I've used various feed lines, one of the earliest was telephone company
direct burial cable that had a spiral wrapped strip of phosphor bronze
for a mechanical and electrical shield. It was destroyed by lightning
that hit the corn crib holding up the SW end of the wire. I know I tried
TV twinlead, but found that it had too effective an air foil and the
small ratio of copper to surface caused it to break from flapping in the
winds far too often. Then I went to 3/4" spaced #18 copperweld (a
commercial feed line) which tended to last longer so long as there was
no ice. Any breeze at all with it or the twinlead covered with ice broke
the feed line into short lengths. The last feed line had several splices
as the result of such ice. I was tempted to make a feed line of #10
copperweld spaced a couple inches, and have the materials but never got
around to doing it. The double extended zepp and feed line are wadded
and tangled in a tall grass patch out back. I need to dig them out for
future use.
I probably had some feed line radiation at 6m and 2m. My first batch of
3/4" spaced line came from W0PFP who found it lossy on 6m due to
radiation when he turned corners with it. My run was stretched straight.
I tied a piece of about 3/16" nylon to half a dozen spreaders (clove
hitch about each) at each end and tied that nylon to the center
insulator at the top and to a concrete block on the ground and raised
the center with a halyard over a pulley until the line was tight.
Back in about 1964, I was experimenting with a monimatch type III or IV
(wire pulled under the braid of a coax) SWR meter. I noticed that for
shorted coaxes that I could detect a change in SWR that seemed to
correspond to the published line loss (at 2m). So to check some K-200, I
built a coaxial balun. Checked it with a short across its terminals and
had about enough added loss to account for the expected balun loss. Then
I hooked in a few feet of K-200 with its conductors bent over and
soldered to make a good DC short. SWR was 1:1... There was no reflection
from that short... Which made it seem as if the RF was just using the
dielectric as a guide (e.g. a 2 wire Goubot line). When I moved a sheet
of copper a couple feet on a side up to the short at the end of the
K-200, I then developed the SWR that I expected for the line loss of the
K-200 and the rest of the test setup. That experiment showed me that the
conductors and dielectric of the balanced unshielded line seemed to be
mere guides for a traveling field, not containers.
K-200 was about 3/4" spacing, about #12 conductors in a big oval of
polyethylene. Very HEAVY DUTY.
There have been many sizes of TV twinlead. The best aren't in the
discount stores where the lead is often miniature, under 3/8" wide with
#24 conductors. The better TV twinleads of the past were more like 5/8"
spaced #18 or 20 in considerable dielectric, UHF twinleads tended to
have a hollow oval of dielectric.
Some EME operators, especially in Europe, believe that using a multi
yagi phasing harness of open wire lines (all straight) results in better
array efficiency than using low loss coax in the feed harness at least
up through 432 MHz. And they test it by working with signals at
threshold where a half dB difference in antenna loss can mean 3 or 4 dB
difference in signal to noise. Their lines tend to be about 1/2" spaced
(with teflon spacers) #12 copper.
73, Jerry, K0CQ
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