What are your thoughts on this? Is it true?
If the sensitivity degrades due to overload it will be instant. I haven't been able to use any portable scanner successfully with a car roof antenna, they have all run into problems. 2-way radios have worked fine though and also most base/mobile scanners as I try to buy and use only the ones that have good RF performance.
There's a lot of RF interference at street level from all kinds of equipment that you don't get higher up at a buildings roof level.
But using an antenna inside a vehicle are not advisable either as you get interference from you car stereo and all the microprocessors that controls a car these days. Best are probably a car roof antenna to have the metallic roof to block most of the internal signals and then perhaps use a low gain antenna, or use some sort of small attenuation in the coax, to not overload the receiver.
One thing to remember are that SDS scanners use a receiver that has a lot of internal mixing products that lets you hear one transmission at 20-30 other frequencies. To avoid listening to those interferencies you can use NAC, CTCSS and other filters but they will still be there and disrupt conversations and block others that are evident when you have another scanner model scanning the exact same channels.
/Ubbe
