, I imagine) The device could only measure one frequency at a time, and would be likely to misread some natural resonances as possible transmitters. False positives might be very annoying, if you are talking about things behind walls, or such. You would have to eliminate any known circuits from the search area first, and maintain control over it subsequently.

So, you are not talking about a hand held object sweeping out a room in a matter of minutes. You are talking about a fairly tedious task involving one or more sweeps at each “likely” frequency, and then a second set of sweeps to verify any positive reading. This is probably fairly expensive equipment, as well.

So, I am sure the NSA can do it, and probably does, fairly regularly in areas it considers to require extreme measures of security. Probably the top of the line in commercial anti-espionage professionals can do so as well. I would charge big bucks if I could do it. I would need big bucks, since the absence of a circuit today is pretty much a placebo assurance for tomorrow. Makes you feel good twenty percent of the time, has no real effect on security.


Triskadecamus, sounds like you’re describing a grid dip meter!


I don’t want a device that finds the frequency of an inactive transmitter, I want a device that can find the transmitter itself, even if it isn’t transmitting.

What is the practical purpose for this?

Radar Detectors! A radar detector can only detect radar if the radar is on and transmitting. To defeat detectors many cops switch their radar into a “hold mode” commonly refered to as “instant on”. The officer waits until he sees a car, then begins transmitting the radar wave. Violators detector sounds off like the 4th of July, but by then it’s too late.

If a device could be manufactured to detect the radar unit itself, and not the frequency it transmits, “instant on” traffic radar would no longer be a threat to the serious speeder, and the inventor of such a detector would become rich.

Is such a detector possible, or is this fantasy?

Well if you’re talking about such a specific application, that might make it easier – at least we know we’re looking for K-band radar, etc.

But the problem is still false positives – you’d basically be looking for a spatially consistent drop in the returned signal, but that could be caused by so many things (metal deposits, passing cars, terrain) that the only way I can think of for it to work would be to also send a very close frequency of radar (but far enough off not to resonate the radar gun), to use as a baseline, to try to control for nonresonant effects.

Of course, this whole idea involves you transmitting radar like a frigging airport beacon, so it’s not going to be the stealthiest of methods, and the cops would just switch over from active to passive, and try to detect you for a change! =)