Third, many times when you add a preamp, the sensitivity of the receiver gets worse, and intermod pops up everywhere. What is the reason for this?
When you add a preamp, especially a broadband one, all signals from police, firefighters, etc. get amplified at the same time. If your receiver created intermod with out-of-band signals stronger than -50 dBm then after adding the preamp with 20 dB of gain, all signals of -70 dBm will now be amplified to -50 dBm. You may have signals with strengths of -30 dBm before adding the preamp. These will now be -10 dBm; very strong signals indeed. All these new strong signals will now start mixing in your receiver and creating intermod that never existed before the preamp. As well, if signals are strong enough they will desense your receiver without causing intermod. The only answer in a case where adding a preamp makes things worse is to reduce the strength of the strong, out-of-band, signals with filtering. In summary, a good gallium arsenide (GASFET) preamp, if used with good low loss RF filters will almost always increase your sensitivity.
