Jonathon Jacob writes in a message to Scott Christensen
 JJ>  BTW: What are PL tones? Are they the ones that sound like 
 JJ> a  phone, or are they the older two tones (county still uses 
 JJ>  those)..
  
First a bit of history.  Motorola and General Electric (now Ericson) started 
sending low frequency tones with 2 way transmissions in the late '50s.  GE 
called them "Channel Guard" and Motorola called them "Private Line" (hence 
"PL") tones.  The idea is that each business (or government agency) on a 
frequency could choose a different tone and if all of the radios were set to 
allow audio out of the speaker only when the radio received "their" tone, you 
could only have to listen to transmissions from your company/agencies units.
  
Today, there are 2 kinds of "PL tones" - the old fashioned low frequency 
audio tones from about 55 Hz to about 250 Hz (yes there are specified 
frequencies - you can't pick a frequency "between" two of the specified 
frequencies) - and since the early '70s a "digital" form which is actually a 
slow speed serial digital waveform mixed with the audio sent from each 
transmitter.
  
Of course, some agencies still run "carrier squelch" like most scanners.
  
There are some problems with the use of PL - mostly having to do with the 
fact that a lot of users just can't be bothered to actually listen to the 
channel in carrier squelch mode so that they don't transmit when some other 
company/agency is already using the channel.
  
But there are advantages too - like a receiver is unlikely to detect it's 
tone/digital stream when there is interference.  Thus the receiver stays 
quiet and doesn't bother the operator with the noise.  (This is the problem 
that the use of PL could solve for you.)  And I have already mentioned that 
you don't have to listen to transmissions from other agencies/companies using 
the same frequency nearby.  Another thing that PL gives you is a clean ending 
of transmissions - no more "squelch tail" - unless your mike is off hook 
forcing the receiver to use carrier squelch.
  
So, most folks use some sort of PL.  All you have to do is find a scanner 
that can decode the digital or tone PL that your fire department uses and use 
that instead of carrier squelch to get rid of your mobile interference 
problems.
  
Realizing, of course, that when the channel is totally clobbered by 
interference you will probably not be able to hear the desired signal anyway.
  
ONE MORE THING...  You know of course that receiver RF stage or first mixer 
overload is the cause of most received interference?  Look for a receiver 
with a FET in the RF stage, bipolar transistors tend to overload worse.  The 
mixer should either be FET or a doubly balanced diode design.  I'm not sure 
if you can find those features in a consumer scanner, but high-end 
professional equipment like Motorola makes usually is designed that way these 
days.
 
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