Wednesday, June 27, 2012

SOMETHING DIFFERENT FOR THE ANTENNA EXPERIMENTER

The weather has improved to a point that I can get outside and play with antennas again.  Other than a twice-daily walk and jog with my xyl, the weather has kept both of us inside.  June on Hawaii Island often brings many days of showers, and this past week was no exception.  So, when the sun finally broke out for several hours, I rushed through my daily chores and proceeded to the postage stamp lot in back of our rental house for some serious antenna work.

Because the inverted vee, loop, and "upper and outer" antenna were doing well, I decided to make a vertical helix for 40 meters using some short pvc pipe, extra #22 AWG wire in the shack, and some 450-ohm balanced line into a balun and the trusty Drake MN-4 ATU.

According to information I found in several ARRL publications, a quarter wave vertical could be created by winding a half wavelength of wire around a sturdy pole and topping the end with a capacitance hat.  I've seen amateurs use vertical helix antennas designed for 80 meters with some good results.  Of course, a good radial field is necessary to make these shortened antennas perform.

Anyway, I wound 66 feet of #22 AWG wire along a 10-foot piece of schedule 40 pvc I had under the qth.  According to theory, that arrangement should give me an antenna roughly equivalent to a 33-foot vertical.  I attached an 18-inch capacity hat to top of the wire helix, connected one lead of 450-ohm wire to the helix, connected the other lead to a 33-foot counterpoise, and joined the entire system to a 4:1 balun and the Drake MN-4.  The homebrew vertical helix had a very narrow tuning range, but it did work on 40 and 15 meters with the help of the Drake MN-4.  I tried the helix on 20 and 10 meters with some degree of success, but the antenna seemed to work best on 40 and 15 meters.  The Drake MN-4 remained cool on all bands and I did not get any rf "bite" in the shack.  I was running 15 to 20 watts cw with no problem.  I tried a few SSB contacts at 50 watts and was pleased to get some good reports.  The vertical helix probably has more losses than my trusty inverted vee, but it does work and it does let you be heard.  Of course, results could be better if I had enough room to establish a decent radial system.

The vertical helix is easy to build, easy to disguise, and very portable.  The antenna is nearly invisible from the street in front of the qth, owning to its small size and proximity to local trees.  As is the case with my other antennas, the vertical helix can be swiveled down to ground level when I am not using it or when storm clouds approach.

I may use another helix on a longer pvc pole to get on 75 and 80 meters--frequencies which are difficult to operate from my location. 

I will let you know how that project turns out.  Meanwhile, have a good day and get on the air with something you have designed and built yourself.

Aloha es 73 de KH6JRM--BK29jx15