[TowerTalk] Open Feedlines

[Tom Rauch]

Hi Eric,

> How far away from the ground does this stuff need to be?

Several wire spacings, and you need to twist the line at small 
fractions of a wavelength to minimize pickup and radiation at higher 
frequencies.

> What are the drawbacks of using it?

Nothing other than radiation losses at high frequencies (if it is real 
open wire line and not twinlead), and the fact balanced lines are 
hard to deal with mechanically and electrically.
 
> My guess is that this is the 6" spaced feedline I have seen before.  I
> would make this and wonder what people use for the spacers, what spacing
> is required, etc.

I used 1500 feet of solid number eight spaced 2.75 inches, and had 
a 4 MHz matched loss of around 0.2dB including matching 
transformers.

Of course on ten meters the line radiated like crazy because I 
didn't twist it often enough.

By the way....moving a balun to the input of a "floating"  tuner does 
not improve system balance. It does reduce the differential mode 
voltage across the balun, but makes no change at all in common 
mode problems unless the tuner is a push-pull grounded center 
point design.

Some articles just stick a balun on the input of a floating network, 
but in that case the balun sees the same common mode stresses 
as if on the output. I think Roy Lewallen, W7EL, did an analysis on 
this and verified it.
73, Tom W8JI
W8JI@contesting.com 

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