have you tried measuring the rf resistorance?

On May 11, 5:52 pm, Owen Duffy wrote:
K7ITM wrote groups.com:





On May 11, 4:42 pm, Owen Duffy wrote:
I am modelling a Double Bazooka constructed of RG58C/U, and am
interested in a method of estimating the effective RF resistance of
the outer of the outer conductor compared to a round copper conductor
of the same diameter.

The structure loss calculated by NEC-2 is about 2%, so it is a fairly
small quantity.

The model so far is of a Double Bazooka resonant at 3.6MHz at 10m
height over average ground, constructed entirely of Belden 8262
(RG58C/U), and fed with 25m of the same line. The model ignores the
effect of the jacket on the radiator, and assumed that it is a round
copper conductor of the same diameter as the sheild of the coax. The
draft results at athttp://www.vk1od.net/DoubleBazooka/Fig01.gif.

Owen

Hi Owen,

A fellow from Times Microwave, I believe it was, wrote an article that
was published in one of the electronics/RF journals back about ten
years ago, about coax, including loss. He included comments about
braids and stranded conductors, I believe. I went looking for the
article some time after I read it, and never could find it again.
Unfortunately, I also never got a positive response from Times about
it when I asked. But if you trust my memory, you can try a value of
about 7% increase in RF resistance as compared with a smooth round
conductor, for braid. That's at best an estimate, but it probably
doesn't matter all _that_ much for what you are doing. I expect it
depends on the angle of the brading, and to some extent on the
frequency; I believe the 7% is for frequencies whose wavelength was
much longer than the "pitch" of the braid, which clearly would be the
case for your antenna.

Ok.

I did try a sensitivity analysis by modelling aluminium (which IIRC has a
skin resistance of about double copper) and it didn't make much
difference to the outcome.



Hope this is some help and not too much arm-waving for your purpose.

Thanks.



You could, of course, get an idea by comparing the loss for various
types of coax where the difference is in the outer conductor: braid
vs solid. But because the outer is typically such a small part of the
total loss, the estimate's accuracy would be limited.

I did think of that, but my estimate is that the outer conductor of such
a cable contributes something like 20% of the R component of an RLGC
model, and I thought it getting to great a reach to try to deconstruct
the total R, deducting the inner R, and then working out a factor for the
inner of the outer R against an ideal conductor of that inner diameter.
Others have hinted at times that I may have gone too far in translation
of published cable specs into an RLGC model.

BTW, the graph has changed, I doubled an admittance instead of halving it
in the code, so the stub was having more effect that it really does.

Thanks again.

Owen- Hide quoted text -

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