Topband
[Top] [All Lists]

Re: Topband: Coax capacitor losses

To: "Peter Wardenier" <Peter.Wardenier@asml.com>,<topband@contesting.com>
Subject: Re: Topband: Coax capacitor losses
From: Terry Conboy <n6ry@arrl.net>
Date: Thu, 23 Dec 2004 10:54:34 -0800
List-post: <mailto:topband@contesting.com>
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



_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband

<Prev in Thread] Current Thread [Next in Thread>