On Mon, 26 Dec 2005 13:43:05 -0800, dansawyeror
wrote:

I have a section of air core Beldin coax, with the center feed separated by a
single wind of plastic insulator. The section is about 65 meters long. I added
enough RG58 to the feed end to get a full wave length. I then measured the
impedance using aa Autek analyzer. It read 30 Ohms.

The coaxpair program predicts for coax with a loss of about 10 db per km an
impedance of about 11.5 Ohms. The cable should be better then this.

Is this difference leaky or bad coax? Is it reasonable to assume water in the
cable? If it is water is there a practical way to get it out?


The hints in this guessing game a
- this might be the popular Belden 9913, lets proceed on that
assumption;
- that 65m is less than a full wave, so the frequency of the test must
less than 3.87MHz;
- the loss at the frequency of interest might be 10dB/km, implies 7MHz
for 9913 (MLL for 9913 at 3.875MHz is around 7.4dB, and less at
~3.6MHz);

Bzzzt... does not compute, inconsistent input data.

Some trivia:

The input resistance to a 65m shorted stub of 9913 at its half wave
resonance at 3.875MHz should be around 2.8+j0 ohms.

The input resistance to a 65m open stub of 9913 at its half wave
resonance at 3.875MHz should be around 900+j0 ohms.

It is anyone's guess what adding an unspecified piece of RG58 will do.

An accurate SWR meter can be used to roughly assess the loss, measure
the loss into a shorted line at the frequency of interest, and convert
the measurement(s) into return loss. The matched line loss is half the
observed return loss. The SWR looking into a 65m shorted stub of 9913
at 3.6MHz should be around 19:1, rho about 0.90, for a return loss of
~0.92dB, and a MLL of 0.46dB. (If your SWR meter does not read 100%
reflected on a s/c applied to the SWR meter terminals, don't waste
your time doing the test.)

Owen
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