[amsat-bb] Re: Question on layout for new preamps

i8cvs domenico.i8cvs at tin.it
Sat Apr 30 21:48:18 PDT 2011

Hi Mike, KB8ZGL

A sequencers provides that preamp switchover occurs before power
amplifier turn on by first removing the preamplifier control voltage and
then providing a relay closure to key the PTT line of your linear amplifier.
This insures that hot switching which could damage the preamp relay
contacts or destroy the GaAsFET does not occur.

In other words when you are receiving the antenna is connected to the
preamplifier input.

When you push the PTT to transmit first of all the antenna must be
switched to the TX line and only then after a 1/2 second delay time
the transceiver and the amplifier must go ON

Doing that no sparks of RF can occur on preamplifier and linear
amplifier relays contacts during the sequence RX to TX switching
and TX to RX switching.

The above delay time of about 1/2 second allow the antenna and
amplifier relays to switch ON and OFF in about 20 milliseconds
without RF current applied to the contacts.

Without the delay time provided by the sequencer the RF current
applied to relay contacts during closing and opening will burn
the relay contacts itself and will destroy the GaAsFET preamplifier.


A description to fully understand how to build a good sequencer how it
works and how it must be connected to antenna- preamplifier- driver
and power amplifier has been published in THE ARRL HANDBOOK
for Radio Amateurs 1991 by Chip Angle N6CA Chapter 31
" TR Time-Delay Generator " pages 9 to 12

I have built it and I am using it with great results to switch the antenna
from my GaAsFET preamplifier to a 600 watt power amplifier for
1296 MHz

73" de

i8CVS Domenico

----- Original Message -----
From: "Michael J. Wolthuis" <wolthui3 at msu.edu>
To: <amsat-bb at amsat.org>
Sent: Sunday, May 01, 2011 2:45 AM
Subject: [amsat-bb] Question on layout for new preamps

> I am adding UHF and VHF preamps to my Satellite setup and I am trying to
> make sure I know the right order of components...
> Here is what I think is right for VHF:
> FT-847 --> VHF AMP  -->  SWR Meter --> SEQUENCER --> PREAMP --> Antenna
> Is that right or does the SEQUENCER go before the AMP?
> For UHF, pretty straight forward I think:
> FT-847 --> SWR Meter --> SEQUENCER --> PREAMP --> Antenna
> Note the SSB Sequencers provide power to the SSB Preamps also, which is
> why I guessed on V it went after the AMP and the SWR Meter.
> Thanks for checking it out before I wire it all up.
> Thanks,
> Mike
> kb8zgl

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