Why would one be able to hear police, fire, and EMS radio on an OKI 900?

[Amok]

Specifically, around analog cell phone channels 900-930?

[amazing_ned]

I'm assuming those frequencies are being used with a trunking system, I don't know the frequencies of those channels so I can't tell you for sure.

Weirder than that, the other night I was scanning and my radio was in the 2m ham band, and I caught a police conversation (ie between officer and dispatch)...wtf?

[immabadspellor_]

Originally, the AMPS cell system was supposed to have been expanded from 666 channels to 1000 channels.  The FCC reallocated channels 800-990 to public service and then started at channel 1 frequency-wise and moved down in 30khz channels.  These are channels 991 to 1023 resulting in the current 832 channel system of today.  The OKI 900 is of the era where phones were still being made to be compatible with a future 1000 channel before the 832 channel compromise was developed so when you listen to channels 800-990, you are really listening to 849 MHz - 854.7 MHz in 30KHz steps per channel so many will be off frequency of the 12.5KHz step standard in that range.  Also public service tends to use a narrower bandwidth than cell phones do so the audio may sound quiet.

For the guy scanning 2m, this was most likely intermod.  This is very common in an area with a lot of radio activity, such as large metropolitan areas.  When two stations in close proximity transmit simultaneously, their signals mix and produce intermod.  For example, lets say a pager is transmitting on 151MHz and a copper is transmitting on 155MHz really close to the pager tower.  The pager is spewing its usual crap and the cop starts calling in for a doughnut order at the same time.  The difference of the two signals is 4MHz so the signals mix and produce intermod at 147MHz (amongst others).  147MHz happens to be the output of your favourite 2m repeater.  You're close by to the cop and the pager and you hear on the repeater's output a mix of pager tones and some cop jabbering about donuts at the same time. 

Scanners have very poor intermod rejection as they are made to listen to a very wide range of frequencies.  This limits what kind of filters they can incorporate as those filters will inevitably affect sensitivity on other portions of its receive area.  Expensive receivers will have different filter banks that get selected depending on which frequency range you're scanning that'll give you monobander performance in a wide coverage receiver.  These aren't your Bearcats though, this would be like a $1000 and up AOR receiver. 

[amazing_ned]

Ahh that's right, thanks a lot for the explanation. I forgot all about intermod, but that would definitely make sense. Great explanation too.

[Amok]

Ditto, that explained a lot.  It would be nice to use this as a sort of ghetto scanner.  Just like you said, the audio is really quiet, and since the OKI doesn't have squelch, I can't just hook my OKI up to some speakers, since if I made them loud enough to hear the police then the static would be deafening.  If I could hook my OKI up to my PC and perhaps use some sort of squelch program which mutes my line-in if there's static, it would work pretty well.

Also, I'm getting a bit O/T, but does anyone know what good SMA-jack antennas would be good for use with my OKI?  Preferably one that works for its entire range of channels and not just for the police.