[TowerTalk] Open wire line

[Tom Rauch]

From:           	K7GCO@aol.com
Date sent:      	Sun, 10 Sep 2000 20:51:49 EDT
Subject:        	Re: [TowerTalk] 80-75M Inverted Vee or Dipole
To:             	towertalk@contesting.com, alsopb@gloryroad.net

> Brian:  Coax actually has it's applications at Cell frequencies.  My
> references were mostly for ham frequencies and many in Europe are using
> open wire line for 6M, VHF&UHF as I have with less loss than with coax.

Balanced transmission lines require very close conductor spacing 
when used at VHF and higher. They also are critical for placement 
and require twisting at fractional wavelength distances to reduce 
radiation. Even 1/2 inch of conductor spacing is enough to be 
noticeable at UHF.

We almost never see open-wire lines used at VHF and UHF for 
those reasons.  

Conventional plastic insulated ladder lines are not nearly as low 
loss as "real" air-insulated line, especially when wet. Wet "ladder 
line" has about the same loss as RG-58. They behave more like 
twinlead than "real" air insulated line.

There are many coaxial lines now that have less loss than some 
"ladder lines", and the coaxial lines maintain that low loss in all 
sorts of weather conditions.
 
> I'd suggest you do you RF home work before you preach with RF at the
> antenna measuements.  I had one feed system on 20M of 120' using open wire
> line and a tuner at had 750W into the link of the tuner, open wire line to
> a 16 ohm 3 element beam (2-1/4 wave step down from 450 ohm) and had 700W
> measured at the antenna--to win a bet.  That's .3 dB loss.  The coax cost
> with .3dB loss here is more than you can afford.

That's on-par with with what 9913 coax typically does. I can afford 
$50.

> The 80/75M feedline configuration I described had less than .1dB loss over
> the whole band.  As I said "dragging kicking a screaming hams into the
> last century."   k7gco

That isn't true. 

The loss on 3.5 MHz of a matched 450 ohm ladder line 120 foot 
long is .12 dB 

The additional loss caused by SWR (you have a ~450 ohm line 
connected to a ~50 ohm antenna) is .373 dB

The total loss in the feedline is about 1/2 dB, using conventional 
"window" ladder line.

Loss would actually be less using 8214 Belden foam RG-8 coax.

The real advantage of ladder line comes when the impedance 
terminating the line is very high, and can't easily be corrected. A 
good example is a multiband dipole.
73, Tom W8JI
w8ji@contesting.com

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