What's commonly done with VHF masthead preamps and such, and I've used these
for decades, is to simply install electronic sequencing in the shack and use
control-line keying (not RF sensed keying). ARR used to sell a sequencer
for this, maybe they still do; but the circuit's been in the Handbook since
the 1970's, and of course designing one's own sequencer isn't difficult. My
4-stage sequencer uses sealed, small, mercury-wetted relays that switch
quickly, have no contact bounce, and are good for millions of operations;
they cost about $10 each. When I push the footswitch or mike PTT bar, the
first thing to switch is the preamp, from RX to bypass; the second thing to
key is the power amplifier T-R relay, from RX to TX; the next thing is the
power amp's bias key line to switch it from cutoff to operate; the last
thing is the exciter, to produce RF. Releasing the switch makes them unkey
in reverse order. The sequence time needs to be more than the relay contact
settling time; I use 30 mS. So, pressing the button causes four relays to
key in sequence, taking up about a tenth of a second; releasing it causes
them to unkey in reverse sequence, also taking up about a tenth of a second.
I built my sequencers per plans from Chip, N6CA, that appeared in the
Handbook maybe 20 years ago. Still work fine, and I use one on each VHF
"There is no reason anyone would want a computer in their home." - Ken
Olson, Chairman & Founder, Digital Equipment Corporation (1977)
> -----Original Message-----
> From: Tom Rauch [SMTP:firstname.lastname@example.org]
> Sent: Tuesday, October 05, 2004 10:10 AM
> To: Jim Brown; Tower Talk List
> Subject: [BULK] - Re: [TowerTalk] ARR Preamp Failures
> > So the question is, is this a common problem? Am I doing
> anything wrong? Could I do
> > anything better?
> This is a common problem. The problem is the system
> configuration requires the radio output RF before the preamp
> switches out of line.
> This means the preamp has to handle nearly the full output
> power of the radio or any amplifier BEFORE the preamp relay
> drops and bypasses the preamp. If they did anything like add
> a diode across the relay or didn't do there homework in the
> switching system, it can be even worse!
> You need to switch the preamp out of line before
> transmitting and switch it back in line after a safe delay.
> RF detected circuits can't do this.
> In some designs, I had to add an intentional MMIC buffer and
> attenuator pad to allow RF switching the preamp out. The
> MMIC is rugged, and a pad of several dB limits the power
> applied to protection diodes and the GAsFET drain. It is a
> real PITA thing to design a preamp that won't fail from
> being banged with high power RF while waiting for the relay
> to transfer.
> 73 Tom
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
> Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
> any questions and ask for Sherman, W2FLA.
> TowerTalk mailing list
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
TowerTalk mailing list