Saturday, July 27, 2013

A 40-Meter Inverted L Antenna. Post #216

During the past few weeks, I've been experimenting with the versatile inverted L antenna for my small house lot.  I find the antenna simple, inexpensive, and easily concealed.

An inverted L is a bent quarter-wave Marconi vertical fed against a system of surface, buried, or elevated radials.  The vertical segment should be as high as possible, with the remaining length running horizontal to a nearby tie off point.  Coaxial feed line can be used for monoband operation or 450-ohm ladder line can be employed if multiband use is planned.


Two vertical structures to support the antenna--one for the vertical segment and the other to tie off the horizontal wire running from the top of the mast.  I had a homemade 20-foot/6.09 meters pvc mast under the house and a convenient Norfolk Pine Tree at the edge of my property to support the horizontal portion of the antenna.

Sixty-six feet/20.12 meters of #14 AWG house wire for the radial system and the antenna element.

Three, 6-foot/1.82 meters wooden stakes.  One stake would support the vertical mast, while one stake would support an elevated radial running from the mast.  The elevated radial would be approximately 5-feet/1.52 meters above ground.  The remaining wooden stake would support the 450-ohm feed line midway between the mast and the garage wall, where a 4:1 balun would be placed.  The ladder line was not allowed to touch the ground.

If you plan on monoband operation, you can use 50-ohm coaxial cable for your feed line.

Four ceramic insulators, one to tie off the horizontal portion of the antenna to the tree, one to attach to the tip of the mast, one to tie off the bottom of the antenna element  on the mast, and one to tie off the elevated radial wire.

A launching system to put the upper portion of the antenna through a tree branch.  I used a slingshot, 50-feet/15.24 meters of fishing line, and a sinker to place the antenna up in the tree.

A W9INN 4:1 balun.

Twenty-five feet/7.62 meters of RG-8X coaxial cable.

A suitable antenna transmatch.  I use a Drake MN-4 in the shack.

Various tools, including soldering equipment, wire cutters, vinyl electrical tape, nylon ties, etc.

Short coaxial cable patch cords to connect the Drake MN-4 to the Swan 100 MX transceiver, the low pass filter, and the dummy load.


Using the general formula 234/f (MHz)=L (ft), I calculated the length of the quarterwave vertical for my chosen frequency of 7.088 MHz.  This came out to 33.01 feet/10.065 meters.  I rounded off the figure to 33-feet/10.06 meters.  With my lowest frequency in the 40-meter amateur radio band, I could have multiband capability between 40- and 10-meters with the use of 450-ohm ladder line as my feed line.

Although many antenna experts recommend a slightly longer length for the radial wire (about 5%), I decided to keep the radial wire the same length as the antenna element (33-feet/10.06 meters).

Once I cut the wire to the proper length, I attached the antenna wire to the monofilament fishing line and shot it over a pine tree bough approximately 35-feet/10.67 meters above ground.  I left the antenna wire loosely hanging from the tree branch until I connected the wire to the top of the mast.

I attached a ceramic insulator to the tip of the mast and then threaded the antenna wire from the tree branch through the top insulator and down the vertical 20-foot/6.09 meter mast.  The antenna wire would be secured slightly above ground once I got the antenna adjust for proper tightness.

I hoisted the pvc mast onto its support stake, tied off the fishing line holding the upper portion of the antenna to the pine branch, and tightened the antenna by bringing the vertical portion down to a point approximately 5-feet/1.52 meters off the ground. Nylon ties were used to secure the wire to the pvc mast. The antenna sloped slightly upward from the tip of the mast.  At the 5-foot/1.52 meter mark on the mast, I attached and soldered the 450-ohm ladder line to the vertical element and the elevated radial.  The radial was tied off at a wooden stake approximately 5-feet/1.52 meters above ground.  The elevated radial ran through the garden and was not visible from the street.

The ladder line was run off at a right angle to the vertical mast to a 6-foot/1.82 meters wooden stake a short distance away.  The ladder line was kept off the ground until it was attached to the W9INN 4:1 balun mounted on the garage door.

A 25-foot/7.62 meters length of RG-8X coaxial cable was connected to the 4:1 balun. The cable was routed through a homebrewed patch panel in the shack window to the Drake MN-4 antenna transmatch.  Short patch cords connected the transmatch to the Swan 100 MX, the dummy load, and the low pass filter.


With the help of the Drake MN-4 transmatch, I was able to get a SWR of 1.1 to 1 on the 40, 20, 15, and 10 meter amateur radio bands.  With an output of 50 watts, I was able to get reception reports between 569 to 599 on cw and 55 to 59 on ssb.  My best band was 40-meters, followed by 15- and 20-meters, where the tuning was a bit more critical.

Performance should increase when I add more elevated radials.  Fortunately, there is sufficient room to run a few more radials through the garden and in the area near the Norfolk Pine Tree.  I ran the single elevated radial as an experiment.  It works, but more radials would surely help the efficiency of the antenna.

Since the mast is colored a dull grey, it can hardly be seen from the street in front of my house.  The antenna wire is nearly invisible, too.

For an inexpensive antenna, the performance is adequate for my purposes.  I get great local coverage and decent DX from my location on Hawaii Island.


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Aloha es 73 de Russ (KH6JRM).

BK29jx15--along the beautiful Hamakua Coast of Hawaii Island.