[AMRadio] Open Wire - was AMRadio Digest, Vol 46, Issue 24


jeremy-ca km1h at jeremy.mv.com
Wed Nov 14 12:11:15 EST 2007


----- Original Message ----- 
From: "Ben Dover" <quixote2 at ix.netcom.com>
To: "Brett gazdzinski" <brett.gazdzinski at verizonbusiness.com>; "'Discussion 
of AM Radio in the Amateur Service'" <amradio at mailman.qth.net>
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 <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
>>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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