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G8MNY > TECH 14.07.03 23:49l 35 Lines 1805 Bytes #999 (0) @ WW
BID : 5663_GB7CIP
Read: DB0FHN GUEST OE7FMI
Subj: Non relay instant RF Switching
Path: DB0FHN<DB0RGB<DB0MRW<DB0HOT<OK0PBX<OK0PHL<OK0PPR<OK0PPL<RZ6HXA<HA3PG<
DK0WUE<GB7FCR<GB7CRV<GB7CIP
Sent: 030714/1954Z @:GB7CIP.#32.GBR.EU #:5663 [Caterham] $:5663_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
Coax lines to do the RF switching. The advantages are instant switching no
power wasting DC control circuits. Some disadvantages are that large signals
can crossmod the Rx & there will be small levels of odd harmonics generated
that should be filtered with a pie section filter before the aerial. A 1/4 wave
of coax teed to the aerial & shorted at the other end should help reduce even
harmonics & reduce out of band crossmoding signals.
RIG -!---!<!---!---------------PA--------------!---!<!---!--------
!---!>!---! !---!>!---! To
! ! Aerial
-================-----------------================-
-----/ 1/4 wave coax \ ! ! / 1/4 wave coax \---------
(electrical) ! --- --- !
! / \ \ / !
! --- --- !
----!---!----
This is the standard circuit. RF Diodes (1N4184 or better) connect the PA when
the drive is > 600mV & drive the aerial through diodes as well, these may need
paralleled diodes for > 10W. In Tx the Rx path is S/C by diodes 1/4 wave from
both the Rig & 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!
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 & 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!
/QSL
73 De John, G8MNY @ GB7CIP
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