The main limitation when trying to receive on the same band with another
transmitter operating is the wideband transmitted noise it creates. This
noise is a combination of phase noise from the synthesizer and amplified
noise from the last mixer. Filtering beyond the last mixer is limited to
low pass units to reduce the transmitted harmonics and has no effect on the
inband noise.
If a transmitter was dedicated to running stations on a specific band and
within some small frequency range (like the low end), a crystal filter
could be inserted in the transmit path at an appropriate stage. Outside
the pass band of the filter, the noise products would be greatly reduced or
eliminated. This would allow a multiplier station to hear well outside the
filter passband. Some antenna separation would still be required to
prevent overload of the multiplier receiver front end.
On many radios there is a convenient place to introduce a 50 ohm filter.
The transverter output is usually close to the output level which would not
overload a crystal filter. A method of getting the filtered signal back
into the transmitter would have to be added to the radio. Even if the
level is a bit high, an attenuator would bring it down to the proper range
and a low noise amplifier could follow the filter. An in/out relay could
return the radio to normal operation.
George
w2vjn@rosenet.net
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