Sunday, February 2, 2014

Simple Ham Antennas: The AG9C Loop Antenna. Post #258

Over the past 37 years as a licensed amateur operator, I've accumulated a wide variety of antenna reference material, including books, magazine articles, and topics discussed on amateur radio forums.  All ARRL members can further augment their antenna research by accessing the digital files of "QST", the offical journal of the ARRL.  All told, there is an almost endless resource of antenna building ideas for most every ham station.

Recently, I began to put some of these "classic ideas" to use on my new property in the Puna District of Hawaii Island.  I now have an acre of space to "plant" my antenna "farm"--quite a change from my present rental home which is hemmed in by neighbors and utility poles.  Although my neighbors have been tolerant of my amateur radio pursuits, I try to keep a low profile.  Namely, my verticals and inverted vees are usually lowered when they are not in use and, because of my part-time employment as a sports announcer for my former radio station employer and my work as a substitute teacher, I can usually operate when everyone else is either at work or asleep.

But with new spaces available for antennas, I'm building better antennas to fit my operating preferences.

So, with that in mind, I've begun a serious program of antenna design and building--something once denied to me by severe space restrictions.

To date, I've  posted a few articles detailing my attempts to build antennas that don't require "tuners" or transmatches.  I have two excellent "tuners" at my disposal--an old Drake MN-4 and a current model from MFJ, the 941-E versa tuner II.  The MFJ is used for my 80-10 meter ladder line-fed doublet, while the trusty Drake is used to "smooth out" the small mismatches on my 40-10 meter inverted vee and the recently built "Fan Dipole".  The 40, 20, and 10 meter "Fan Dipole" can be used without a "tuner", since the swr for each dipole is less than 2 to 1 across each band.

On Saturday, I decided to build another "tunerless" antenna.  This time around, I consulted the late William I. Orr (W6SAI).  In his "HF Antenna Handbook", he describes a fascinating loop antenna designed by his close friend Bob (AG9C).  I built a copy of this intriguing antenna this past Saturday at the new place.  I found it to be a "very forgiving" design for my location.

MATERIALS:

I configured the AG9C as a vertical loop antenna with the horizontal portion of the loop running between two self-supporting telescoping fiberglass masts.

Two tall supports.  I used two 33-ft/10.06 meters MFJ telescoping fiberglass masts.

Two ceramic insulators to shape the horizontal delta loop.

One hundred thirty nine feet/42.37 meters of #14 AWG house wire for the loop.

Two five-foot/1.52 meters wooden stakes to support each mast.

One 4:1 current balun.  The bottom of the loop will be connected to each terminal of the balun.  I had a spare W9INN balun in the shack.

One plastic storage bin to support the balun and to enclose the balun when operating is done.

Fifty feet/15.24 meters of RG-8X coaxial cable with UHF connectors.

Two small battery clips.

Six feet/1.82 metes of RG-8X to run from the window patch panel to the station equipment.

Several 3-ft/0.91 meters lengths of RG-8X coax to interconnect station equipment.

ASSEMBLY:

I built the loop antenna on the ground.

I laid out a roughly equilateral triangle on the ground.

I fastened a ceramic insulator to the top of each mast.

The antenna was threaded through each insulator.

I slowly hoisted the horizontal loop into position, with the horizontal part of the delta loop measuring 46.33 feet/14.12 meters.

The remaining parts of the horizontal delta loop were brought down to the 4:1 balun sitting on the raised plastic storage box.  These sections were attached to the terminals of the balun with two small battery clips.

The RG-8X coaxial cable feed line was attached to the balun and run through the patch panel in the shack window.  A 6-foot/1.82 meters piece of RG-8X was run from the patch panel to the Drake MN-4 transmatch.  The transmatch was bypassed except for the SWR meter and power level indicator.

Several short pieces of RG-8X coax were used to interconnect the transmatch, low pass filter, and dummy load to the Swan 100 MX transceiver.

INITIAL RESULTS:

I was quite surprised how well the vertical  loop worked without the Drake MN-4 or MFJ 941-E versa tuner.  The swr was less than 2 to 1 across the 40, 20 and 15 meter bands.  With the MFJ 941-E versa tuner in line, I was able to use the horizontal loop on 80, 30, and 10 meters.  It should be noted that I found, like AG9C, the input impedance of the loop on 80 meters was high, as it is a half-wave resonance.  None the less, a transmatch easily matches the feed line to the transmitter.  As AG9C states, "antenna radiated power is reduced, but adequate, over the CW portion of the 80- meter band."  I won't be using the antenna on 80 meters, anyway.  I'll  defer to my ladder line- fed 80 to 10 meter doublet for 80/75 meter operation.

I kept the Swan MX 100 below 50 watts and had some excellent contacts with the South Pacific and the mainland United States.  This was a fun antenna to build.  I had all of the materials on hand, so a trip to Radio Shack or to Hilo Ace Hardware wasn't necessary.  Best of all, I didn't have to install a ground radial system.

REFERENCES:

Orr, William I. (W6SAI). The W6SAI HF Antenna Handbook.  CQ Communications, Inc.  Hicksville, NY, 11801.  Fourth Printing, 2005. pp. 5-4 to 5-5.  This book is full of interesting antennas that can be built at low cost in space restricted areas.

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Aloha es 73 de Russ (KH6JRM)

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



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