[TowerTalk] 160-m. Inverted L Question

[john at kk9a.com]

You can feed an inverted L directly, however if having a lower SWR makes you 
happier a hair pin coil will work very well.  They are simple and 
inexpensive and also bleed any static from your antenna.  I use one made at 
P40A with great results.

John KK9A


To: towertalk at contesting.com
Subject: Re: [TowerTalk] 160-m. Inverted L Question
From: Terry Conboy <n6ry at arrl.net>
Date: Thu, 13 Dec 2007 10:56:30 -0800
List-post: <mailto:towertalk at contesting.com>

At 04:18 PM 2007-12-12, Dick-K9OM RLVZ at aol.com wrote:
>I've been doing research on 160-meter Inverted L's and would appreciate 
>your
>thoughts on the following:
>
>1) Some people report that the 160-m. Inverted L can be fed directly with 
>50
>ohm coax with good results.  Is that true?
>
>2) Some people feed their Inverted L's with fixed 500 or 1,000 pF
>capacitors.  What advantages does this offer, such as: lower SWR
>at  resonance, and/or
>increased bandwidth?  (again these are "fixed" capacitors  not variable)
>
>3) Can you offer a good resource for the 500 or 1,000pF  capacitors that 
>will
>handle a Kw?
>
>I realize a variable capacitor is best as you can move around the band
>better... but for this temporary 160-m. Inverted L... I'd prefer
>Options 1 or 2
>above.

Another approach, that gives a 50 ohm match without capacitors or
transformers, is to use an inverted-L that is about 3.5 feet shorter
than resonance and place a shunt coil (about 4.7 uH) across the
feedpoint.  This forms a virtual L-network to step up the naturally
low impedance of an inverted-L.  I use this approach with a four-leg
inverted-L with a very short vertical section.  The loss in the
matching coil is well under 0.1 dB.

Using the shunt coil match (also known as beta or hairpin match)
actually gives a very slightly wider SWR bandwidth than the
series-cap matching approach.  In addition, the shorter horizontal
wire of the shunt-matched inverted-L decreases the high angle
radiation.  Of course, if you're mostly interested in short-haul
contacts, the longer wire needed with the series-C match would be an 
advantage.

The gain at low angles is very similar regardless of the matching
approach used.  The ground losses are indeed lower with the series-C
approach, due to the higher feed Z, but all of the extra signal takes
off at high angles.  I'll forward to Dick another message containing
patterns and SWR numbers comparing series matched, resonant, and
shunt matched inverted-L's that I ran recently.

73, Terry N6RY