Gas tubes usually fail in a shorted mode, although a "mother of all
strikes" could cause them to fail open.
RF Voltage can cause what's called glow mode if the tube is turned on
by a over voltage condition and the RF power is high enough. This may be the
case here (RF Voltage could turn the gas tube on by itself too) as the device
has been out of service for a while. 10W should not cause the problem under
normal circumstances. Every time the tube fires (or goes into glow mode) the
voltage needed to cause this condition drops because of gas contamination and
the deposition of conductive materials on the envelope. However, this should
take a few years to occur. Again, since you don't see a short, the tube is
probably
good. You will need a device called a hi-pot tester to verify the tube's
condition.
The voltage associated with the RF wattage can be obtained using the formula
V=(1.414) (X) ([SQR ROOT] {Wx50}).
X is the reflection coefficient related to the VSWR
of your system. For a VSWR of 1.5:1, X=1.2.
There's a nomograph in the ARRL Handbook
relating all these properties.
Hope this helps!
73, Bob AA0CY
<AA0CY@CONTESTING.COM>
----------
From: Pete Soper[SMTP:psoper@encore.com]
Sent: Sunday, September 28, 1997 7:50 AM
To: towertalk@contesting.com
Subject: [TowerTalk] lightning suppressor problem
I have two ICE model 301 coax suppressors that have been out of service for
a while. Trying to tie my new tower feedlines into them I find that they are
transparent on receive but cause a severe mismatch on transmit. Ohming out
the components I see the gas tube is an open circuit as expected, the bleeder
resister has its marked value and the series capacitors check out too. The
toroid inductor appears to be fine. The inside of the box is pristine and
both units have been in a "ground window" box and out of the weather.
I wonder what the problem could be? With both units, swapping them in instead
of a barrel connector and transmitting 10 watts causes the SWR to jump from
1.2 to 3.5:1 (on 20m). My Autek sees no change in SWR with the suppressor
added to the circuit. This suggests that the gas tubes are firing, doesn't it?
Do gas tubes have failure modes like this? Maybe contaminated by an air or
moisture leak? Do any Tower Talkers have a source of spare parts?
While I'm at it, one quick comment about "weather proof boxes". I took an ICE
balanced line suppressor out of service after it had been strapped to a ground
rod and in the weather for a year or so. It's built in a plastic box with a
rubber gasket. It had a couple tablespoons of water in it (below the level of
the components or I guess I'd have noticed some particularly tricky tuning).
Wish I'd drilled a weep hole in it as a buddy did with his.
Regards,
Pete
KS4XG
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