It is the water or dirt on the PE web which causes the increased loss.
Way back, when UHF first was broadcast (remember the external
tuner/downconvertor boxes?) , there was a version of 300 ohm twin lead
that was encased in foam and then a PE jacket so the weather didn't
affect it as much, or with proximity to other materials. I think
another version had a hollow tube between the conductors. I haven't
seen that for a long time.
If 93ohm coax is used then 186 ohm balanced feeders can be made. There
used to be a lot of cheap RG62 around from earlier computer systems, not
so much any more. I think most is CCS center conductor same as a lot of RG6.
Grant KZ1W
On 8/2/2016 9:31 AM, Jim Thomson wrote:
From: Grant Saviers <grants2@pacbell.net>
To: Tom Osborne <w7why@frontier.com>, towertalk@contesting.com
Subject: Re: [TowerTalk] Flaky dipole
First, I have to admit to a strongly held opinion that "ladder line" is
one of the worst feedlines one can chose for getting the rf to the
antenna. By "ladder line" I mean the windowed polyethylene insulated
"twin lead". The "low loss" mystique is misleading.
Yes, it is low loss when suspended in free space in a vacuum. If it is
wet or dirty or in contact with most anything the loss goes WAY up.
see http://www.k6mhe.com/n7ws/Ladder_Line.pdf and
http://www.karinya.net/g3txq/wet_ll/
## can u get the 300/450 ohm line..without the windows ??
Seems to me that 300 ohm TV type line back in the 60s /70s
had no windows, I remember it well. It was also available
in a windowed variety.
## along the same subject, back in the 70s, solid twinlead
was available in 75 ohm. Came in 2 x formats, small and large.
The large stuff would easily handle high power. Back then,
the kw rated 75 ohm twinlead was fed directly to the dipole,
then the other end had a PL-259 installed on it. Dunno if its
still available or not. With no windows in the line, water wont collect.
## On a similar note, a buddy used paralleled 50 ohm coax,
to make 100 ohm, shield, balanced line for his quad project.
Shield tied together at each end. 100 ohm Z between the
2 x center conductors. Made from 2 x 213-U taped together
every few inchs. Can also be done with 2 x RG11 to make
150 ohm, balanced, shielded line.
Jim VE7RF
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