Product Manual

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20-62 VHF & UHF TRANSMITTER
Product Manual
Sea Air and Land Communications Ltd, 10 Vanadium Place, Addington, Christchurch 8024, New Zealand June 2020
Aerial Mismatch
Overview
The aerial mismatch feature of the 20-62, when enabled, allows changes to the aerial, changes
in RF output level and changes in drive to the power amplifier to be monitored. Based on the test
result performed on each transmission an error message can be returned, along with control over
the error output.
This feature allows reporting when an aerial is disconnected or damaged. If a problem is
encountered along with error reporting the power is reduced to a level that is not likely to
damage the transmitter. The feature must be self-calibrated for each installation before use.
Operation
The feature can only be configured used serial commands.
Serial format: _MISMATCH_CONFIG=E,TTT
where:
E ‘1’ when the feature is enabled, ‘0’ if the feature is disabled.
TTT Tolerance (decimal value between 0 and 255). The default value is 50, which should
yield good results in most cases
e.g. _MISMATCH_CONFIG=1,50<CR>
Calibration
The feature requires calibration before first time use. To do this:
1. Connect up the system and aerial.
2. issue the command _MISMATCH_CONFIG=1
3. Confirm the 20-62 responds with “OK”
4. 20-62 will auto calibrate on next transmission.
5. Test by transmitting a message, eg “CA1234560 1 test” (Salcom protocol). The 20-62 will
respond with something like “RF:415:SP:762 CAL DONE”.
6. Internally if the output power increases by the tolerance configured (50 by default) or the
RF power module drive varies by 50, then an error is produced.
7. Test the calibrated system by:
• Connecting a different aerial or
• Disconnecting the aerial.
When a mismatch is detected, an output similar to below will be produced:
390-415 : 874-762
ER19 RF_LEVEL
In this example “390-415” shows the recorded RF output level and the calibrated value. The
difference is only 25 (415-390) and not enough to trigger a mismatch alarm. The second values
shown (874-762) shows the recorded power module drive setting and the calibrated setting. In
this case the difference is 112 (874-762). Since this is above 50, an error is produced.