Your transmission line, being an odd multiple of ¼ wavelength, will transform
your nominal 70Ω antenna to about 1300Ω so your observed SWR seems to be what I
would expect. If the line were an even multiple, then 70Ω would be presented
at the radio end with a 1.4:1 SWR into your 50Ω system.
-Mike-WA6ZTY
From: Tony <dxdx@optonline.net>
To: TowerTalk@contesting.com
Sent: Thursday, October 25, 2018 10:14 AM
Subject: [TowerTalk] Ladder Line Fed Dipole Issue
All:
I have an 80 meter dipole fed with 300 ohm ladder line that needed to be
moved to a different location. The antenna is 110 feet long and the
ladder line is attached to a DX Engineering 1:1 tuner balun with a short
10 foot run of coax. The length of the ladder line is cut to an odd
multiple of the 80 meter wavelength.
The SWR on 80 meters was very high so I trimmed the ladder line a few
inches which lowered it from 25:1 to 18:1. I continued to trim a few
more inches and noticed no change: changing the coax length had no effect.
My question is: is it normal to expect an SWR to be that high on 80
meters with this setup? Should I continue trimming the feed line to find
a length that presents a more reasonable SWR or look for other issues
with the antenna system? The ladder line does not touch the ground, it's
not folded onto itself and it's not near any metal object.
Tony
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