Writing An Article

From Adventure Radio Society

TAPE MEASURE DIPOLE

OR The Variable Band Portable Tape Measure Dipole

OR The Resurrection of the Tape-Tenna

by K4ALE, Bevin Alexander, Lynchburg, Virginia

Although I am generally looking for lightweight backpacking antennas to use with my KX-1, Small Wonders tranceivers, or the like, the problem is that antennas that are efficient on more than one band are technically hard to build, and often a pain to erect on the trail. I was hoping to find something that could work equally well on 20m, 30m, and 40m.

Wandering around home improvement stores is fun for techno-geeks like amateur radio guys, even when they really are not so technical. On a recent visit to Harbor Freight, I saw their 33 foot carpenter's tape measures on sale for about $4 apiece. I had recently seen an ad in an old QST from the late 1970s for the now extinct "Tape-Tenna," and it occurred to me that this might be a good project.

Eight dollars later, I was at home rooting around for alligator clamps and a generic coax dipole connector from Wireman. By the next morning, the two tape measures were glued together, back to back, using some of the new high-tech "foaming" super adhesive. This adhesive is waterproof and amazingly durable. Through the middle of the joint [running "up" and "down"], I included a short length of thin, strong braided nylon cord. I had salvaged this cord from some window blinds. The cord is about 1mm thick, but extremely strong. Overnight, the glue set. I had enough cord protruding from the top and bottom to tie two loops: one above the tapes to hang from a tree support, and one below to attach the coax-dipole connector. I soldered the alligator clips to two lengths of good stranded wire, and then to the two leads from the coax-dipole connector. The connector was tied to the lower loop, and out to the trees in the backyard for some testing.

I keep a length of nylon rope over a branch in a tree about 23 feet off the ground as an antenna "test platform." Using the standard dipole formulas as a starting point, I pulled the two tapes out to that suggested "leg" length. I started with 10m first. I then scrapped off some of the paint on the tape at the suggested points, clipped on the coax connector, and hauled the antenna up to the tree limb. The legs are held "extended" by the locking mechanism of the tape measures.

Using an MFJ antenna analyzer, what to my wondering eyes should appear, but a 1.1:1 SWR a little bit below the target frequency. More scrapping at intervals of 4 inches, and soon I had an antenna that appeared to be resonant along the entire breadth of the 10m band. Apparently, the one-inch wide radiator contributes to a broad bandwidth.

I proceeded to do the same for 15m, 20m, 30m, and 40m. At 33 feet of maximum length of tape, 40m is the lowest frequency band for which these tapes can be "cut", but that is fine for me on the trail. SWRs were at 1.1 or 1.2 at their lowest on all 5 bands, and never exceed 1.5 or 1.6 at band edges, even with the height of the center only 23 feet off the ground. The legs were held up by light cords attached to the ends of the tapes, tied off at eye level, making the antenna a slightly inverted V. The wound "excess" inside the tape reels does not appear to affect the performance much, if any.

The next test was how it worked on the air. Using a battery powered SGC SG-2020 at about 20 watts, I quickly made both SSB and CW contacts on 20m, 30m, and 40m to stations over 600 miles away, even under very noisy and "low" propagation conditions. It works and is now a top choice for portable antenna use.

Advantages:

    - multi-band coverage
    - easy installation with one main support point and two "tie downs" for the tape ends
    - small size when rewound
    - idiot resistant

Disadvantages:

    - the antenna has 66 feet of 1-inch steel tape, which weighs about 2 pounds

You will need lengths of nylon line for the center and ends, coax as a feed line, and the means of getting the center up in a tree. I carry a small cloth bag, which I fill with sand or gravel at the destination. It is easy to toss this over a volunteering limb, with a little practice. It is a good idea to attach the bag to the support rope first, however, and you should practice swinging the bag over the limb with an underhand swinging motion. This is how arborists launch lines into trees, and it works well for radio dudes, too.

Summary: This antenna is not the first choice of those doing serious or ultra-light backpacking. However, for a hike or short backpack trip where the goal is more focused on good radio operation, the flexibility and efficiency of this antenna justify the extra weight. I have since used it on a hike to a mountain top on the Appalachian Trail, which passes within 20 miles of my home QTH. I was very pleased with the performance as a QRP antenna, and it equaled or exceeded my other single band designs. I prefer the ability to get a good SWRs on the three bands on my KX-1, and resonance of the radiators is far more important to good propagation than matching whatever I have up with the on-board ATU. Although I have built and used both the excellent half-square and vertical bazooka designs elsewhere on the ARS Wiki, this antenna performs as well or better on all bands. If a planned trip is by canoe, this antenna is small and waterproof, and is easy to erect from trees along the river bank.

Enjoy.

73,

Bevin Alexander, K4ALE