----- Original Message -----
From: "Ben Dover" <[EMAIL PROTECTED]>
To: "Brett gazdzinski" <[EMAIL PROTECTED]>; "'Discussion
of AM Radio in the Amateur Service'" <[email protected]>
Sent: Wednesday, November 14, 2007 9:59 AM
Subject: RE: [AMRadio] Open Wire - was AMRadio Digest, Vol 46, Issue 24
-----Original Message-----
From: Brett gazdzinski <[EMAIL PROTECTED]>
Sent: Nov 14, 2007 8:12 AM
To: 'Ben Dover' <[EMAIL PROTECTED]>, 'Discussion of AM Radio in the
Amateur Service' <[email protected]>
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
lossless??
Not at all. Coax VSWR loss is completely dependent upon frequency and at 80M
it is so low as to be meaningless. Using RG-213 as an example a 4:1 VSWR
will introduce a huge 0.374dB additional loss. If the transmitter can tune
into that load a tuner is not necessary. Ive had no problem tuning a
LK-500ZC at 1200W into that VSWR (going from 3.5 to 3.9 MHz)and the coax
voltage is well below breakdown. I use CATV RG-11 foam for all my flexible
cables and losses are even less than RG-213. With cheap little cables and
higher frequencies the losses begin to add up.
Go to www.ocarc.ca/coax.htm and run your own analysis and get weaned off of
the myths, many of which are CB originated.
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.
Only with cable that is being operated near its max in the first place.
RG-8X wont break down with a Valiant at 4:1 but I wouldnt run a KW thru it
even at 1:1.
I also wont us import connectors; Ive had PL-259's and barrels burn up at
1200W into a dummy load.
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!
All tuner applications do that unless it is a pure resistive load.
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.
Pure mythology not supported by facts at HF and MF frequencies with typical
coax runs.
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.
Same comments as above.
In addition, VSWR on open wire will radiate and can cause all sorts of
problems from RFI/TVI, telephones, and strange pattern distortion not
encountered with coax using a sleeve choke at the antenna.
Whether it is coax or open wire the tuner losses themselves will be more
than the feedline in most cases, even with the best designs.
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...
It is just ONE way to do it.
I'll drink to that!
That may be part of the problem <GRIN>
Carl
KM1H
73,s
Mr. T., W9LBB
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