[TowerTalk] Inverted L Dimensions
Anthony J. Cioffi (N2KI)
n2ki at amsat.org
Wed Sep 22 04:58:15 PDT 2010
This is a design I found and it works right from the start. I built it for 160 meters using 14 gauge stranded bare copper and
have great results. The feed point is made from aluminum L stock. Drill a hole for a panel mount SO-239(not chassis) to fit.
Do not directly connect the vertical element to the SO-239. The first strong wind will rip it apart. To address this,
anchor the vertical element to the tree using an insulator, rope and then a lag eyebolt into the tree. This will secure your
vertical end so you can safely solder a wire to the center pin of the SO-239 and the wire without straining the feed point.
Connect the elevated (10') counterpoises to the aluminum plate with a terminal lug and self-tapping screws. Extend the
counterpoises and support them along the way by affixing them to a tree with electric fence post insulators. They are
perfect for this application. I have attached photos of the feed point idea and the insulators. I suspect the server strips
the attachments. Scott, I emailed you direct so you will have the photos.
Here is a copy of the text from the document, by WA2VUO.
The Inverted "L" Antenna
So, you REALLY want to get on the 160 Meter band, but the neighbors won't let you run your 250' long
dipole over their swimming pool? And, you say that the State DOT looks down on your attempt to tie the other end across the
State Highway? Well, fear not, as there is a compact solution to your MF antler woes, and it is an Inverted "L". [It can also
be cut for 80 or40].
The Inverted "L" is a 1/4-wave Marconi, fed against ground or a counterpoise such as radials. The
antenna displays a low feed impedance, which allow you to feed it directly with 50-ohm coax cable. The vertical section
should be as high as possible, with the remainder of the antenna run horizontally to a support, such as a tree or a mast. The
antenna is fabricated from wire, with the heavier the gauge the better. [The larger the wire diameter, the wider the
bandwidth]. One method of installation would be to use a mast as the vertical "wire", with the horizontal section firmly
bonded to the top of the mast. The wire is cut to a resonant 1/4-wave length, based on the formula:
L(Ft) = 234/F(MHz)
If your ground is REALLY good, like your house is on stilts over the ocean, a counterpoise won't be
needed, but for our GREAT ground here in Western NY, figure on using at least one counterpoise, if not a few. The length of
the counterpoise wires will be cut to a 1/4-wavelength by the formula:
L(Ft) = 246/F(MHz)
Basically, the counterpoise should be high enough to not get in the way, but could be as high as your
installation allows. Mine is run around the base of the house, just below the siding, except around the back door. At that
point, I ran the wire under the bottom of doorframe. The wire I used is #18 stranded insulated hook-up wire.
Well, any antenna discussion is not complete without a chart of some sort, so here are the dimensions
for an Inverted "L", listed in tabular form:
Frequency Radiator Length Counterpoise Length
1.85 MHz 126.5' 133.0'
1.95 MHz 120.0' 126.2'
3.60 MHz 65.0' 68.3'
3.90 MHz 60.0' 63.1'
7.15 MHz 32.7' 34.4'
From: towertalk-bounces at contesting.com [mailto:towertalk-bounces at contesting.com] On Behalf Of Scott MacKenzie
Sent: Sunday, September 19, 2010 19:26
To: towertalk at contesting.com
Subject: [TowerTalk] Inverted L Dimensions
I have decided to take the plunge and put up a simple antenna for 160M.
It has to be simple and not excessively expensive;
I have limited room for antennas on my lot, and very limited room for
I do have multiple tall trees that I can use to hoist a wire 60 feet + in
My soil, if you want to call it that, is poor and rocky (typical Eastern
My application is casual DXing and working stateside.
Based on this, it looks like an inverted L is probably my best choice. I
want to feed it with 50 ohm coax. Now the questions are pretty simple:
What dimensions are best - 5/16 or 1/4 wave? Both are indicated in the
What is simpler to match?
Am I better off making a loaded vertical approximately 60 feet long?
I appreciate your help.
Scott aka KB0FHP
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