"Owen Duffy" wrote in message
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" wrote in news:6d1a078a-e5f4-
:

...
I think the problem with using the double coax is the very large
capacitance it adds to the feed line, effectively becoming a low pass
filter. Could be mistaken about the cause, but not the symptoms. I
have actually used the system to make some local 6 meter contacts.

The coax is, and always is a transmission line, and at the length you
described cannot be approximated well as a shunt capacitance.

What you have is a cascade of two line sections, one of say 600 ohms,
then one of 100 ohms, and each is probably operating with standing waves,
so there is impedance transformation.

To illustrate, lets say your feedpoint at 3.6MHz with a certain loop
antenna was 100+j0, and you had say 30m (100') of 600 ohm open wire, the
impedance looking into that would be around 240-j675. If you feed that
with say 6m (20') of LMR400 twin, then input Z would be around 60+j260
and loss would be about 40%.

The synthesised shielded pair is relatively lossy, and low Zo. Most
people use this configuration thinking that the shielding prevents
external fields from common mode current, but they are quite wrong. See
http://www.vk1od.net/transmissionline/stcm/index.htm .

Though the traditional approach has been to use a 4:1 voltage balun at
the rig to feed these things, there is good argument to use a 1:1
Guanella balun (current balun), and it can be located outside the shack
and inboard shield effectively grounded to deal with common mode current.
You still need to minimise the length of coax operated at high VSWR, and
it would not be necessarily absurd to think about low loss coax.

Approximating coax as a shunt capacitance might be reasonably accurate
for some applications at audio frequencies, but it is probably not for
most RF applications.

Owen

Owen, my thinking was to make a transmission line with a characteristic
impedance that gradually changed from 600 ohms down to as close to
200 ohms as practical, just to avoid the impedance bump at the junction
of the open wire and shielded balance line. I've found a few
references that state the taper should follow a log response; and also
if the line needs to be a wavelength or longer pretty much dashes that idea.
It would be a lot easier to construct a line with charateristic impedance of
400 to 600 ohms, and deal with narrow spacing issue for only a relatively
short portion of the entire span.

I didn't expect the shielded cable to be immune to common reradiation
problems, but that issue could be dealt with separately if it was a
problem. Pete