Well, since posting my inquiry yesterday, I received a lot of expanations
for the practice of twisting balanced transmission line. Here is a summary
of the majority of replies:
1) Twisting the lines reduces motion due to wind loading effects.
2) Twisting the line is analagous to twisting telco and CAT-5 LAN cabling;
which tends to reduce common-mode noise through common-path phase
cancellation.
3) Twisting the line has a tendency to cancel the effects of current
imbalance when the conductors pass near metal or other conductive objects.
Perhaps there is some merit to all the expanations. What bothered me was
that in two commercial AM broadcast facilities, and after personally
witnessing non-twisted open transmission line at the Bethany
Voice-of-America transmission facility (before its destruction three years
ago), the "art" of twisting transmission line was something I've only seen
in amateur, not professional applications. BUT, none of these facilities
used their antennas for *receiving* purposes. So, perhaps the real derived
electrical benefit is a manifestation of lower received noise due to
common-mode electrical interference. And, twisting the line arguably helps
to reduce physical motion in the presence of wind.
I'll try to conduct a search of this technique using my QST CD ROM archives.
If I find anyhting, I'll post it to the group. Oh, I did find once
reference to the use of "transpositioning blocks" by the legendary Don
Wallace, W6AM. In that reference, he not only condoned the use of
transpositioning, he also indicated that it is the duty of every amateur to
drop his antenna every Saturday morning and, using steel wool, remove the
oxidation that results on the copper wire surface! In the 1920s, it was
thought that an antenna's radiation efficiency would be greatly improved
(due to the skin effect of R.F.) with a smooth, non-oxidized copper surface.
-Paul, W9AC
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