Sunday, July 29, 2012

Simple Antennas for Amateur Radio Operators--a continuing series

KH6JRM's Amateur Radio Blog

Post #176

A compact 40-meter vertical

There are times when a small vertical comes in handy.  You could be in a space restricted environment such as I where neighbors are almost back to back or you could be looking for an easily portable antenna useful for mini-DXpeditions or a casual day of operating from a park or beach.  There is a solution to this often vexing problem.

If you have the resources, you may want to consider screwdriver antennas, adaptations of various mobile antennas (ham stick), or event the handy Buddipole sytem.  But if you're on a restricted budget and willing to "roll your own", you can find all the materials you need at the nearest hardware store.

What I wanted to build was as a top-loaded "vertical helix" that could be erected in my backyard and easily hidden by bushes and trees bordering my qth.  Based on various readings in antenna literature, I found that if you wound a half-wave length of ordinary AWG #22 gauge hook up wire in a spiral along a pvc or wooden pole, the antenna would behave as a quarter-wave vertical for your chosen frequency.  As in any vertical, a good ground system would be needed.  In my case, I had a 10-foot piece of schedule 40 pvc pipe (2" diameter) under the house which could be used to wrap the wire.  For my 40-meter helix, I wound 66-feet of #22 wire along the length of the pipe and attached a 4-foot piece of an old CB antenna to the spiral to provide a bit of top-loading.  At the bottom of the pvc pipe, I attached one lead of some 450-ohm twin lead to the spiral or helix and connected the other part of the twin lead to 4, 33-foot radials.  The twin lead ran into a W9INN 4:1 balun, which was attached to approximately 25 feet of RG-6 coax (that's what I had in my storage box).  The coax was attached to the Drake MN-4 tuner, which was connected by a short piece of coax to the Swan 100-MX.

I adjusted the old Swan transceiver to 7.040 Khz, checked out the signal on the dummy load, used the Drake MN-4 to reduce the swr, and fired off a CQ.  Everything seemed to work alright, although the bandwidth was quite narrow.  From everything I've read about HF vertical helices, the impedance of the antenna is around 5 ohms, so this mismatch may preclude the use of coax for multi-band use.  The antenna works on 40, 20, 15, and 10 meters.  Performance does not match what I get from a dipole or inverted "vee".  But, with ladderline, I can get contacts on those bands, usually a s-unit below what I get on the dipole.  Your results may very, depending on the number of radials you use and the proximity to nearby objects.  As you may have guessed, this system is quite primitive and can use improvement.  The good thing about this antenna is that it's easy to build and easily hidden.  I plan to attach a better ground system when the weather improves.  I can break up the pvc support pipe into two, 5-foot sections, which fit comfortably into my van for portable use.

Now that my vacationing neighbors are scheduled to return on 31 July, it's time to take down by temporary 40-meter "long wire" (see last post).  This distant cousin of an off-center-fed dipole did fairly well under marginal propagation conditions.  I will roll up the longer 100-foot and shorter 35-foot sections and put them back in the storage chest.

I trust that your weekend was pleasant and that you had some time to work with homebrew antennas--that's half the fun of amateur radio.

Until next time,

Aloha es 73 de KH6JRM--BK29jx15