Detecting 108MHz-137MHz transmit

[Oznog]

I would like to be able to detect when a nearby transmitter in the 108MHz-137MHz range (aircraft radio) is transmitting. The signal should be quite strong, in fact circuit sensitivity must be low because we don't want to detect the signal from another transmitter or RF noise. I don't need to demodulate the audio out of the signal- no, this is just a cheap on/off switch that lights up when the nearby transmitter is on. This needs to be a simple RF detector without a commercial antenna or anything. It's a plastic case so an on-board antenna should be possible and highly desirable. I might need to make an installation that does have an external antenna option but if so it's going to need to be a pretty cheap implementation.

Hmm... one problem that occurs to me is that I need to detect a nearby transmission anywhere in this range, but a very low-Q receiver, even with low sensitivity, might have trouble with broadband noise over this whole range having the same total energy as a strong single-channel transmission, yet a simple receiver would not be able to analyze it based on channel energies. Well, actually, I should clarify- a dsPIC30F is involved in the circuit anyways. It does have an impressive ADC and mathematical analysis capabilities, but I'm not sure what its abilities are to analyze the RF band like this.

What are my options here?

 

[audioguru]

You are detecting the amplitude of an RF signal. Static from light switches or from a sparking motor is an amplitude of RF. Lightning is also.
Good Luck.

 

[crutschow]

Police scanners minimize the problem of broadband noise by scanning the band with a swept, narrow band receiver. Perhaps you need to consider something like that.

 

[KMoffett]

The scanner idea might work, if the local transmitter is stronger than others around, and had audio modulation. I once built a crude aircraft band spectrum analyzer. The receiver section was an aircraft band receiver kit from Ramsey Electronics: **broken link removed**. This uses a NE602 with voltage controlled tuning. I replaced the tuning pot with a sawtooth voltage circuit to sweep the tuning. The sweep voltage also was fed to the horizontal input of an O-scope. The receiver's audio output was fed to the scope's vertical input. If the audio output was fed to an adjustable amplitude comparator and monostable, it might be used to trigger Oznog's light.

(This was many years ago, so I don't have a schematic for the sweep circuit...in case you ask. )

Ken