Design your next small transmitting loop with a free multi-turn magnetic loop calculator from DG0KW and others.
Now that I have the remote control circuits underway, I turned my attention to understanding multi-turn small transmitting loop performance. I conducted my research with some calculators, a spreadsheet and a whole lot of printed theory.
You should be able to achieve two main benefits from a multi-turn STL. First, reduced size. My earlier experiments with a five-foot (1.5 meter) diameter copper loop taught me how unwieldy such a structure will be. On the other hand, a 1 meter diameter, two turn loop is lighted, less top heavy and way easier to mount. Plus, you get twice the inductance, which provides my second benefit. Substantially less tuning capacitance is needed for the same frequencies.
But the downside is less efficiency and self capacitance. A smaller multi-turn loop has substantially more self (stray) capacitance between the windings than a single turn exhibits. You need to take this extra capacitance into account when trying to tune the loop. Second, because of the reduced loop area, you get less radiation resistance and more loss resistance. Hence, less efficiency.
I used a spreadsheet to model loop parameters from five different design articles. Unfortunately, these articles rarely dealt with multi-turn effects. So I went looking for a good STL multi-turn loop calculator online. My best finding was the DG0KW Magnetic Loop Antenna Calculator from Klaus in Germany. Make sure to download the latest 1.1 version as it supports English as well as French and German.
I found this multi-turn loop calculator, shown above, works well and provides consistent and improved results compared to my hand calculations.
Multi-turn Loop Calculator from DK0KW
As you can see above, the DK0KW software provides all the expected calculations, plus the ability to specify number of turns and turn spacing. All told, a one-meter, two-turn loop should work on 40, 30 and 20 meters with a 65 pF variable capacitor. By comparison, a single turn loop of the same size needs more than three times the tuning capacitance.
Overall efficiency should be between 6-12 dB down from a dipole, which is 1-2 S-units. So, I think I will go with that design.
By the way, VK3CPU also provides a wonderful online magnetic loop calculator with great visualization.