Topband: Re: Coax capacitor losses

[Tom Rauch]

> This leaves the -maybe surprising- conclusion that Q is
independent of the coax
> stub's length! Once again: this applies to short stubs
only; up to some 10m @
> 1.8MHz is considered 'short' in this respect.
> At a given frequency (i.e. given beta), the Q is only
directly influenced by the coax
> cable attenuation (alpha).
>
> However, for one reason or another, some transmission line
simulators erroneously
> seem to show an ever increasing Q (up to infinitity...)
for the stub length approaching zero!
> This unrealistic outcome is present in the handy online
version (applet) by W9CF,
> which is based on TL.EXE, please refer to
http://fermi.la.asu.edu/w9cf/tran/
>
> Concluding: some care is needed using transmission line
simulators for this application
> (Zl=infinite, L~0), for it may lie outside the permissible
scope of the simulator.
> Validity of simulations can easily be checked by taking a
quick look at the original
> transmission line equations.

Hi Peter,

If you are correct, every test instrument I have is wrong.
Since my instruments are currently calibrated by a lab just
two weeks ago, then the calibration lab is also incorrect.

Measuring 12 feet of RG-58 coax on 7 MHz my display tells
me:
R=.709 ohms
X= -j46.824
Q= 66.05

TLA predicts:
R= .57
X = -j47.33
which would be a Q of 83

Changing length to 6 feet by cutting the cable I measure:
R= .3078
X= -j116.09
Q= 377

TLA predicts:
R=.31
X= -116.2
Q would be 374 based on TLA

The discrepancy between TLA and what I measure is very
reasonable. As a matter of fact using W9CF or TLA over the
past years I never find significant error. Components and
systems also behave very much like TLA predicts, including
loop antennas and capacitors for loop antennas I have
designed.

My Conclusion:
If you are correct, then my equipment (which is certified to
be to industry standards) and TLA are both grossly
incorrect. My money would be on the fact you are missing
something in your analysis, and it looks like it is the fact
all conductor loss resistances are in series with the
impedances seen further downstream in the transmission line
as the line is made longer.

The Internet is a very dangerous place to obtain reliable
technical information, because it lacks peer review and
verification.

73 Tom