Thanks for the note, Dick...
Bigger PINs are available but the cost goes up pretty sharply if one tries
to do it "brute force." My preference is to protect and match - like your
idea at the end. I use multiple L networks, one for each band where my
antenna SWR goes much above 1.5:1 - packaged in a 3" deep chassis and
switched in automatically under control of a DAS antenna selector tied to
the 87A.
Some day Alpha/Power may seriously attack a digitally-controlled matching
system to handle up to ~5:1 VSWR across the HF range, as quite a few
commercial/industrial applications exist. The challenge is to keep the
reliability very high and the cost much lower than that of the amp!
73, Dick W0ID
-----Original Message-----
From: Dick Green [SMTP:dick.green@valley.net]
Sent: Friday, March 19, 1999 11:12 PM
To: Richard W. Ehrhorn; amps@contesting.com
Subject: Re: [AMPS] Alpha 89 SWR protection
> Load SWR protection is generally similar in both the ALPHA 87A and 89.
Both
> respond to reflected power, rather than actual SWR, primarily in order to
> protect the T/R PIN diodes from abnormally high RF voltages which can
> result from high Prefl/SWR. They are programmed to trip the amp to
standby
> when Prefl exceeds about 250W, which corresponds to about 2:1 SWR at 2.2
kW
> Pfwd, and to 3:1 SWR at 1 kW Pfwd.
Ah. Thanks, Dick. After four years of owning one, I finally understand the
reason for the 250W reflected power trip used in the 87A. I never thought
about the PIN diodes! It's not been a problem for me because I prefer to
work with well-matched antennas and have them for all but one band segment:
75M phone (a place I usually avoid anyway.) The subject came up when I
dragged out my old SB-221 to serve as a temporary amp for my second contest
rig, and "rediscovered" during ARRL DX Phone that the old amp has no
problem
tuning into the 3:1 load presented by my 80M inverted vee operating on 75M
phone. In considering a legal limit replacement for the SB-221, I was
thinking it would be nice to have that feature on one of my amps for the
rare DX contact on 75M or the occasional phone contest.
By the way, this brings up an interesting note about comparison shopping
for
amps. I had a check mark in the "Matches 3:1" column for the AL-1200
because
a number of owners reported no problem with such loads. But after receiving
your reply, I checked the specs on the Ameritron's PIN diode QSK-5PC add-on
and discovered that the CW limit is about 1.5KW into a SWR of 1.5:1 or
less.
At 3:1, the limit is 750W. No doubt, the reports I received were from
non-QSK users. The recommended limits are about 25% lower than the limits
you computed for the PIN diodes in the 87A and 89. Also, I didn't see any
mention of protection circuits for the QSK-5PC PIN diodes. I'm speculating,
but perhaps the lower SWR limits are intentionally conservative to
compensate for the lack of protection.
The reflected power trip in the 87A and 89 have been the source of the only
real complaints I have ever heard about these amps. I'm sure you would sell
lots more if they didn't have that particular feature (there are many
mismatched loads out there -- especially 80M vees and dipoles!) Are there
PIN diodes on the market that can tolerate more reflected power, or is a
lower tolerance for mismatched loads the tradeoff we have to accept for the
quiet and long-life offered by PIN diode QSK? I've used vacuum relay QSK
and
PIN diode QSK, and while both work, I really prefer the PIN diodes.
Hmmm... instead of finding an amp to match the antenna, maybe it would be
wiser just to pop a relay-controlled match on that vee...
73, Dick, WC1M
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