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
<http://www.radioworks.com/ct-4.html> 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
externally.)
-Chuck, W1HIS
|