Several companies make whats called a "radar detector detector" All radio devices use whats called an IF or intermediate frequency. Say a radar gun is putting out 10.245Ghz and the IF of a detector is 10.7Mhz the radar detector uses a local oscillator to generate 10.234.3 to make the the 10.7 diffrence. Put diffrently 10.234.3 is mixed with the incoming signal of 10.245.0 to make a hetrodyne diffrence of 10.7 MHz. With this in mind a radar detector "detector" could use a local frequency that is 10.7 higher than your detector and when it hetrodynes with your 10.234.3 it could make its own IF thus signalling the officer that you were using a radar detector. Im using the old X-band frequencies as an example but most if not all radio devices today use the IF conversion principal. Another example is your FM radio. Tuned to 100.3 MHz its local oscillator must produce a frequency that is 10.7 MHz higher so it would actually be 111 Mhz (111-10.7=100.3) the station is transmitting on 100.3 Mhz but to have a tuned RF tuner that would be selective enough to only pass the 100.3 signal with enough bandwidth to carry the desired frequency and reject the station at 99.7 would be large and cumbersome not to mention tempermental plus trying to discriminate a signal at this rate using frequency modulation would be technically impossable. FM is rate of change so you would need a discriminator tuned to each frequency division on the dial. Same holds true for radar guns that use dopplar shift as all do.

IF Experement? Take 2 FM tabletop radios and place them next to one another. Tune one for a station close to 104 Mhz and turn it up a little. Next take the other table radio and turn the volume down and tune it across the FM band until the first radio goes silent. Turn up the volume and you should faintly hear the station you were just listening to. What you have done is tuned radio-2's local oscillator to the primary carrier frequency that radio-1 was receiving. Now move radio-1 off frequency slightly until you loose the station. Now if one radio is high tune the other radio low and you should get the station once again in a new part of the dial, this is the A+B=C hetrodyne mix and you have just created an IF on your tabletop. Most if not all FM broadcast receivers in north america are whats called high side single conversion. The 10.7 IF is fairly clean, offers good audio recovery and bandwidth and low distortion. Communications radios sometimes are double and a few are triple conversion. The bandwidth is much less than broadcast FM so they can up the selectivity by going say from 45 MHz to 10.7 Mhz to 455 KHz. Now you have 3 conversions to stop unwanted signals. Radar detectors are almost universially single "low side" injection A-B=C IF with a 10.7 IF. At this frequency radar detector mfrs dont have to rely on complex waveguides and delicate receivers. A single GUNN diode slotted horn does the job of both making the IF mix and the local oscillator in one small footprint.
Got a radar detector that you dont know if it works correctly? Hold it near a known good detector. If both local oscillators are running correctly the one will receive the other in a direct fashion and set them both off.


I will not however explain how to make a local oscillator out of 3 table radios to blank out your neighbors television. It can be done and I used to do it during my fathers football games as a kid. Was very effective during superbowl touchdowns!

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