Nearly every issue of QST inspires me to do something new or different. Now I am going to try another small transmitting loop.
I always keep a stack of QST magazines lying around, typically the last five years. During free moments, I browse QST looking for knowledge or ideas. This week, I found that QST inspires my next magnetic loop project.
My recent experiment with a flying saucer resonator vertical was okay, but not great. I found the idea worked, but without a great ground system it’s a mediocre performer. Trying to cover 80-30 meters with an eight foot vertical antenna was a bridge too far.
Skimming through April 2019 QST, I found an article by Steve Adler, VK5SFA, describing his two-turn 160-80 meter STL. This won first place in the 2018 ARRL Antenna Design Competition. If you aren’t an ARRL member, you can read all about Steve’s design on his web site. Now, even a two-turn loop for 160 is huge with a ten foot diameter. But maybe I can adapt his ideas to a smaller loop for 80-30 meters (3.5 to 10 MHz).
So, here we go. I am going to start out with a two-turn one meter diameter loop which should work on 40-20 meters with pretty good efficiency. You can see two examples of this idea above. On the left, is Steve’s 40 meter design shown on the Amateur Radio Experiments Group web site. On the right is a similar effort from KB0YH.
My design will include remote motor control over WIFI. One motor will be a rotator, so I can shift the loop null over 180°. The second motor will tune a butterfly capacitor to resonate the loop. I have done a similar proof of concept previously with a five foot copper loop, but this will be a fresh design.
QST Inspires a Multi-Turn Compact Loop
You will find two big benefits from a two-turn loop, compared to a single. First, compactness. Loop inductance increases significantly with multiple-turns. Higher inductance reduces the amount of capacitance required for resonance. Two turns with a one meter diameter should give me 11 μH versus 2.5 μH for a one turn loop. To get that much inductance from a single turn requires a ten foot diameter!
Second, a multi-turn loop increases radiation resistance much faster than loss resistance. You need higher radiation resistance for more efficiency.
At least all this seems to work in math. Time to try it out. Also, I am not sure about the effect of multiple turns generating stray capacitance and what effect that would have.