Is anyone listening

Voice Card  -  Volume 23  -  Robert Card Number 1  -  Sun, Feb 9, 1992 3:27 PM

Something to keep in mind if you use a cordless or cellular phone.


The new "Cellular" mobile (and portable) telephone service can be a very interesting source of entertainment. For some reason, most people with mobile telephones don't seem to realize that their conversations over these devices are anything but private! You can hear some really juicy stuff on mobile telephone frequencies.

Due to the nature of the cellular service, particularly the number of frequencies and very low power (and therefore short range) of cellular mobile phones, hobbyists are going to find it a little more difficult to catch cellular conversations than the old-style mobile telephones. For one thing, cellular frequencies occupy a whole new part of the spectrum not readily covered by most scanners. And, even though we now have at least one manufacturer producing scanners which cover the 800MHz range, there are 1,332 frequencies allocated for cellular use!

Well, that's the bad news... now for the good. Only a very small fraction of those 1,332 frequencies need to be monitored to catch the action. In fact, you might be able to get your "fill" on as few as five frequencies! That's right, only 5!

How can this be?, you ask. Well, read on. To understand how you can find happiness listening to 5 out of 1,332 (that's about three-tenths of one percent, by the way) you have to know how the cellular system works.

First, cellular frequencies are divided between two separate "systems", called "System A" and "System B", each having 312 "user channels" and 42 "control channels". Each channel uses two frequencies with mobiles transmitting on one frequency while simultaneously receiving on the other. System A is allocated to existing "wire-line" service companies (e.g., your old "Bell" telephone company, in most cases) and System B for the "competition", or more specifically, non-wire common carriers.

Monitoring the mobiles' transmitting frequencies is not very practical because the mobile transmitters run only 3 watts and have very limited range. Besides, you would only hear one side of the conversation that way. The base transmitters, however, run about 12 watts with antennas about 150 feet high and transmit both sides of the conversation.

So what?, you say. That's still some 312 (or 624 for both systems) frequencies to monitor. Not so!, says cellular guru! Here's why... Even if you have both systems A and B operating in your area, not every "cell" operates on all available channels. In fact, most cells will be transmitting user conversations on less than 50 frequencies.

Now here's where it starts getting a little complicated, but hang in there! Each cell-site may have as few as 5 active channels. The base transceivers are frequency-synthesized and can be configured to operate on any of the 333 channels allocated to either system. Additional transceivers are added in multiples of 16 as demand in each cell grows. Given the geographical dimensions of the actual "cells", and your particular location, there may be anywhere from one to five cell-sites (more, closer to big cities) that are actually within your receiving range. Your own particular receiving range, of course, will depend upon your antenna, equipment, elevation, etc., so you'll have 'o experiment to find out just how many you actually can hear.

Okay, so now we've got it down to something like 25 to 125 frequencies! Still too many channels? Read on! Not too many of us are going to want a staple diet of telephone conversations. And based on the algorithm used to automatically select channels in each cell, plus a few other factors, here's how you can reduce the number to a level that "fits" your particular interests.

First thing to do is to purchase that new scanner with the 800 MHz coverage, or, get a converter to allow reception of those frequencies on your old one. Next, you need to spend some time in the "search" mode to find exactly which of those estimated 25-125 frequencies are actually in use within your own receiving range.

The range of frequencies you will want to search are:

System A Base:   870.030 - 879.360 MHz
System B Base:   880.650 - 889.980 MHz

From here on a lot of the "work" will be up to you. If your scanner has a search mode, it probably also has an activity or call counter on each memory channel as well. Once you have a list of active cellular frequencies, program as many as you can into your scanner's memory channels and let it count the activity on those channels. Once the count reaches maximum (most scanners' counters only go to 99) stop the monitoring and write down the count totals beside the appropriate frequencies on the list you made earlier.

Then, load up the next "batch" of frequencies and repeat the procedure until you have checked all the frequencies on your list. A review of the list with call counts added should reveal just what kind of activity you can expect. Of course, the more detailed your analysis, i.e., the more searches and activity monitoring you do, the more representative will be your results.

Anyway, once you have the list, complete with activity, simply decide which of those frequencies you should program into your scanner based on activity, and, how much "telephone soap opera dialog" you think you can stand.

Finally, if you've just got to know it all, here's the entire list of cellular frequencies (step between channels is 30 KHz):

System A Mobile 825.03 - 834.36 MHz
System A Control834.39 - 835.02 MHz
System B Control835.05 - 835.62 MHz
System B Mobile835.65 - 844.98 MHz
System A Base870.03 - 879.36 MHz
System A Control879.39 - 879.99 MHz
System B Control880.02 - 880.62 MHz
System B Base880.65 - 889.98 MHz