Scanner Desense

[SquierStrat]

Hello, i was reading in the 2 "fm trap" threads and read a little about scanner desense. From how i understand it, if i were to take a GRE scanner (i pick GRE because of their VHF sensitivity), put a VHF specific antenna, and put it in an area where there was tons of VHF interference, the scanner would eventually become weaker in the VHF band. So my questions are:

Basically, is this true?

If it is true, then what if i had that scanner monitoring only UHF or 800mhz frequencies at the time. Would it still desensitize it?

And also, what if i had it monitoring a handful of VHF frequencies, but had the attenuator turned on, on just those specific frequencies?
(both of these scenarios being with the GRE scanner, with the vhf antenna, in the high vhf interference area)

 

[n5ims]

You can think of desense is like having a conversation with someone. If you're both in a quiet area you can understand eachother quite nicely talking quietly. If you're both in a noisy bar you may not understand even if you're shouting. Your voice levels during that conversation would be like your scanner's reception while the noise level would be like the unrelated (but nearby) transmitters.

If one (or more) nearby transmitter is very strong your receiver won't pick up weaker signals near as well as when that transmitter isn't transmitting. If you have multiple strong transmitters nearby, the chances of one transmitting is greater. If that transmitter is an FM or TV broadcast station you can be sure it's transmitting nearly all the time so your problems will be constant.

A filter will reduce the signal levels in the frequency range while allowing others to pass through without being reduced. Your strong FM broadcast transmitter just became a weak transmitter (to your scanner) when you put the filter in.

If you notice, I was always talking about transmitters above, not receivers. Receivers will not desense another receiver, only a transmitter can.

A strong VHF signal often won't desense a UHF or 800 MHz signal (or at least quite as much) since the radio's own filters kick in. They help filter out signals far away from those being received. Some receivers may have fairly narrow filters (they only allow a small range of frequencies in at a time) while others may have rather wide filters (they allow a large range of frequencies in at a time). Band-specific radios generally have the narrow filters while wide-band radios, like scanners, often have the wide filters.