12
Very roughly the Diplexer works as follows;
In order that the in-band radios can operate side by side without de-sensing each other, a device called a diplexer (or duplexer) can be used. This is basi-
cally two independent filters. One is tuned to the frequency of one radio and the other to the frequency of the second radio. Diplexers are not required when
cross-band re-broadcast is used.
In the present case, one side is always tuned to 147.350 MHz as this is the common uplink channel used by all of us at this time. This is the ‘Low’ side
of the diplexer. The other radio would be programmed with your Team Working Channel, the ‘High’ side. In practice, this works a little like the left and right
channels on a stereo amplifier. The two radio frequencies are kept apart and not allowed to interfere with each other. The price you pay for this is that the
tuning of the diplexer is both precise and narrow. If it were not then they wouldn’t work!
On the 147.350 MHz side, this isn’t a problem as only this channel is required, but on the TWC side, ideally we would like to be able to use the rebroadcast
on all TWC’s. Sadly, due to the narrow tuning, this isn’t possible and the best we can achieve is that if your TWC is an ‘A’ channel as in 63a or 156.175 MHz,
then you can use your re-broadcast device on any other ‘A’ channel. The same goes for the ‘B’ channels as in 85b or 161.875 MHz. These teams can have
their rebro operate on any ‘B’ channel. When operated away from the channel for which it’s been tuned, the diplexer will degrade the performance of the
radios.
The only channel available to both types of rebro is 158.650 MHz or 53a. This is approximately mid-way between both sets of channels and would be avail-
able as a common channel should this ever be needed. It’s still not ideal and the performance of your system will inevitably be degraded.
Simultaneous receive and
transmit From a single
aerial. The Diplexer
effectively ‘steers’ each
frequency to the correct
radio
Filter tuned to
the TWC
Aerial socket of
the TWC radio
Filter tuned to
147.350 MHz
Aerial socket of
the 147.350 MHz
radio
Electrical screening between the
two sets of coils
To Next Page
Loading...
