Total posts : 45366
The term “loopstick” can refer to any coil wound on a ferrite rod. It can be either a fixed or variable inductor. We are discussing the popular variable inductor in this thread, which was the basis for many published hobby projects many years ago. However, your diagram shows a fixed inductor, L1, tuned by a parallel capacitor. As you explained in your post, when you built the circuit in the diagram, you replaced the fixed coil shown with a variable loopstick. I’m glad you still have this inductor. It is a real gem, and very useful.
As I stated in my previous post, I measured an equivalent series resistance of 12.3 ohms when my loopstick was adjusted to the maximum inductance of 287 uH. I also stated in my post that that a 71 uH coil I made with an Amidon T-106-2 toroid has an equivalent series resistance of 2.3 ohms. Four of these coils in series would have nearly the same inductance as the maximum inductance of my loopstick, and have an equivalent series resistance of 9.2 ohms. This is slightly less than the loopstick resistance. So, there is an efficiency advantage to using the Amidon coils in series, but not as much as would be expected, considering how tiny the loopstick is. Whoever designed the loopstick had really put a lot of knowledge and effort into it. Until now, I had wrongly considered the loopstick to be trivial.
The circuit diagram in your post is very interesting. It uses the combination loading coil and “L” network design used in the SSTRAN. This kind of tuning network for short antennas was featured in the Mobile Antennas section of several editions of the ARRL Antenna Book. It seems to have originated from a 1951 QST article. It looks like the designer of your circuit intended to make an efficient class C amplifier, but, it seems to me that there would not be enough RF drive to the final transistor stage to get respectable efficiency. Thanks to some posts about some efficiency tests performed by Neil late last year, the efficiency of Part 15 transmitters has become a particular interest of mine. I contributed several posts about this subject on the “New products under proposal” thread initiated by Wilcom Labs, which is
My reason for wanting to look into using a loopstick was to get a continuous-tuning inductor for matching the output stage of a Part 15 AM transmitter to a 3 meter antenna. I also want to get a continuous-tuning capacitor at the input to this tuning network. The capacitance required is rather large. The present approach is to select a large value fixed capacitor (several hundred pF) in parallel with a trimmer capacitor. I think it is inconvenient to select any fixed capacitor. Fortunately, I have a four-gang air-variable capacitor with a total capacitance of 728 pF. I connected a 100 pF air-variable capacitor parallel to the larger capacitor to serve as a trimmer. Unfortunately, air-variable capacitors have become about as scarce as variable-inductance loopsticks. I will be hard-pressed to find the components I need If I try to build any more transmitters.