Thursday, December 25, 2014

Portable Vertical HF Antenna Proof of Concept

Yesterday was the first day of my Winter vacation, and I took a ride down to the Delaware Ham Radio Outlet. My goal was to get some parts needed to construct a portable vertical antenna for 20M to 6M, based on the “FXTenna” by KD5FX.

KD5FX’s design centers around the MFJ-1979 telescoping stainless steel whip, which extends to 16.9 feet. I also wanted to try a 20M Hamstick-type antenna, so I picked up an MFJ-1620T HF Stick. Both are commonly used for portable ops, and have a 3/8”x24 threaded male stud at the base end. This is the same thread as most CB antennas.

Along with the MFJ-1979 and –1620T, I bought three 3/8”x24-to-SO-239 studs, an Anderson Powerpole-to-alligator clip with a fused negative cable, and a package of Bongo Ties for cable organization. I also went to the Radio Shack a few doors down and picked up a couple crimp-on PL-259s.

After getting home and grabbing something for lunch, I fabricated a mount for the antenna elements, mostly using stuff I already had in my shop. The 3/8”-24-to-SO-239 stud was attached to a couple pieces of steel strap that I salvaged from the trash at work. These were held to a 19” long piece of 1/4” drill rod.

Since the radiating element of a vertical antenna needs something to work against, I made six 17’ long radials from 18 gauge speaker wire, which had ring terminal added to the ends. Some heat shrink tubing was then placed over the terminals and a couple inches down the wire, to reinforce the connection.

The radials attach to the steel strap with a machine screw and a couple nuts.

Since pictures are worth a thousand words, take a look below.

Mount next to collapsed MFJ-1979 and a yardstick for scale.

As you can see, once the kinks (described below) are worked out, this will make a nice, portable antenna.

One thing I learned the hard way was that in order to prevent creation of a Gordian knot, it’s important to wind up each radial individually, hence the multiple Bongo Ties.


Detail of top of mount:


Detail of mount bottom:


It turned out that the steel straps weren’t rigid enough to support the weight of the MFJ-1979, so I swapped on the MFJ-1620T. The provided marginal support, and the wind today didn’t help:

The ends of the radials were secured in place with large nails driven into the ground.

Unfortunately, the mount wound up being too flimsy. It would probably work if I guyed the antenna, but I want something more rigid so that guys are not necessary. Also, the hose clamp connection between the straps and the drill rod stake are not very secure. I accidentally pulled the mount right off the stake at one point.

So, design defects aside, how’d it work as an antenna?

With my Yaesu FT-817ND powered by a 12V battery and pumping out a whopping 5 watts, my PSK-31 signal reached northern Indiana, Columbus, OH, and Lexington, KY, according to I only saw a few other signals on the waterfall, but the 20M band wasn’t doing very well, per comments I saw in PSKer.

Still, not bad. In fact, it’s a downright promising proof of concept.

Things to do:

  • Make the base more robust by using a CB antenna mirror mount attached to the stake. I may replace the steel drill rod with a lighter weight aluminum rod.
  • Grind a point on the end of the stake, to make it easier to drive into the Earth.
  • Replace the nut securing the radials to the mount with a wingnut so they can be removed without tools. I didn’t have one of the correct size handy.

Incidentally, the ring terminals, shrink wrap, and hose clamps all came from Harbor Freight variety packs. These are cheap and handy to keep around for when you need a miscellaneous part.

Another experiment for the future is to get a second 20M hamstick and an MFJ-347 mount, then construct a hamstick dipole that can be hoisted from a tree in a vertical orientation, or raised on a painter’s pole for horizontal polarization. Yet another idea is to cut a length of wire, put an alligator clip on one end, then connect it to the top of the MFJ-1979 and run it horizontally as an inverted-L antenna. I might be able to get it to work on 30M and 40M this way.

This kind of experimentation is part of the fun of ham radio, and builds your knowledge base so that you can provide communications under less than optimal conditions, or using improvised equipment.

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