N2SMH's Z-Match Lives!!
I recently got an opportunity to stick my 20-meter delta loop up in the antenna tree and try it with the Z-match. The short version is that it worked very well.
If you'll remember...I made up the loop in the wake of Bill, W2DP's Field Day success. I've also got the balanced-feed doublet I used on FD myself. My St. Louis tuner was woefully inadequate with both antennas. That's why I scrambled around looking for a tuner design better suited for use with balanced-feed wire antennas and finally settled on the Z-match design of Charlie Lofgren, W6JJZ as published in the June 1995 QRP Quarterly.
First, I simply fed the tuner's input with the Autek RF-1 at 14.060 MHz and attached the 300-ohm twin-lead feed from the loop to the tuner's output. In no time at all, I had an SWR reading of 1.3:1. The next test was to actually transmit through the tuner and loop. Out came the roll-a-table, the Sierra, and the Cascade. The results were the same. I stuck my OHR wattmeter in the line as an SWR bridge. I wasn't able to completely null out the reflected power, but it was darn close. Actually, I had even better results using the Cascade up around 14.250 MHz. There, reflected power was in the single-digit milliwatt range.
With so little reflected power and a healthy 2 W surging from the loop (oriented for an east-west pattern and corner-fed with apex up), I had little choice but to try and make some contacts. Was there some kind of QRP test yesterday? I worked four stations, and three of them were well-known QRPers and/or denizens of QRP-L (N4BP, AB5UA and K1OJ). All contacts were made in the general vicinity of 14.060 MHz. Two stations were in Texas, one in Oklahoma and one in Florida. So the tuner and the loop have been duly baptized with RF.
You may recall me crying about the instability of the St. Louis tuner with the loop. That combination yielded wacky SWR readings that seemed to change as the wind blew. No such problems with the Z-match! Stable, sure, rock-solid SWR readings were the order of the day. And, I got my 1.3:1 or better match at the low-capacitance setting (only one of the two air variables on the input side in the circuit) and with the caps right around the middle of their ranges. I was able to get the same match with either of the two output windings. As advised by W6JJZ, I used the high-inductance output winding for maximum efficiency.
The only thing I didn't try was to see if I could match the loop on bands other than 20 meters. Well, the loop is still up in the yard, and the weather is still nice...
Glen Rock, NJ
(Posted 8/25/97 on the NJ-QRP listserver)