[Antennas] Re: 160M INV L

[Roy Koeppe]

'Anonymous' asked this, so I'm sharing my humble answer with the
reflector:

"I have been off of top band for a couple of years but just getting back
on.
Am using the Inv L, about 70 feet vertical and about 20 radials.  Direct
feed with
coax at the base.  Could you tell me more (pretty much as much detail as
possible)
about how I could go about using a shunt coil wound on a powdered iron
toroid?

I am not happy with the match I have on my antenna and find it is almost
taxing
my antenna tuner (in shack) to the limit to tune out most of the swr.  I
would love
to be able to turn the antenna switch to the Inv L and cut the tuner out
of
the circuit.
I did find years ago while experimenting with my first Inv L that the
more
radials
the higher the SWR although the "resonant freq" didn't change.  I'm not
much of
a
technical guy (but wish I was)."


Yes, the input Z at the feedpoint of such an INV L as your described
will be on the order of 13 ohms. That surprised me the first time I
measured it. And the more radials you add, the lower the input Z, and
thus the SWR at resonance will be. The very simple, shunt coil, aka,
Z-match, aka, inductive-match is so basic and simple that it seems like
over kill to describe it in detail, but since it's your first time at
it, here goes:

Acquire a powdered iron core such as a T-200. This application is not
demanding, being at a 50 ohm, non-tuned point. T-200 will handle 1.5 KW
here, but a bigger core is great. Wrap on about four turns, spacing them
over 2/3 of the core. Connect one lead to the antenna feedpoint and the
other to ground. Your coax center conductor connects to the junction of
inductor and antenna (power is fed across the inductor). This winding
can be small gauge wire, like #20 because it is a temporary test. Later,
replace it with #16 or #14.

Check your SWR. The antenna resonance point will have moved downward in
frequency slightly; however the SWR should be lower than before. Now you
must juggle between resonance point and the lowest SWR, either adding or
subtracting turns, plus changing the antenna element length to maintain
resonance where you want it. Be certain you're in a good mood before
beginning this. A perfect 1:1 SWR is possible, with a nice broad curve.

In CA, my lot measured 130 ft. by 100 ft. and I ran the 36 radials out
to the perimeter in all directions, being buried 1-inch deep. Everything
just squeezed in using three supports, being highest in the middle, at
85 feet there. Radials obviously were not a quarter wave long.

Good luck & 73,

Roy        K6XK/0         Iowa