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G8MNY > TECH 18.07.04 19:36l 67 Lines 3058 Bytes #999 (0) @ WW
BID : 1752_GB7CIP
Read: GUEST
Subj: Non relay instant RF Switching
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0BEL<IK6PYS<W4JAX<VK4TUB<GB7YKS<GB7YFS<
GB7CIP
Sent: 040718/1826Z @:GB7CIP.#32.GBR.EU #:1752 [Caterham] $:1752_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Apr 04)
Coax lines to do the RF switching. The advantages are instant switching no
power wasting DC control circuits.
SERIES DIODES
RIG ÄÂÄÄÄ´<ÃÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄ PA ÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄ´<ÃÄÄÄÂÄÄÄ AERIAL
³ ³ ³ ³
ÃÄÄÄ´>ÃÄÄÄÙ ÀÄÄÄ´>ÃÄÄÄ´
ÀÄ=================ÄÄÄÂÄÄÄÂÄÄÄ=================ÄÙ
ÄÄÄÄÄÄÙ ¬ wave coax ³ _³_ _³_ ³ ¬ wave coax ÀÄÄÄÄÄÄÄ
electrical ³ /_\ \_/ ³ electrical
ÀÄÄÄÁÄÄÄÁÄÄÄÙ
SHORTING DIODES
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 ¬ 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!
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 & 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, & on Tx there will be small levels of odd harmonics generated.
L.P.F. The Tx harmonics & 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.
PA _________()))__________ Aerial
SWITCHING ³ L ³
C === C ===
³ ³
ÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄ
For 2M C=23pF & L=1-3 turns 1cm dia to give a SWR 1:1 match into a good load.
1/4 WAVE PASS. Further improvement can be made with a electrical ¬ wave of coax
teed to the aerial & shorted at the other end. This will reduce even harmonics
from the Tx & reduce some of the out of band crossmod signals on Rx. It also
provides a jolly good DC short to aerial static!
FROM ______________________ Aerial
FILTER ³ ___________
(')___________)<-short
³ Shorted
³ ¬ Wave
ÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
Start with slightly longer piece of coax than a ¬ wave x coax Velocity Factor.
eg 48cm. Tee this across the aerial socket & leave open circuit. With a good
aerial & 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 cut into the inner
insulation & solder up a short circuit.
/QSL
73 De John, G8MNY @ GB7CIP
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