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G8MNY > TECH 01.04.04 18:55l 52 Lines 2418 Bytes #999 (0) @ WW
BID : 51033_GB7CIP
Read: DB0FHN GUEST OE7FMI
Subj: Non relay instant RF Switching
Path: DB0FHN<DB0FOR<DB0SIF<DB0EA<DB0ACC<DB0GOS<ON0AR<ON0AR<GB7CIP
Sent: 040401/1150Z @:GB7CIP.#32.GBR.EU #:51033 [Caterham] $:51033_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Graphics updated 11/03)
Coax lines to do the RF switching. The advantages are instant switching no
power wasting DC control circuits.
SERIES DIODES
RIG ÄÂÄÄÄ´<ÃÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄ- PA ÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄ´<ÃÄÄÄÂÄÄÄ AERIAL
³ ³ ³ ³
ÃÄÄÄ´>ÃÄÄÄÙ ÀÄÄÄ´>ÃÄÄÄ´
ÀÄ=================ÄÄÄÂÄÄÄÂÄÄ-=================-Ù
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!
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 large signals can crossmod on Rx the due to the
diodes, & on Tx there will be small levels of odd harmonics generated. These
can be filtered with a pie section filter before the aerial.
An electrical 1/4 wave of coax teed to the aerial & shorted at the other end
will reduce even harmonics in Tx & reduce some of the out of band crossmoding
signals on Rx. It also provides a jolly good DC short to aerial static!
PA _________()))__________ Aerial
SWITCHING ³ L ³ ³ ________
C === C === (')________)<-short
³ ³ ³ Shorted ¬
ÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÁÄ Wave Coax
For 2M C=23pF & L=1-3 turns 1cm dia to match 1:1 on load.
Coax length can be teed across the aerial socket & left open circuit, cut coax
length to null out Rx signals, then shorted out to make the even harmonic
traps.
/QSL
73 De John, G8MNY @ GB7CIP
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