[TowerTalk] Some More Thoughts about Open WIre LIne

[James R. Duffey]

I usually alternate antenna builds between balanced feeders and coax. When I
need to replace balanced line, I remember the balanced feeder hassles and
replace it with coax for convenience. When I replace coax I remember how
nice it is to use the antenna on multiple bands with balanced feeders and
put up the balanced feeders.

I have given up on homebrwing balanced line for two reasons: 1)almost any
other easily available insulator than ceramic disintegrates in the high UV
at this altitude (nearly 7000ft asl), and 2) it requires more labor and
patience than I feel it is worth. I don'tt think that the ceramic is not too
usable as an insulator as it is very heavy. I use the commercial "450 Ohm"
ladder line or the "300 Ohm" ladder line now. In the last 10 years or so
this has been made from high density polyethylene dielectric which holds up
well at this altitude.

None the less, I supppose every ham should, at one point in his ham career,
build and use his own balanced feeders. It is a rite of passage, just like
building regenerative receivers.

I don't see much advantage in feeding a single band antenna with balanced
line when the feedline runs are typical.

To answer W1JR's question, Teflon, was developed in WWII by duPont and
made commerically available thereafter, so I doubt if any Teflon balanced
wire spreaders were available in the 30s. Bakelite and Phenolic were widely
available "plastics" in that era, so the spreaders may have been made out of
that material, but I doubt it.

The radiation from balanced feeders can be approximately modeled by
replacing the feedline with a dipole whose length is equivalent to the
spacing of the balanced feeder. So the 0.75 inch spacing at 70 cm is roughly
equivalent to a 0.03 wavelength dipole. This isn't much shorter than some
antennas used on 70 cm walkie talkies! It is no wonder that you noticed some
radiation from the feeders. It would be equivalnet to an 8 ft antenna on 80
M, and lots of people make contacts with mobile whips this length.

Just my $0.02 worth. - Dr. Megacycle KK6MC/5

James R. Duffey KK6MC/5
30 Casa Loma Road
Cedar Crest, NM 87008


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