They DO have a reliability problem. Don't forget that the OFF diode has to
be back biased by at least 3 times the RF voltage at the relevant power
level: that means approximately 800V at the 1kW level and this without
taking the higher SWR's into consideration! The real rugged ones _ like
those manufactured by Chelton Microwave- are specified at up to 3000 bias
for high power operation. I wouldn't lie to have to add such a source and
think that you have also to switch it! Not so easy, believe me! I'd bet for
vacuum relays any time for QRO use!
73,
Alex
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Paul Christensen
Sent: Wednesday, April 02, 2008 5:33 PM
To: amps
Subject: Re: [Amps] Relays
> Alpha began using PIN diodes later in life, did they not?
> Any reports on that picture of reliability?
I can't comment on the failure rate statistics, but I can tell you I have a
real love-hate relationship with high-power PIN diodes. I know that they
will not hold up as well as a vacuum relay under adverse operating
conditions (e.g., certain points on the SWR circle). On the other hand, I
have an Alpha 86 with PIN diodes that has performed exceptionally well under
some pretty harsh conditions and I've not yet experience a failure.
Probably for resonant or near-resonant antennas under a 2:1 VSWR, they are
fine. And, as ling as the SWR protection circuitry is paying attention, I
am not sure why the diodes fail unless they are being operated at full power
at a VSWR of 2:1 for extended periods of time with modes like RTTY.
I like the robust design of vacuum relays, but I also like the completely
silent switching offered by PIN diodes. I do see that Alpha has just
released a PCB for the 86 that replaces the PIN diodes with a Jennings RF-1
vacuum relay. It's a 100% drop-in replacement for the existing T/R board.
Cost of the board alone is $500.00 from Alpha. I plan on ordering one to
keep on the shelf, but I have every intention of keeping the 86 in stock
condition for as long as possible.
My current Alpha amp line-up consists of a 77Dx, 70V, and 86. The 77Dx uses
a Jennings RJ-1, while the 70V uses a Jennings RF-1. Neither relay has
required replacement and the RF-1 is now 38 years old and has definitely
worked for a living.
Paul, W9AC
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