Topband: Re: Coax capacitor losses

[Peter Wardenier]

> Coax is not to be used on 160m as a capacitor? Or can it?
I think the truth is somewhere in the middle, depending on your personal
taste.
 
First, let's repeat some of the math on lossy transmission lines, or
better, have 
a look at http://www.sm.luth.se/~urban/master/Theory/4.html.
More specifically, consider formula (82) for the input impedance of a
lossy transmission line of arbitrary length.
You only need to recall that 
-   alpha [Neper/m] = attenuation per unit length [dB/m] / 8.686 
-   beta = 2*pi / (free space wavelength [m] * coax velocity factor)
 
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.
 
Having such a coax stub in *series* with a slightly long quarter wave
vertical (like we have
at the PC5M 160m contesting site), adds  ~0.5 dB of loss, compared to a
'good'
capacitor. For RG-213 (assumed attenuation: 1 dB/100m), the loss is
about
halved (~0.25 dB). If this is acceptable, is up to you!
 
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 ;-)
 
73, PA3AUC
 


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