[Elecraft] Thanks! OT: Vertical doublets / baluns / tuners

[Stuart Rohre]

Guy,
With all due respect, in the 50's and 60's there was true kilowatt 72 ohm
twin lead of good dielectric properties and receiving type 72 ohm twin lead
as well, also of good properties. It was marketed by Amphenol and probably
made by Belden.  There is still true low impedance twin lead available in
U.K. --I have some.  The American made ones by known manufacturers were
fully specified for RF use.  The 72 ohm "receiving" type actually worked
with 100 watt rigs quite well.

Well respected operators on this list have reported in the past that zip
cords of today work fine for them in short lengths such as used on camping
trips.

Yes, zip cords can vary in precision of manufacture.  But quality ones can
be had in the lower loss dielectrics.   If the plastic is same as that used
in coax, that should be suited for RF feeders. Much insulation today is PVC,
and that can function in a satisfactory manner

The ham old saw about zip cord feeder losses came from some ARRL tests of
long ago, probably on thick rubber insulations loaded with carbon, but in
any case not specified which kind of negated the usefulness of their test.
Zip cords come in all colors of insulation, and that filler may affect the
RF properties.

How long were the pieces you measured, and could you verify the dielectric
or origin of the zip cord?
I have seen some clear insulated import zip cord that looks promising.
Others seem to have little copper, but the spacing looked uniform, a
condition for a characteristic impedance.

One of the questions to be tested is how well does high quality speaker
cable zip cord function as feeder.  As hams need to be prepared for
emergency communications, it is useful to have alternates to commercial mail
order parallel lines in case of interruption of supply lines such as 9/11
when air travel was suspended for some days.
-Stuart
K5KVH