Thursday, April 12, 2012

Simple Antennas for the Hawaii Island Amateur Radio Operator--a continuing series

One of the joys in making your own antennas is to discover what others have done in similar circumstances.  A visit to the antenna forums on QRZ.com and eham.net can give you a lot of useful information, particularly if you must operate from a deed-restricted area or from a small backyard as I do.  I am amazed at some of the clever antenna designs that my fellow amateur radio operators have used to get around HOAs, CC&Rs, and lack of space.

Another good source of ideas for those of us in space-restricted areas are books and articles dealing with small antennas, stealth antennas, and concealed opertions of various sorts.  Although most of us can get by with the antenna books published by the RSGB, ARRL, and CQ Communications, there are a few sources I would recommend from personal experience.

The first book on my list would be "The Short Vertical Antenna and Ground Radial" by Jerry Sevick (SK), W2FMI.  This small volume offers several useful and effective designs for the small backyard.  Presently, I am using one of Sevick's vertical helix antennas near my wooden garage.  I wound 33 feet of #14 gauge wire on a 10-foot pvc pipe, stuck it on a 4-foot stake, attached eight, 10-foot radials, and ran some 450-ohm balanced line to a 4:1 balun, which was connected by a short length of RG-6 coax to the trusty Drake MN-4.  Apparently, by winding a half-wave's worth of wire around the pvc mast in the form of a helix, it approximates a regular quarter wave antenna.  I added some top loading with a 10 inch pie tin (aluminum) and I was in business.  The bandwidth is quite narrow, but the Drake MN-4 can make the antenna cover most of 40-meters.  I can use the helix from 40 to 10 meters if I have to.  The tuning is sharp but manageable.  I suspect the antenna has some pronounced ground loses, but it does get out and I do make contacts beyond Hawaii with RST reports ranging from 539 to 579 (cw).  Not great...but this antenna does allow you to get on the air.  One of the benefits of this homebrew skyhook is its ability to blend in with the trees bordering my lot.  You can't see the antenna from the street.  I run anywhere from 05 to 30 watts with this antenna without any rf "bite" in the shack.  I think I will keep this antenna as an emergency backup or for portable operation.  I can divide the vertical helix into two, 5-foot sections and pack the antenna wire and radials in a small bag.

Another book I have found valuable is "HF Antennas For All Locations" by Les Moxon (SK), G6XN.  Although the book may be a bit complex for those of us non-technical types, it contains enough examples of simple antennas to keep us busy for years.  Moxon writes well and he supports all of his designs with the appropriate drawings and mathematical analysis.

And, of course, there are those antennas that I just build and test for the sheer joy of seeing if they work.  As you may suspect, some of my great ideas have been less than spectacular, but that's how we learn.  Antennas are the one area where every amateur can do something on his or her own initiative.  Half the fun of building antennas is getting parts locally and modifying well-known designs to fit your particular situation.  Over the past several decades, hardware and home improvement stores have given me most of the parts required for my simple backyard antennas.  As for coax and wire, I've become friendly with cable installers and former radio station associates, who often supply me with cable end runs and various lengths of wire from studio rebuilds.  I've learned to appreciate RG-6, since most of these cables are available free or at minimum cost from installers.  With the proper connectors, RG-6 is suitable for antenna feed lines, patch cords, and impedance transformers.  As for antenna wire, I've used everything from #22 gauge hookup wire to "zip cord" used for wiring lamps and electrical appliances.  I wouldn't recommend "zip cord" for feedlines because of line losses, but it does make for good radiating elements in a dipole.  The plastic insulation on the cord provides some protection from the elements.

The important thing is to use your imagination and locally obtainable materials to "roll your own" antennas.  Your designs will improve as you gain experience.  Along with some antenna design software and your PC, you can quickly fashion an antenna that will give you the performance you require at a minimum cost.   Besides, you will have the pride in knowing you built it yourself without "breaking the bank."

That's about all for today as I prepare to return to my teaching duties on Friday.  Have a good weekend and build an antenna that you can call your own.

Aloha es 73 de KH6JRM--BK29jx15