I read the comments of "ridgescan" who then credited the comment he read by ""wise871" on the advantages of using an FM trap and so I thought I would try it. I recently got a Bearcat 15X scanner and one of my ardent long-term hobbies besides amateur radio [K2AJL] is flight simulation which has come a LONG way in its sophistication but my point here is that through the flight sim hobby, I naturally like to listen to the "real McCoy", as they say, on the airband but even with an external and dedicated airband antenna, the number of scanner lock-outs was right up there because all that was present was RF hash on far too many airband frequencies. To make a long story short, I installed a $4.99 [plus tax] Radio Shack FM trap [with the necessary BNC adapter] and the difference was absolutely amazing! Amazing! Then, [sort of like the old classic axiom, you know, "... when all else fails, read the manual ... " only in this case I used the internet as the "manual", anyway, when I did some research on FM interference, you learn at once that it's hardly limited to 'just' powerful FM broadcast station interference on VHF but an even worse culprit are paging systems a la mode and a veritable laundry list of FM wireless gizmos which can greatly interfere or literally knock-out airband VHF frequency reception to include airband TRACON repeaters.
One last word -- there are, of course, better [read: pricier] FM traps, some even adjustable, but I would recommend first going the inexpensive way and try the $4.99 RS FM trap if ONLY to see the difference and if you 'do' see a decided difference in reception [as I did and was literally amazed at the difference and, BTW, of my former =18= purposely locked out airband frequencies, NONE are presently locked out after the FM trap was installed!], then you can look around, if even needed, for the better quality or indeed tunable or, shall we say, more professionally engineered FM traps ... and the decidedly extra cost therein. For me, $5 did the trick. Doc Tony, K2AJL
