Non-relay instant RF Switching
By G8MNY

This is the standard circuit. RF Diodes (1N4184 or better) connect the PA when the drive is > 600mV and drive the aerial through diodes as well, these may need paralleled diodes for > 10W.

In Tx, the Rx path is S/C by diodes a quarter wave from both the Rig and aerial, therefore presenting high Z to both. If the S/C diodes are doubled to 2 pairs then a preamp can be added between them!

To find the electrical 1/4 wave length in your coax see 1/4 WAVE PASS below.

Although this design is really for FM/CW, the power lost in the diodes as a % of the total signal is very small. eg +/- 1V Peak in say 33V peak for a 10W PA or 70V peak for a 50W PA signal. This will cause spreading and crossover type distortion on SSB modes, but this additional distortion may be very small compared to the typical 6V of distortion or -22dB 3rd order IP of a 50W PA!

FILTERING IMPROVEMENTS
Some disadvantages are, that a large signal can crossmod on Rx due to the diodes, and on Tx there will be small levels of odd harmonics generated.
L.P.F. The Tx harmonics and higher out-of-band Rx frequencies can be filtered with a low pass filter "pie section filter" before the aerial. This will attenuate signals @ 18dB/O above the cut off frequency. For 2M C=23pF and L=1-3 turns 1cm diameter to give a SWR 1:1 match into a good load.

1/4 WAVE PASS.
Further improvement can be made with a electrical quarter wave of coax teed to the aerial and shorted at the other end. This will reduce even harmonics from the Tx and reduce some of the out of band crossmod signals on Rx. It also provides a jolly good DC short to aerial static!

Start with slightly longer piece of coax than a quarter wave x coax Velocity Factor - eg 48cm. Tee this across the aerial socket and leave open circuit. With a good aerial and Rx connected, cut down the coax length a bit at a time to null out a Rx signal. Check the null is good with the coax in it final position (around the PA etc). Then, without shortening the coax, make a cut into the inner insulation and solder up a short circuit.

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