Leave it to Murphy. Family and business commitments on 26 April put the skids on an all-out effort at KI6SN in the 2003 QRP To The Field competition.

Yeah, there would be a few free hours to get on during the NorCal QRP Club-sponsored event, but packing a knapsack and heading into the field was out of the question. Even if I’d be operating from home, though, why not at least make it interesting and simulate trail conditions?
           
A discussion on the QRP-L Internet Mail Group about the utility of end-fed antennas at QRP output levels was the catalyst to make this year’s QRPTTF a challenge at KI6SN. The plan was to use simple half-wave wires with quarter-wave counterpoises strung around the garage and on fences and bushes to work 20- and 40-meters. Granted, it’s not exactly on the level of a dust storm or driving rain or snow. But a hurdle nonetheless.
           
To make sure that the antenna systems and the transceivers would see eye-to-eye, I needed to build a trail-friendly antenna tuning unit (ATU) specially for end-feds.
           
There have been lots of designs published in QRP periodicals and on the Internet. I wanted a small, lightweight ATU with two-band capability and a built-in resistive SWR bridge with LED indicator. That’s a great package for the field.
           
The ingredients for this stew came from a quartet of sources: the LED-based SWR bridge is from designs by Dan Tayloe, N7VE, and Charlie Lofgren, W6JJZ; the transmatch from Bill Jones, KD7S; and modifications for two-band capability from David Bixler, WØCH.
           
The accompanying schematic shows the simplicity of the ATU. A double-pole / double throw (DPDT) switch – S1 – is used at the input to toggle between the unit’s OPERATE and TUNE modes. One side puts the SWR bridge in line for tune-up. The other bypasses the bridge for on-air operation.

An SPDT switch at S3 toggles between the 20- and 40-meter windings on the matching inductance. Another SPDT switch at S2 gives the operator – via ALTERNATE OUTPUT – the option of using antennas other than end-fed wires by bypassing the end-fed matching network.

Two 100-ohm resistors in parallel were used to achieve each 50-ohm leg of the bridge. A Radio Shack LED (RS 276-309) serves as the SWR indicator.

It took a morning at the homebrew bench to put the ATU together. I jammed the transmatch and the SWR bridge into a Radio Shack plastic project box 2 inches wide, 4 inches long and 1 inch deep. A 100 pF tuning capacitor with a shaft was used across the output inductance. If a screwdriver-adjusted trimmer capacitor is used, the whole thing could be built in a much smaller box.

A piece of double-sided printed circuit board served as both the panel for the unit’s controls and the ground plane for the parts that were mounted inside the box. The circuit is simple and practically fool-proof to build. You’ll see there are two T50-2 toroids to fill with wire. Both have primary and secondary windings, and the output inductor is tapped. Even for beginners, though, they’re pretty easy to make.
           
When the time came for smoke testing, it was simply a matter of attaching a transceiver at the input of the ATU and a half-wave antenna and counterpoise to the output terminals.
We started testing on 40 meters late in the afternoon on the eve of QRPTTF. A couple of QSOs with 579 signal reports indicated that the lash-up was working.

Cowabunga, dude!

On Saturday morning, it was time to give the unit a test under QRPTTF contest conditions. Propagation on 20 meters wasn’t particularly good, but there were enough stations in the hunt to make a go of it.

On 14 MHz, the antenna was made of a 33-foot, 4-inch long piece of insulated hook-up wire strung from the operating position, across the inside of the garage, looping to the top of the open garage door and sloping down to our Dodge Caravan. The end of the antenna was secured under the driver’s side windshield wiper blade.

The 16-foot, 8-inch counterpoise snaked along the garage floor and out onto the driveway.

With low expectations, I was amazed to make contact after contact during the contest. Running about 3 watts from a NorCal-20 transceiver from Southern California, I was able to work 13 states, including Washington, Arizona, Colorado, Minnesota, Oklahoma, California, Oregon, New Mexico, Nevada, Idaho, North Dakota, Texas and South Dakota. Signal reports ranged from 339 to 589. Not bad for an antenna whose height peaked at 6 feet and drooped to about 3 feet off the floor. And some of my best signal reports came when the family dog was snoozing on top of the counterpoise. But that’s a topic for an entire antenna engineers’ conference.

From the start, the antenna tuned up nicely, with the LED dimming as the tuning capacitor swept into a matching condition.

Switching to 40 meters was as easy as replacing the 20-meter antenna with a 66-foot, 6-inch wire for 40 meters combined with a 33-foot, 4-inch counterpoise. Flip the switch to change the tap on the ATU loading coil and you’re ready to go.

This time the 40-meter antenna was stretched across the inside of the garage to the top of the garage door, bent right along a wooden fence and then took a hard left along a cinderblock wall to some bushes. Height peaked at about 6 feet, but the average was closer to 3 to 4 feet.

Again, on tune-up the antenna dipped beautifully as S3 was toggled to the 40-meter tap and the tuning capacitor found a good match – the LED dimming to near darkness. Signal reports received on 7 MHz were solid 579 to 589s.

If you’ve never considered an end-fed wire / counterpoise combo in your station configuration, I’d highly recommend giving it a try. In the end, the results can be pretty remarkable.

Controls for the end-fed ATU are mounted on a piece of double-sided PC board.

Components are soldered “ugly” style on the underside of the ATU’s front panel.

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Richard Fisher, KI6SN is Execituve Editor of The ARS Sojourner. He lives in southern California and is an avid QRPer and homebrewer.