OpenBCM V1.06 (Linux)

Packet Radio Mailbox


[C.D.G. OpenBCM]

 Login: GUEST

G8MNY  > TECH     24.06.19 11:00l 91 Lines 4117 Bytes #7 (0) @ WW
BID : 9564_GB7CIP
Subj: Non relay instant RF Switching
Sent: 190624/0854Z @:GB7CIP.#32.GBR.EURO #:9564 [Caterham Surrey GBR] $:9564_GB

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.

                             ³\                    4 or 6 
     SERIES DIODES           ³ `\                PARALLELED
     ³         ³             ³  /'               ³         ³
     ÃÄÄÄ´>ÃÄÄÄÙ             ³/'                 ÀÄÄÄ´>ÃÄÄÄ´
     ³                                                     ³
 ÄÄÄÄÄÙ    ¬ wave coax    ³  _³_ _³_  ³   ¬ wave coax     ÀÄÄÄÄÄ
           electrical     ³  /_\ \_/  ³    electrical
                         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 short circuited by the diodes
a ¬ wave from both the Rig & aerial, therefore presenting high Z to both.

It can also be used with a PA & preamp, if it is non linear FM PA, & possibly
even a low gain linear/preamp combination by adding a 2nd pair of diodes.

     SERIES DIODES             ³ `\
     ³         ³               ³  /'                ³         ³
     ÃÄÄÄ´>ÃÄÄÄÙ               ³/'                  ÀÄÄÄ´>ÃÄÄÄ´
     ³                             /³                         ³
     ³                           /' ³                         ³
 ÄÄÄÄÄÙ  ¬ wave coax  ³ _³_ _³_ `\  ³ _³_ _³_ ³ ¬ wave coax  ÀÄÄÄÄÄ
           electrical ³ /_\ \_/   `\³ /_\ \_/ ³   electrical
                           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 & 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!

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/Octave above the cut off frequency.

   PA     _________()))__________ Aerial
SWITCHING        ³   L   ³
              C ===     === C
                 ³       ³

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 socket & 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!

FILTER   ______________ ³   Socket
             Shorted  ³
              ¬ Wave  ³

Start with slightly longer piece of coax than a ¬ wave x coax Velocity Factor.
e.g. 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 its final position (around
the PA etc). Then without shortening the coax make cut into the inner
insulation & solder up a short circuit.

Why Don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP

Read previous mail | Read next mail

 27.06.2019 12:44:56lGo Back Go up