On Sat, 23 Sep 2006 00:32:04 -0700, Roy Lewallen
wrote:

Owen Duffy wrote:

A widely accepted line loss model is that attenuation = k1 * f^0.5 +
k2 * f, your approximation makes k2=0.
. . .

In that model, k1 is attenuation due to conductor resistance, which is
proportional to the square root of frequency as long as the conductor
thickness is at least several skin depths. K2 is dielectric loss, which
is proportional to frequency for good dielectrics. So this model is good
for common transmission lines like coax or dry twinlead, at HF and
above, but not necessarily valid for loss due to water. I commented more
about this in another posting.

Roy, I agree re the wet lines application. A further issue is that of
standardisation of the wet line.

Wes' work and yours shows that there is a degradation, but there is
uncertainty regarding the scale of degradation, and effects like salt
build up in marine locations could be expected to influence results at
the start of rainfall and after torrential rainfall for instance.

I do often use TV ribbon for temporary / portable antennas, tuning the
antenna by adjusting the feedline length. I tend to avoid ground
dependent antennas for this purpose, and a dipole with TV ribbon has
some advantages, but as you note in your article, and as we have all
observed, the "tuning" changes with rainfall more than would be
observed with a coax feedline.

Since your article was published, RG6 has become popular for TV
feedline, and is cheap as chips. It performs very well, and I find
myself using it for portable antennas. RG6 is a little heavier than
RG58C/U (~30%), a little thicker, but has relatively low loss,
approaching that of RG213 as a dipole feedline.

I wonder if anyone has every critically appraised various forms of RG6
for through braid leakage and IMD? I know there is variability in
quality, some seem to not locate the centre conductor in the true
centre of the dielectric, and aluminium wire braid can be a
disadvantage for solder-ability. (Cecil will correct the spelling if
you don't know what I mean!)

Owen
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