My 160m base insulated tower (120ft) is series fed. I knew that I would
run into "issues" when I added a 2el 40m yagi up on top along with feedline,
rotator and the associated cables. An "isolator" which I believe to be a piece
of Teflon coax with a bunch of ferrite beads over it seems to have done the
trick with the 40m coax. I thought I had the rotator line problem licked when I
purchased 18 ferrite beads and installed them over the rotator line.
Theoretically it should have worked and the minimal change on my swr on 160m
made me think I was using a good approach. I still had some deflection on my
direction meter on the rotor box though. Then came the 160m contest last
weekend.
After the contest was over I noticed that my swr low point had shifted way
down below the band.(1730) I turned on the 40m antenna rotator control box and
it would not indicate a direction. I climbed the tower to see if the cable had
melted or burned open. I could not find any visible problems until I opened up
the section of PVC conduit where the section of ferrite beaded line was located
at the base of the tower. The beads had heated up enough to melt the rotor
cable straight through to the conductors. The jacket of the cable became fused
to the insides of the beads. What a mess. The rotor cable had now become an
integral part of the 160m system thus the shift in the swr lowpoint.
I was pretty convinced that I'd have to change to a shunt wire feed
system however physically cumbersome it would be. But then I spoke with W8JI!
He suggested 40-50 turns of rotor cable around a 4 inch air form such as PVC
conduit. I built this the day before yesterday and I no longer have any
deflection on the meter when TXing on 160m with full power.My low swr point
moved up to 1900 but a quick tweak of the vacuum variable has brought it back
down to 1850. Where there is a will, there IS a way. Thanks Tom! I was NOT
looking forward to
re-doing the feed on the 160m antenna for the 3rd time.....
73, N4UK
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