Topband: Inverted L Matching Question

[Tom Rauch]

> > Is it evil juju to use a coax stub to match this antenna
> > (especially since I can run a KW on 160)?
>
> Probably no evil juju.
>
> Melting a coax stub some years ago prompted me to delve
into the matter.  My
> melted coax stub was feeding a shunt fed tower.  The input
inductive
> reactance looking into the stub is very high

The lowest Q and highest loss way to "make a capacitor" is
to use coaxial or parallel wire in a stub.

Not only do you have the I times X = voltage issue, the
voltage is transformed along the length of the open stub to
an even higher value than at the antenna terminals! If you
have 4 amps flowing through a 100 ohm reactance formed by a
"stub capacitor" I can guarantee you  the open end peak
voltage is greater than 100*4=400 volts RMS times 1.414

As we raise the impedance of the line, especially if we need
a low value of reactance, the open-end voltage becomes
higher yet and the losses also go up. You really have to
treat the stub like a transmission line with infinite
termination SWR, since that's what it actually is.

http://www.w8ji.com/omega_and_gama_matching.htm

The longer the stub in terms of wavelength (less reactance),
the GREATER the voltage step up along the length of the stub
and (as a general rule) the higher capacitor series
resistance becomes. This means a low reactance load is not
always a cure for stub voltage breakdown or heating.

The vast majority of heating relates to conductor loss
resistance, not dielectric heating. The dielectric melts
because the conductors heat it,  or in some cases because
corona heats it.

I'm not saying a stub won't work, but they are a PITA in
many cases. Ask Dennis, '0CKD, about his stub experiences.

73 Tom