Topband: 2 wire beverage question

[Tom W8JI]

> This is very convenient for me.  Can I just put up two of these
> side by side a foot apart and have a differential impedance of
> around 600 ohms?

A two-wire Beverage operates in two modes, common mode (antenna) and 
differential (transmission line) mode.

The common mode works correctly and non-critically no matter what we do, and 
is pretty much independent of what type of wire is used or how the wire is 
used. The differential mode is the more critical issue, because differential 
mode and the transformer system at the far end supplies termination 
impedance to the far end.

While almost anything will "work" to the extent of making some happy, the 
transmission line impedance of some twisted pairs can change by 50% or more 
when wet compared to dry. Also, if the line adds significant signal 
transmission loss in reverse mode, flaws in line balance become more 
apparent. This is because differential loss increases while antenna mode 
losses do not.

This doesn't mean it quits working, it just won't have the front-to-rear 
ratio of a good installation. I looked at this all in the late 1970's and 
early 1980's, and sometime after that I wrote an article  detailing a system 
using ladder line (which was the best wire) in an obscure northern Ohio 
newsletter called the "Amp Letter". I used real 450 ohm air line in that 
antenna.

I've never looked at the care of wide-spaced wires close to earth. The 
closest I've look at was a 1500 foot long air insulated transmission line 
with just under 3" wire spacing here at my QTH. With under 3" spacing and 8 
to 10 feet height, loss and impedance was stable under all weather 
conditions. Without twists in that line, it had noticeable signal leakage on 
mid HF and higher. With twists every 20 feet, it was only a problem up near 
ten meters. On lower bands, it was almost like coax for ingress.

I'm not sure what, if any, problems would appear in other line 
configurations. I suppose results would depend on how fussy you are and what 
is around the antenna.


> What would be even better would be a very light weight twisted
> pair, but is such as thing available, now that phone lines are
> ruled out?  I guess 300 ohm twin lead is ruled out because of
> the rain degradation issue. Probably too much wind load too.
> The twisted pair in CAT5 cable seems like a good RF candidate
> if an easy way could be found to liberate one pair from 400 feet
> of cable.  Still might be affected by rain.

Antenna results are a mixture of many things. While they work the way they 
work, we often "invent" or exaggerate some positive or negative aspect.

If we want stable coax-like performance, the balanced line mode has to act 
like an acceptable transmission line in all weather. It stands to reason the 
very worse lines are twisted pair lines with fibrous coverings. The water 
easily fills the area in the line between and around conductors, and it 
stays there. Wider spaced lines, especially those that push the water away 
from conductors and between conductors, are much better. Open wire lines 
(real ones) are better yet, because there is not very much for water to 
collect on.

I think 300 ohm or 450 ohm transmitting twinlead (which is never 450 ohms, 
and actually has more loss than the ARRL claims) is a good compromise, but 
almost any type of line can be made to "work". It is all in the degree of 
what we look for. I wouldn't be afraid to use twinlead lines, but I'd be 
reluctant to use 50 year old audio paired cables or unshielded twisted pair. 
I'm getting old and lazy........ but just not that cheap yet. :-)

73 Tom