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Re: [TowerTalk] Inverted L for 160 Meters

Subject: Re: [TowerTalk] Inverted L for 160 Meters
From: "Rob Atkinson" <>
Date: Mon, 15 Oct 2007 18:06:45 -0500
List-post: <>
hi Tom,

I have not read the QST article but I'll take a stab at ur questions since
no one else has.

*        Do I understand correctly, that C1 in the diagram is a simple
variable capacitor that ranges from 100 pF to 800 pF?  (I found one, and
purchased it)

*        Is it for tuning the antenna for minimal SWR?

The short answer is yes but see below....

*        How did the author arrive at the wire length of 165 to 175 feet?

It looks to me that the idea was to make the active part (the L part
of the inverted L) longer
than 1/4 w. so the feedpoint Z would have some inductive reactance
(Xl).  By doing this, you
can have a variable cap at the f/p for matching--it tunes out the Xl
and it is all you need.
Making the L long in other words makes the matching network simple--no
roller inductor, only a
cap.  In fact, at one time Heath made a motorized variable cap in a wx
proof enclosure just for this
purpose, so you could tune an inverteed L from the shack.

I know these are pretty basic issues/questions, but I built this antenna
today.  I want to be sure I've built it correctly, and moreover, I want to
be sure I understand how to tune it correctly.

Of course--you have the new antenna jitters--excitement + nerves--I think they
are always there with every new antenna design a ham tries.   The first time I
put up a horizontal loop I was so excited I dropped my MJF analyzer on
a concrete
floor -- fortunately it wasn't damaged (which speaks well for MFJ).

*       Right now, I can get a minimal SWR of 3:1 at 1.810 MHz by tuning the
variable capacitor for min SWR.

Hmmm...since you have no radials down the cap is probably causing ur
problems, but
that doesn't mean you don't need it.  I'd add some radials, around 20
for starters,
and check the match again and see if things are better/worse.

*       I don't have any radials installed at present.  The coax braid is
grounded to my tower ground (three 8 ft rods bonded together)
*       I plan on adding some radials (laid on the ground) tomorrow.  Should
they be of any particular length to perform optimally?

Well, ~120 feet would be nice if you have enough wire, but more shorter radials
are better than a few long ones, i.e. 60 sixty footers are better than
30 that are
one hundred twenty feet long.

If you get down 30 or 40 and you still can't tune the feedpoint you
are gg to have to
mess around with the length of the L.  It's not real
complicated--basically just mess
around with it until things begin to look good.  GL


rob / k5uj

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