If you click on a plane on the Skyview map it will report the power of the incoming signal called RSSI (relative signal strength indicator). The value go from 0 dB (strongest signal) to about -40dB (weakest signal). Planes with a signal level from 0 to -2 dB are overloading the receiver and are hard to decode. Signal levels lower than -32dB are usually too weak to decode.
You can lower the signal strength (gain setting) until you see the strongest signal (closest planes) with a RSSI of around -2dB.
If the antenna is inside the building you have a reduction of about 6 to 20dB in signal power due to the walls. You can use a very high gain setting (40-50) and see very close by planes. You need the extra gain to counter the effects of the walls.
If the antenna is outside then a low gain (around 20-30) and you can see planes within a km or less. Incoming Signals higher than 0dB will not be decoded and you will not see anything from these planes. You need to lower the gain so the close by signals are not overloading the receiver.
so you can use 49.6 or -10 for aircraft 5 nautical miles or more ?
btw what about aircraft being directly overhead > FL100 ?
The receiver sweet spot is where it can decodes messages. This corresponds to incoming signal strength (RSSI) from about -2dB to about -35dB. If a plane signal is outside that range it can’t be decoded. Some people refer to this as the receiver donut effect (the receiver decodable messages look like a donut shape).
The gain settings move the inner and outer detection range.