Sunday, July 7, 2013

A simple 80 meter inverted "L" antenna. Post #209

Because my back yard is rather small, I haven't been able to erect a decent 80 meter dipole antenna.  In the past, I've used an inverted vee with elements measuring 67 feet/20.42 meters on a side.  Although the antenna worked reasonably well, it barely fit in back of my house and was clearly visible to my neighbors.  Another approach was needed.

I decided to build an inverted "L" antenna, since I had some room to go up (vertical) and some room to go horizontal (flat top segment).  If I used a lightweight wire (#20 or #22 gauge wire) and placed a mast among some trees bordering my lot, I could have a working 80 meter antenna with some degree of stealth.

An inverted "L" is a form of bent vertical, with the vertical section running up a mast for 1/8 wavelength (or more, if possible) and a horizontal wire running for 1/8 wavelength from the top of the mast.  Like all verticals, I would need a ground radial system to maximize efficiency and cut losses.

Although I could feed the antenna with 50-ohm coaxial cable, I decided to use 450-ohm ladder line in conjunction with a 4:1 balun and an antenna transmatch to operate on 80, 40, 20, 15, and 10 meters.

MATERIALS:

Using the general formula, 234/f (MHz)=L (feet), and the chosen frequency of 3.500 MHz, I cut 67 feet/20.42 meters of #22 AWG wire for the length of the 80 meter bent vertical.  The actual length was 66.85 feet/20.38 meters, but I rounded off the length to 67 feet/20.42 meters.

A slingshot, fishing line, and a sinker to launch the horizontal portion of the antenna into a tree notch approximately 30 feet/9.14 meters above ground.

A 33 foot/10.06 meters telescoping fiberglass mast (MFJ brand).   This mast would support the vertical portion of the inverted "L".

Six ceramic insulators to secure radial wires, the horizontal wire to the tree, and an insulator at the tip of the mast to route the remaining wire down the mast.

Four, 7 foot/2.13 meter wooden stakes to support a rudimentary radial system.

One, 5 foot/1.52 meters wooden stake to support the mast.

Four, 30 foot/9.14meters pieces of #22 AWG wire for the radial system.  These rudimentary radials are not a resonant 1/4 wavelength, because my lot is not large enough to accommodate a full 67 foot/20.42 meters radial in any direction.  As it was, I had to "snake" and bend the 30 foot/9.14 meters radial wires through the garden and along the border of my property.  My space is a bit restricted.

Fifty feet/15.24 meters of 450-ohm ladder line, a 4:1 balun, and an antenna transmatch (Drake MN-4).

Twenty five feet/7.62 meters of RG-8X to run from the 4:1 balun to the Drake MN-4 in the shack.

Small patch cords to connect the antenna transmatch to the rig (Swan 100-MX, the low pass filter, and the dummy load.

Nylon ties, vinyl electrical tape, soldering kit.

Log, note paper, pencil, J-38 key, and a Shure 444 microphone.

ASSEMBLY:

I first launched the antenna element into a convenient branch of a Norfolk Pine Tree bordering the south end of my lot.  The slingshot, fishing line, and sinker made the launch fairly easy.  I tied off the fishing line to an old fence post near the tree.

I then threaded the antenna line through the insulator at the top of the mast.  The insulator was secured to the mast by several nylon ties.  The horizontal portion of the antenna was 37 feet/11.28 meters.

The remaining portion of the antenna, 30 feet/9.14 meters, was secured by nylon ties to the mast.  That segment would be the vertical portion of the antenna.  The end of the antenna was 3 feet/.91 meters above ground.

The mast was hoisted onto its support stake.

One wire of the 450-ohm ladder line was soldered to the vertical portion of the antenna, while the other was soldered to four, 30 foot/9.14 meter elevated radials.  The radials were approximately 3 feet/.91 meters above ground.  One  radial was placed along the path of a east-facing garden and another was laid along the south-facing border of my property line.  One radial led into an abandoned lot bordering my property.  The fourth radial ran along side a pvc water pipe near the north facing side of my house.

Once the antenna was raised, it had a horizontal component of 37 feet/11.28 meters and a vertical component of 30 feet (9.14 meters).

INITIAL RESULTS:

For a compromised antenna system, it performed as well as my 80 meter inverted vee.  Although the radial system is far from ideal, the inverted "L" has been able to gather both local and DX contacts on 80, 40, 20, 15, and 10 meters.  With the help of the 4:1 balun, the 450-ohm ladder line, and the trusty Drake MN-4 antenna transmatch, I've been able to keep SWR on all bands below 1.7 to 1.  Antenna tuning is a bit challenging at times, but I can use 80, 40, 20, 15, and 10 meters without much difficulty.  Obviously, the radial system must be improved. But, for now, I'm enjoying the late night "adventure" on 80 meters.

REFERENCES:

http://www.hamuniverse.com/slopinginvl.htm.
http://www.hamuniverse.com/w7lpninvertedl8010.html.
http://www.youtube.com/watch?v=uuD2mBZKHO9.
http://www.dxzone.com/dx16525/allband-inverted-l-antenna.html.
http://www.amateur-radio-wiki.net/index.php?title=Inverted-L_antenna.
http://www.praisescribe.com/InvLAnt.pdf.

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Aloha es 73 de Russ (KH6JRM).

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