Fishing Pole Tuner

Fishing Pole Tuner

Here are the details of the tuner that I built for use with my "Fishing Pole Antenna."

Recall, the antenna is about 35 feet of insulated wire (20m half wavelength) wound on a 20-foot "Black Widow" fishing pole, and is tuned as a random wire. The antenna is worked against a single counterpoise, which is about a quarter wavelength long. The antenna and tuner work at least from 20m to 10m; further testing is needed for 40m and 6m (which may be outside the range of the selected components).

The tuner is a simple L-network, consisting of a tapped toroidal inductor, a variable capacitor, and a switch to select taps on the inductor. To describe the schematic (see below): Input is via an SO-239. Output is via banana jacks, one for the antenna and one for the ground (counterpoise). The inductor is in series between the input and the output. The capacitor is across the output, that is, it is connected between the antenna jack and ground. The switch selects taps on the inductor.

Parts list: The switch is a Radio Shack #275-1385 single pole, 12 throw rotary. The capacitor is a receiving-type air variable, about 25 to 250 pf. (The low end appears to be the most critical value, at least on the higher bands. My first try was with a 40-400 pf, and it would not go low enough.) Almost anything will work here; I have used mica compression trimmers at QRP power levels, or you could build a home-brew capacitor along the lines of the one shown in a recent QST. The inductor is 37 turns of #22 enameled wire wound on a T-94-2 toroid core (0-15 microhenries), with taps a 2, 3, 5, 7, 8, 11, 15, 18, 24, and 29 turns. (Ten taps plus the two ends = 12 connections.) The inductor is about the same diameter as the switch, and is mounted directly on the back of the switch. (This turned out to be easier than I thought!) The first several taps give fairly closely spaced, relatively low values of inductance, the last few give 5, 7.5, 10, and 15 microhenries. (Note: measured values of the larger taps agreed well with the formula, but the smaller taps disagreed considerably. By formula this inductor should have about 43 turns with the first tap at eight turns, but based on measurements I removed the first six turns to get suitable results.) The box I used is a Serpac #211 made of light gray ABS with dimensions of 4 x 2.5 x 1.5 inches. Brass shim stock (0.010 inches) was used inside the front of the box to provide a common ground for components. Layout is not critical. (In the "good old days," you might have built such a tuner on a pine board, with nails for binding posts.)

For /PM work, the tuner attaches directly to the rear of the FT-817 with a male-male coupling. If this configuration is desired for use on the operating table, consideration should be given to placement of the SO-239 so that the tuner box does not extend below the bottom of the xcvr. Of course, a short coax jumper can be used also. For /PM use I simply strip a few feet of wire from the antenna pole and run it to the banana jack on the tuner.

Tuning is simple. I find this works best with the preamp on (IPO off for the FT-817), since this gives more background noise to use as a tuning indicator. First, peak L and C for maximum noise. Then transmit a short carrier and adjust L and C for minimum SWR. (The RS switch is make-before-break, so there is probably no harm in switching under load, but use your own judgment here to protect both your rig and the switch.)

That's all there is to it. It took me hours to build this, mostly because I proceeded very carefully with the layout to maximize compactness and to minimize ruined components! If I had had several spare boxes I probably could have worked faster, but I only had one. The end result is a very nice looking piece of equipment that complements the appearance of the FT-817. (I still ended up with two "dummy" screws on the front panel to fill misplaced holes. See if you can spot them.) BCNU/PM!

Here are the pictures:

Front view of tuner, attached to FT-817.

Rear view of tuner, attached to FT-817.

Inside view of tuner.

Inside view of tuner. Note RS switch.

Schematic.