Topband: Inverted L Matching Question

[Larry Higgins]

----- Original Message -----
From: "George McCrary" <k8qm at nc.rr.com>

Subject: Topband: Inverted L Matching Question


> 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.  That means the required
capacitive reactance would also be high.  The line current at 1 kw, 50 ohms
would be about 4.5 amps.  That current then flows through the capacitor into
the shunt feed conductor.  4.5 amps flowing through a very high capacitive
reactance means a very high voltage.  Even small coax can withstand very
high voltages.  The meltdown occurs because dielectric heating heats the
dielectric in the coax enough to melt it.  An arc then occurs to finish off
the stub.

Your inverted L is quite a different story.  If your capacitive stub tunes
the antenna, then the feedpoint of the antenna must have an inductive
reactance.  But in this case, the reactance is low.  The line current is
still 4.5 amps, but it's flowing through a very low reactance.  Hence there
is only a  low voltage acrosss the capacitor.  This explains why you didn't
melt your stub.

If you want more protection, AB7E's suggestion to use 450 ohm ladderline
would give you a nice comfortable safety margin.  Vacuum capacitors are
overkill on series feed (but a must on shunt feed!).

73

Larry, N9DX