|[AMRadio] Open Wire - was AMRadio Digest, Vol 46, Issue 24|
quixote2 at ix.netcom.com
Wed Nov 14 09:59:03 EST 2007
>From: Brett gazdzinski <brett.gazdzinski at verizonbusiness.com>
>Sent: Nov 14, 2007 8:12 AM
>To: 'Ben Dover' <quixote2 at ix.netcom.com>, 'Discussion of AM Radio in the Amateur Service' <amradio at mailman.qth.net>
>Subject: RE: [AMRadio] Open Wire - was AMRadio Digest, Vol 46, Issue 24
>Someone correct me if I am wrong, but the coax
>loss is listed at no swr.
>If you operate off the resonant frequency, the swr will go up
>and may be quite high if your antenna is cut for 3880 and
>you operate on 36XX. Say the swr is 2:1 or more, wont the coax
>get warm and lossy, and the open wire line still be virtually
Yup, ya got it Brett. Coax is, by definition, meant to be a FLAT
transmission line... meaning, terminated at it's characteristic
impedance. With standing waves on it, that nice dielectric starts
getting warm and gooey... and if you're running the coax near
it's max rating, there's the additional hazard of exceeding the
breakdown voltage rating at the voltage loops along the line.
In TV and FM broadcast installations it's not real uncommon to
see sections of coax hard line (made of 3.125" ID or LARGER copper
pipe!) where at intervals along the line (1/2 wavelength) the
pipe outer conductor has a bluish discoloration from heating at
the current loops!
BTW... with broadcast hard line, the dielectric is usually dried
air (provided by a dehydrator at the transmitter end of the line)
or dry nitrogen gas. That has a lower dielectric loss than the foam
or spiral polyethylene insulators used in Heliax lines. Both hard
line and large Heliax are quite commonly used.
>I used to have resonant dipoles for 80 and 40, and they worked well,
>but if I went off frequency much I needed a tuner anyway.
Understandable... but when a dipole is fed with coax and a tuner is
used, it's one of my pet peeves. The sole function of the tuner is
to HIDE the high VSWR from the transmitter; it does NOTHING to fix
problem that's causing the high SWR in the first place! At high power
the coax takes a beating, and the line losses are elevated by the
dielectric loss... and the bottom line is that efficiency is way down.
With open wire, the tuner ALSO hides the high SWR from the transmitter,
but because of the inherently low loss of open wire line the power loss
is greatly reduced. You don't take anywhere near as bad an efficiency
hit in the antenna system.
>My open wire line was free, I used scrap #14 wire and scrap
>plastic spreaders with plastic wire ties.
My preferred method is similar to yours, except that the spreaders are
sections of fibreglass rod cut from cheap electric fence posts. The
posts are 48" long, and 3/8" in diameter, and they're strong as hell,
as well as heat resistant so I can solder on tie wires to hold the line
conductors in place. The posts are cheap enough... in single units from
the local "Cowboy's K-Mart" (Farm and Fleet), they're under $1.50 each.
The price goes down in bundles of six.
>Surely if you want to operate multi bands and frequencies
>at high power, open wire line is the only way to do it...
I'll drink to that!
Mr. T., W9LBB
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