This was my second STL that I built. After learning a few things from my first build, the 30-17 Octagon STL, I had some ideas for the next one.
First was material used to make the radiating loop. This time I used 1/2" soft copper tubing. This allowed me to make the loop from a single piece of copper, thus eliminating the eight 45° elbows and solder joints. This eliminates the losses in the loop due to the multiple solder joints. It is also very easy to form into a perfect circle.
Second was the use of a vacuum variable capacitor manufactured by Jennings with a range of 5-465 pF. This would not only increase the available tuning range of the antenna, but allow me to run 100 watts SSB, CW, or digital without any worry.
Lastly, I wanted to try a different method of feeding the loop. I chose the gamma match for this loop. The use of a gamma match method will result in the radiation pattern slightly favoring the side of the loop the gamma tube is on, however my testing has shown this to have a very negligible effect.
Tuning the capacitor is done with the same type of 2 RPM geared motor I used on the 30-17 Octagon Loop. I mounted the motor above the radiating loop in order to keep anything that might interfere with the RF being radiated out of the interior of the loop. Rather than let the control wire hang straight down through the center of the loop, I ran it on the inside of the copper tubing and drilled a small hole in the bottom of the tube and inserted a grommet, then pulled the wire through. Due to skin effect, all the RF is contained on the outer layer of the copper tube, and not on the inside, so the control wire is essentially invisible to the antenna.
The performance of this antenna is far and above that of the first one. My criteria for this antenna was to be able to tune all of the 10 meter band, and still have good efficiency on 20 meters. Efficiency at the top of the 10 meter band is calculated to be 93% (-0.30 dB), while at the bottom of the 20 meter band is calculated to be 50% (-3.0 dB). More than acceptable.
Looking farther down the bands, efficiency on 30 meters is calculated to be 24% (-6.18 dB), and on 40 meters 8% (-10.88 dB). On 40 meters, that's almost 2 S-units. Keeping in mind though that while I never intended this antenna to be useful on 30 and 40 meters, I have made several SSB contacts on 40m, and FT8 contacts on 30 and 40 meters. While it will tune on all five 60m channels, efficiency is a paltry 3% (-16 dB).
(During a recent local NVIS Field Day in early 2023, I did test this antenna with fantastic results on 60m and 40m NVIS)
The small transmitting loops only need to be about one loop diameter above the ground. In other words, my 3' diameter loop only needs to be high enough to get the bottom of the loop 3' off the ground. And this is one of the big reasons I really love these antennas; if you live in a location that does not allow for an antenna, a mag loop can blend in very easily, can be hidden in plain sight, all while basically being at ground level. Just keep in mind potential RF exposure to those that might get near it, and the potential for serious injury if someone where to touch it while you are transmitting.