[TowerTalk] SWR Problem

[Tom Martin]

About three weeks ago, as I tried to work a YK on 40 CW, my SWR on the 40 meter shortened dipole jumped from 1:1 to 5:1.  At the time we were experiencing a heavy, wet snowfall.  The antenna is the driven element of a Mosley S-402 at 80 feet.  I assumed that it was possible that snow had accumulated on the feed point causing the jump in SWR.  I shut down the amp and missed the YK.  I'm glad that I have an Alpha 99!  The amp went to the Fault mode and saved the tubes.

After we had some dry weather, I tried again and the SWR was a steady 5:1.  I checked coax connectors at the base of the tower and even cut off some coax to see if there was any water in the line.  Nothing was found.  A few years ago, on the same antenna, but with the reflector attached, I cut the coax and water ran out.  Since that time, I have replaced the coax to the driven element and removed the reflector.  I installed a 4 element SteppIR and didn't want more wind load.  The rotating driven element has done very well on 40 DX.

Last week, during a very warm spell for April here in the U.P., I tried the antenna and the SWR was perfect.  I even worked a few stations, using the Alpha at 1 KW with no problem. After a few contacts, the SWR went to 5:1 again even at 25 watts.  

In order to work the S04, I erected a delta loop, corner fed with the apex at 70 feet.  It worked very well.  However, the 40 meter driven element was 2 S-units better on receive!  I am presently transmitting on the loop and receiving on the 40 driven.  My fingers are cramped from switching back and forth! I can't get to the antenna without a bucket truck. Unfortunately, none of the trucks in my area go to 80 feet!   A climber wouldn't get to the antenna either, since it's 8 feet above the SteppIR.  I sure don't want to go through the process of removing the rotor and lowering the mast.  That is unless, K4FMX wants to climb.  HI!

The question I have is why does the antenna receives so well with a terrible SWR?  Is there just one thread of copper attached at the feed point that allows it to receive but when power is fed to it, it heats up and the SWR goes high?  Is it possible that the wet snow and high power combined to cause a break in the center conductor of the coax at the feed point?  I've checked the connection with binoculars and it looks OK.  Also, when I installed the antenna last October, I made the connection as waterproof as possible and formed  loop above the feed point so that water wouldn't run back toward the pigtail.

This is sure frustrating!

73,

Tom W8JWN 

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