Re: Topband: Coax capacitor losses

[Terry Conboy <n6ry@arrl.net>]

At 05:29 AM 2004-12-22, Peter PA3AUC wrote:
> Now take as an example an open-ended, 10m long piece of RG-58 with an
> assumed
> attenuation of ~2 dB/100m at f = 1.8 MHz, and a velocity factor of 0.66.
> Inserting Zo = 50 ohm, ZL = 'infinite' , you'll find:
> Zi = 3.9 - j 77.7 ohms, which you can think of a 1.14 nF capacitor with
> a Q of ~20.
> Not very good, indeed!  Note: I did the calculations using MathCad.

I wanted to see if I could reproduce this result with tools at my 
disposal.  I used TLW (version 1.0 from N6BV) and XLZIZL (from AC6LA) and 
got a much lower series resistance.  Both of these programs had a selection 
for Belden 8259 (RG-58A).  For a 10 meter stub at 1.8 MHz with a 10 Megohm 
termination (they don't allow an open circuit), TLW gave Zi of 0.76 - j 
77.77 ohms and XLZIZL gave 0.82 - j 77.75 ohms or a Q of about 95.  Still a 
poor Q for a capacitor.  In a matched nominal 50-ohm circuit, this would be 
about 0.07 dB loss (24 watts out of 1500).

> Putting two half-length coax stubs in parallel  (5m each in this
> example)  does *not*
> reduce the power loss as compared to the original full-length stub.
> You can easily do the math yourself ;-)

Doing the math myself (aided by XLZIZL), I used a 5.443 meter stub of 
Belden 8259 (RG-58A), which gives Zi of 0.42 - j 155.5 ohms or a Q of about 
370, a great improvement (but still a marginal capacitor Q).  (A 5 meter 
stub gives somewhat more than twice the reactance of a 10 meter stub, thus 
the odd length.)  It would seem that paralleling two of these stubs would 
give Zi of 0.21 - j 77.75 ohms or about 1/4 the loss resistance of the 
single 10 meter stub (under 0.02 dB or about 6.3 watts out of 1500).

Did I miss something?

73, Terry N6RY



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