Total posts : 45366
Yes, thank you, Joe, for “passing this information along to anyone who may make use of it.” Your post was very useful to me. I am constructing a Part 15 transmitter with higher efficiency than is available with commercial transmitters. I think that the kind of antenna tuning circuit used in the SSTRAN will work well in my system. The SSTRAN uses a tapped loading coil to tune out the capacitive reactance of the antenna, and to provides the inductance for an “L” network for tuning for maximum output to the resistive component of the antenna circuit. I think, however, that it is very inconvenient to use taps. It would be much easier to use a continuously variable inductor. I tried using a slug-tuned inductor using a commercial coil form, but the tuning range of the inductor was too small. I then made my own slug-tuned inductor by using a ferrite antenna rod as the slug. There was plenty of tuning range, but the core loss caused by the ferrite rod was too high. The equivalent series resistance of my inductor was even higher than the expected ground resistance of the antenna system.
I have a variable-inductance loopstick coil of the kind that I think you are using. It is about a quarter inch in diameter, and has a 1 1/8 inch ferrite rod attached to a long brass screw. The screw is used to adjust the position of the ferrite rod with respect to the winding on the coil form. If I hadn’t read your post, I would have never thought of testing this loopstick. It seemed to me that its dimensions were too small to give good performance. However, the results were much better than I expected. The inductance tuning range was 31.6 uH to 287 uH. What surprised me the most was how low the equivalent series resistance of the coil was. At minimum inductance, the resistance was only 3.14 ohms. The DC resistance of the winding is 1.7 ohms. As expected, the equivalent series resistance increased as inductance was increased, because of increased core loss. However, the equivalent series resistance at maximum inductance is only 12.3 ohms, which is quite small, considering how small the dimensions of the inductor are. It’s too bad that these loopsticks are hard to find these days. I’d sure like to have some more of them, in order to be able to build more transmitters.
The maximum inductance of 287 uH, while high, is not high enough for all antenna tuning situations. It would be useful to add an inductor in series with the loopstick inductance. A situation where the required loading coil inductance is less than 100 uH is very unlikely, so I made a 71 uH inductor by filling an Amidon T-106-2 iron powder coil form with a single layer of #24 magnet wire. Combined with the loopstick, this coil produced a minimum inductance of slightly over 100 uH. The equivalent series resistance of this inductor was 2.3 ohms. Filling the same kind of Amidon toroid with a single layer of #28 magnet wire produced an inductance of 212 uH. This inductor, however, has a high series resistance of 23.3 ohms. It’s surprising that the loopstick actually has a lower equivalent series resistance for the same inductance than the supposedly low-loss toroid. I also tested the Amidon T-106-3 coil form, which has higher permeability than the T-106-2, and gives more inductance for the same number of turns. This coil form gave excessively high equivalent series resistances.