Message 26681 in TECH.WW
From: G8MNY
Date: Mon, 27 Jul 26 11:35:00 Z
Newsgroups: TECH.WW
Subject: Non relay instant RF Switching
Message-ID: <414_GB7CIP>
Path: N2MH4|DK0WUE|PI8ZTM|CX2SA|ED1ZAC|GB7CIP
R:260727/1144Z 34428@N2MH4.#SENC.NC.USA.NOAM LinBPQ6.0.24
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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Oct 05)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Coax lines to do the RF switching. The advantages are instant switching no
power wasting DC control circuits.
SHORTING DIODES
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 short circuited by the
both.
It can also be used with a PA & preamp, if it is non linear FM PA, and possibly
even a low gain linear/preamp combination by adding a 2nd pair of diodes.
SHORTING DIODES
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. e.g. +/- 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/Octave above the cut off frequency.
PA _________()))__________ Aerial
C === === C
For 2M C=23pF & L=1-3 turns 1cm dia to give a SWR 1:1 match into a good load.
teed to the aerial socket 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!
e.g. 48cm. Tee this across the aerial socket and leave it 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 its final position
(around the PA etc). Then without shortening the coax make cut into the inner
insulation and solder up a short circuit.
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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