I needed to change the resonance of my 73kHz antenna tuning
arrangement to be resonant on 136kHz for cross band contacts.
The tuning arrangement here is a sort of three-stage booster,
comprising a commercial variometer ( probably for about 500kHz)
followed by the 136kHz coil then the 73kHz coil, all in series. I
felt that all I would have to do would be to short out the 73kHz
coil. I knew that the voltage at the top of the coil was high (it
will arc a 15mm gap) so I started looking for suitable relay.
In spite of good advice on the reflector from Rik and Jim (M0BMU) I
still pressed on with the original idea.
Ohm input series tuning can be used (an additional series capacitor between
RX and loading coil). This capacitor can be short-circuited to switch from 136
to 72kHz. A simple
low-voltage relais will do.
After a lot of spectacular failures the message finally sunk in. I
fitted a series receiver type airspaced capacitor between the first
stage of the coil where the RF voltage is relatively low. It is
shorted out on transmit by the high voltage relay sent to me by Andy, G4JNT.
This arrangement has the advantage that it can be tuned up accurately
for maximum signal on 136kHz, giving me at least 2 S points extra
gain compared with when it is not tuned.
--
Regards, Peter, G3LDO
<[email protected]>
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