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Re: [TowerTalk] 40-2CD SWR problem

To: <>, "'Tower'" <>
Subject: Re: [TowerTalk] 40-2CD SWR problem
From: "Eric - VE3GSI" <>
Date: Sun, 10 Feb 2008 21:29:40 -0500
List-post: <">>

>From your message:
> I don't think it's in the switching relays (SO2R box.)

I sort of suspect the relay box is the problem. I have seen 'lazy' relays in
non radio applications do exactly what you are describing, that is, poor
continuity until hit with a current load. It should be easy enough to
eliminate the SO2R box as the problem, try putting the antenna circuit in
question on a new port or swap ports with another antenna.  Also try
manually selecting the antenna via the SO2R box a few times before hitting
the transmit switch and see if there is a change.

I am not familiar with your antenna so if someone has knowledge on it, now
is the time to chime in.

73 Eric.

> -----Original Message-----
> From: Dick Green WC1M
> Sent: February 10, 2008 8:37 PM
> To: Tower
> Subject: [TowerTalk] 40-2CD SWR problem
> 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 so.
> - 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 100W-200W.
> - 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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