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Let me re-post a link provided earlier here on part15.us I think by Rich Powers, because it demonstrates how to easily build a useful tool for making general analysis of a transmitting antenna
Now to the subject of loading coil, which I also posted only today on the Big Talker thread, for very much the same reason as yours, Radio Bon Esprit. Namely, there are times when a less than ideal length antenna must be used, such as in a small physical space. The classic solution becomes a matched loading coil.
For 13mHz a loading coil will contain far fewer turns than with medium wave, because the wavelengths are shorter. As you know, the coil makes up for the missing antenna length.
I have downloaded and printed every description I’ve found regarding medium wave loading coils, and given all the variables, the general pattern usually advises using a 100′ spool of magnet wire or insulated wire and starting to add taps at about 60-turns, every two turns or so. Then, using a clip, attach the antenna to the tap which provides the most resonance to the antenna.
Another way that’s given to find resonance, and this would be easier to do on a smaller 13mHz coil, is to gradually unwind until a resonance is found.
Probably all we need to begin experimenting is a “ball-park” estimate of how many turns to start out with on a test coil.
The device linked above, the light bulb on a stick, could be used as an estimator of how well the coil was doing.