FM signals, which are between TV channel 6 and TV channel 7, can interfere with the reception of TV signals in several ways. They can cause fundamental overload of tuners and preamps in the VHF band, and even in the UHF band if they are strong enough. The second harmonic of an FM signal can interfere with a VHF-High (7-13) TV signal. Very strong FM signals at around 88 MHz can cause adjacent channel interference to TV channel 6.
This is an analysis I did for dbseeker:
thread:
http://forum.tvfool.com/showthread.php?t=14415
my post:
http://forum.tvfool.com/showpost.php...2&postcount=15
THREE TYPES OF OVERLOAD
There are three types of preamp or tuner overload, in order of increasing signal strength:
1. The strong signals almost cause enough intermodulation distortion (IMD) to interfere with the reception of weak desired signals, but the spurious signals are at or below the noise floor. This is the point that holl_ands uses in his preamp charts to obtain max SFDR (Spurious Free Dynamic Range). No damage will happen.
As the strongest signals continue to increase in strength, more of the weaker signals are damaged until you reach:
2. The strong signals cause overload to the preamp or tuner that makes it impossible to receive any signals. No damage will happen. The strongest signals are still there, but they can't be decoded because the IMD products have damaged them so that they contain more errors (high BER....bit error ratio/rate) than can be corrected by the FEC (forward error correction).
3. The signals are so strong that the input transistor is toast. You are not likely to encounter OTA signals that strong, unless you live next door to a high power transmitter and you have your high gain antenna aimed at the transmitter's antenna.
As a general rule, tuners can tolerate stronger signals than preamps before overload. The difference in strength is approx. equal to the preamp gain.
Forum member holl_ands has made a preamp chart that shows the maximum input signal for preamps. Many of the preamps are no longer available, but it gives you the general idea.
http://imageevent.com/holl_ands/file...=0&w=1&s=0&z=4
Intermodulation Distortion
The IMD creates new spurious signals within the preamp itself that can interfere with the reception of your weakest desired signals if the spurious signals are stronger than the noise floor of the weakest desired signals. The spurious signals are caused by the interaction between two or more of your strongest signals.
IMD is not the only distortion that can be created within the preamp; you can also have distortion caused by signals so strong that the top of the strong signals are clipped, which causes compression of the signals. This can be seen if you increase the input to the preamp by, for example, 10 dB and the output increases by less than 10 dB.
Spurious Free Dynamic Range
The Spurious Free Dynamic Range needed is the difference in strength between your strongest signal and your weakest desired signal, using the dBm Pwr scale on your tvfool report. This difference is expressed in terms of dB, not dBm, because the original units are the same. To this you must add 16 dB that is required for the SNR of the weakest desired signal.
Another way to think of it is from the top of the strongest signal down to the bottom (noise floor) of the weakest desired signal. The top of the spurious signals must be at or below the noise floor if they are not to cause interference.
You can also use the NM scale for your calculations, but I prefer to use the Pwr scale because most tuners drop out around -85 dBm.
The second harmonic of an FM signal can interfere with a VHF-High signal if its frequency is 1/2 the frequency of the tV signal:
Testing for DTV Interference
http://www.tvtechnology.com/digital-...ference/202503
Strong FM signals at the low end of the FM band (88 MHz) can cause co-channel interference to CH 6 TV signals that are just below 88 MHz.