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[TowerTalk] Rising VSWR

To: <>
Subject: [TowerTalk] Rising VSWR
From: (Chuck Counselman)
Date: Tue Jan 28 08:44:02 2003
At 11:50 PM -0600 1/27/03, Tom Branch wrote:
>...On 80 and 160 meters, I see a rise in VSWR after calling CQ for a 
>while or running RTTY.  This would lead me to balun saturation... 
>They are 160 - 10 meter baluns from the Radioworks and rated at 5 
>kW.  I also have a line isolator near the shack that's rated for 160 
>- 10 meters rated at 2 kw....

I have two RadioWorks type T-4 line isolators, both of which 
overheated while I was transmitting 1.5 kW.  One caught fire.

Dissection revealed that they were wound with 0.2-inch (o.d.) coaxial 
cable and potted in plastic foam.  The coax had PTFE dielectric, 
which was nice but did nothing to reduce the I-squared-R heating of 
the skinny center conductor of the coax; and the foam is thermally 
insulating!  So it was 100% clear why they had overheated.

RadioWorks' power rating for these devices 
<> is:

   ">1500 watts if SWR < 3:1, SSB/CW duty cycles*
   "*Not rated for AM, RTTY or other high duty-cycle modes."

Since you say that you were running RTTY, you have probably been 
overheating your line isolator.  Is its PVC pipe housing still 
perfectly straight, or has it developed a slight curve like that of a 
banana?  Both of mine had gotten curved.

Your line isolator overheating will not raise your VSWR, but your 
balun(s) overheating will; and RadioWorks' baluns are probably 
constructed as their line isolators are.  (They look similar 

-Chuck, W1HIS
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