I have installed a 160 mtr delta loop off a 90 ft tower. I am having an
unusual problem (I think!!) with the feed and changing SWR as power is
applied.
The antenna resonates about 1837kc with a good swr (1.05 to1). With
about 250 watts applied the swr slowly rises to about 1.9 to 1 on
1837kc. This takes about 5 minutes or so. With 1kw out the process is
alot faster (about 20 seconds). The low SWR point moves up in freq and
ends up about 1.3 to 1 at 1895kcs or so.
I think the problem must be in the coax feed (or match). The match is a
18' 3" section of RG213 at the feed point and then a 61' 2" piece of 75
ohm Belden 8213.
Then Belden 9913 to shack. I have checked the SWR at the end of the
match (75 ohm to the Belden 9913 and it is the same as in the shack.
The only other thing in the line is a 1:1 W2AU balun at the end of the
matching line (between the 75 ohm cable and the Belden 9913 that runs to
the shack.
Obviously the impedeance of something is changing; but I can't figure
out what. It seems to me that it has to be in the matching section. I
have gone out and felt the 75 ohm stuff and can't detect any warming. I
can only reach the end of the 18 fty section of RG213, but it feels
cool.
Does anybody have any idea what is going on; or more important how to
solve the problem?. It certainly acts as if something is heating up.
Because after I unkey for a few minutes, everthing it back to normal
Thanks
Dick Williams K8ZTT
Franktown, CO
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