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May 05, 2014 - 13:29
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 Base frequency (how to find)
(By Tigger)
To work out a base frequency for a system using a logical channel plan
Scan, and find voice channels that are using MPT. Run TrunkView, feed it with audio from a voice channel and look at the 'Call clear-down' messages.

Hopefully you will see something like:

09:18 - CLEAR| Call clear-down                | Ch: 787  Freq: 000.0000 MHz
             |   Return to Control Ch: 895    |                            
or

09:18 - MAINT| Call maintenance message       | (021)  Org: 1425  Oper: 6
             |   Ch: 787   Freq: 000.0000 MHz |                          

From that and your scanner readout you will know:

1. the frequency  (voice frequency 180.6375 MHz assumed here)
2. the channel number (787)
3. the control channel number (895)

The 'MAINT' message is not always transmitted.


Calculate base frequency
Base frequency = Voice frequency - (channel x step)

Almost all European systems use 12.5 kHz channel steps, so the above example works out:

Base frequency (channel 0) = 180.6375 - (787 x 0.0125) MHz
= 180.6375 - (9.8375)
= 170.8000

So now you know the base frequency: 170.8000 MHz.


From the more useful 'Call clear-down' message you would also know that:

Control Channel frequency = Base frequency + (channel x step)
= 170.8000 + (895 x 0.0125)
= 170.8000 + (11.1875)
= 181.9875

As you can see, the above (real) example is a Band 3 system with standard numbering (as used by Fleetcomm in the UK).

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