Tom,
I realized that the field extends beyond the dielectric into the air, and
so I played with lower values of permittivity to see how much changing that
would affect the results.
But you are certainly right, the best way to do it is to directly measure
it. :-)
The samples were from Milt, N5IA. I'm sure he would be happy to send you
some if you'd like to test it.
73, Mike
www.w0btu.com
On Mon, Jul 23, 2012 at 6:36 PM, Tom W8JI <w8ji@w8ji.com> wrote:
> > Gary, was your 140 ohm WD-1A made from .0393 (1mm) diameter wires, spaced
> > .118 center-to-center? My math says that should be a little over 140
> ohms.
> >
> > But the stuff I was talking about was a different diameter and spacing
> > (.033 diameter and .0685 center-to-center spacing.) The same math says
> > it's
> > ~115 ohms, but I had no way of measuring a piece less than a foot long.
>
> Since the field extends outside the dielectric, all bets are off. Even if
> the strongest area of electric field is between the conductors, we really
> don't know the percentages of what dielectric are involved.
>
> That's why a measurement is best. Tuning between a frequency where length
> is
> and odd 1/4 wave and even 1/4 wave electrical and adjusting for most
> constant SWR would hit pretty close.
>
> I think to make any meaningful tests, I'd use either 50 feet or 100 feet of
> line. Anyone with a movable power meter, antenna tuner, and transmitter, as
> well as an antenna analyzer, could probably measure loss and impedance as
> well as any VNA.
>
> 73 Tom
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