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TUCoPS :: Radio :: cellconv.txt

Building A Cellular Converter




SCANNER CONVERTERS FOR CELLULAR TELEPHONE
Written for P-80 Systems by THE RESEARCHER

     This article is presented for information only. The new Electronic
     Communications Privacy Act makes it illegal to monitor cellular
     telephone calls.

     A UHF TV tuner can be used as a converter to listen to cellular
     telephone calls. Salvage a UHF tuner from and old TV set. Connect
     it to a power supply. Typical voltage requirements are 12 - 25
     volts. If the set is still working, measure the voltage before
     removing the tuner. Connect the output cable from the tuner to the
     external antenna input of a scanner or tunable monitor. Tune the
     scanner or monitor to a frequency between 41 and 46 MHz which is
     the IF output of the tuner. If you are within a few miles of a base
     station, a pair of test leads clipped to the antenna terminals of
     the tuner will serve as an antenna. Turn off the squelch on the
     scanner or monitor and carefully tune through UHF channels 70 - 83.

     It is easier to use a tunable monitor than a scanner for this
     application because the monitor allows you to compensate for drift
     in the tuner. Either will provide an adequate means of checking out
     the cellular activity in your area.

     There are several crystal controlled converters available which
     will convert cellular frequencies to the UHF range of many
     scanners. There is a slight problem involved with these. The
     spacing between cellular frequencies is 30 KHz. Most scanners have
     a stepping interval of 12.5  Khz at UHF. This means that on most
     channels the frequency tuned to by the scanner will not be a
     perfect match. I have been assured by one of the converter
     manufacturers that this is not a serious problem. When using this
     type of converter, the scanner can be used in scan or search modes
     as usual.

     I wanted to find out how much discrepancy exists between the output
     of the converters and the tuning intervals of most scanners. I took
     the specs of a typical converter and put my computer to work doing
     the calculations. What follows is the result of this examination.
     Listed are the cellular base frequencies followed by the
     frequencies after conversion followed by the closest tuning point
     of a scanner with a tuning interval of 12.5 KHz. Each of the base
     frequencies listed is paired with a mobile frequency located 45 MHz
     lower. The mobile frequencies are not listed.

     All 666 base frequencies were checked. Only the first 33 of these
     are listed. The pattern repeats throughout the list.

CELLULAR FREQ.  CONVERTER OUT  CLOSEST SCANNER FREQ.
-------------   -------------  --------------------
   870.030         486.030           486.0250
   870.060         486.060           486.0625
   870.090         486.090           486.0875
   870.120         486.120           486.1250
   870.150         486.150           486.1500
   870.180         486.180           486.1750
   870.210         486.210           486.2125
   870.240         486.240           486.2375
   870.270         486.270           486.2750
   870.300         486.300           486.3000
   870.330         486.330           486.3250
   870.360         486.360           486.3625
   870.390         486.390           486.3875
   870.420         486.420           486.4250
   870.450         486.450           486.4500
   870.480         486.480           486.4750
   870.510         486.510           486.5125
   870.540         486.540           486.5375
   870.570         486.570           486.5750
   870.600         486.600           486.6000
   870.630         486.630           486.6250
   870.660         486.660           486.6625
   870.690         486.690           486.6875
   870.720         486.720           486.7250
   870.750         486.750           486.7500
   870.780         486.780           486.7750
   870.810         486.810           486.8125
   870.840         486.840           486.8375
   870.870         486.870           486.8750
   870.900         486.900           486.9000
   870.930         486.930           486.9250
   870.960         486.960           486.9625
   870.990         486.990           486.9875


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