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[TowerTalk] 40-2CD SWR problem

To: "Tower" <>
Subject: [TowerTalk] 40-2CD SWR problem
From: "Dick Green WC1M" <>
Date: Sun, 10 Feb 2008 20:36:46 -0500
List-post: <">>
Hi All,


I have a strange problem with my Cushcraft 40-2CD. The symptoms are:


- The first time I select the antenna and transmit, say at 50W, the SWR is
very high -- somewhere between 10:1 and infinite.

- Depending on which radio I use, sometimes if I send a dit at about 100W,
the SWR returns to normal, about 1.5:1.

- The reason it depends on the radio is that one (FT-1000D) folds back power
dramatically when SWR is high, while the other (Orion) doesn't. Usually have
to use the Orion.

- Sometimes even 100W isn't enough, and I have to transmit at slightly
higher power with a linear to clear the high SWR -- perhaps up to 200W or

- I use an Acom 2000A amp, and it never trips due to the high SWR during
this procedure, presumably because the reflected power is relatively low at

- If I keep transmitting, the SWR stays low.

- After 20-30 minutes of inactivity, high SWR returns. If I give it a shot
of power, SWR returns to normal.

- This *seems* to happen more often when the temperature is cold, but I'm
not sure there's a definite correlation.


I've experienced the  opposite of this problem: SWR low at low power and
high at high power, when I've had contaminated or damaged coax or a carbon
path inside a connector. But the symptom here suggests that something is
shorting the circuit until just enough power is sent down the line to
interrupt and/or vaporize it. I've thought maybe there's water in the cable,
but that seems unlikely -- the waterproofing is good. I don't think it's in
the switching relays (SO2R box.) The only explanation I can imagine is that
something is shorting the two tubing halves inside the ceramic insulator of
the driven element, like water (turning to ice), or frozen carbon or
something. Maybe insect bodies? Before I put the antenna up, I reinforced
the center tubes with smaller diameter tubes. I thought I was careful not to
let them extend beyond the outer tubes inside the insulator, but maybe I
inadvertently reduced the size of the gap between the elements, and it's so
close that frozen moisture mixed with carbon is making a circuit t between
the element halves.


I've seen something like this problem once before, on a delta loop. High SWR
and dead receive, hit it with a quick dit of low power and everything is OK.
But it never took high power. I wasn't able to track that one down, and it
seems to have disappeared after I replaced the wire (insulated before and
after) and restrung the antenna (same balun, same coax, same relays.)


Has anybody seen symptoms like this? Any theories?


73, Dick WC1M




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