Loading Coil for 13560 kHz

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

http://blog.makezine.com/archive/2010/02/seeing-radio-waves-with-a-light-bul.html

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.

I posted a spreadsheet earlier for MW loading coils. The author of the original math stated that the calculations are dependable only to about 3 MHz.

With that disclaimer in mind, here is what that set of formulas predicts

Freq. = 13.56 MHz, Coil Form = 1.5" diameter, 2.0" long

Unloaded matching 1/4 wave vertical would be 18' high, a half wave dipole then would be around 36 feet.

Visit
http://www.k7mem.150m.com/Electronic_Notebook/antennas/shortant.html for a great HF short dipole designer

As we say in America, "We are in the ballpark." Just in case France doesn't know that figure of speech, it means we have reached the best part of the topic. SCWIS has given a starting formula for building a loading coil for the 21-meter band, home of the 13mHz frequencies.

Meanwhile,

I've been searching through a big thick book from William I. Orr, W6SAI, the Radio Handbook, 20th Edition, from 1975. It covers everything to do with amateur radio, except there is no section on loading coils for the short indoor antenna. BUT, there is a great section on MOBILE AND PORTABLE EQUIPMENT, and, as it turns out, the mobile whip on a car or truck is an undersized antenna totally dependent on a loading coil for efficiency.

I will pick out a few points that might add to this discussion.

For example, it says the transmission line from transmitter to antenna should be "exactly odd multiples of one-quarter wavelength long electrically." If the antenna is located right at the transmitter there will be no transmission line.

Another notable fact is that the majority of whip antennas at 14mHz and below are CENTER-LOADED, meaning that the loading coil goes at the center of the vertical antenna rather than at its bottom.

A CAPACITY HAT may be added to a loaded whip antenna to improve the efficiency. "The capacitance added above the loading coil requires a reduction in the number of turns in the coil to reestablish resonance.

Here now is something different than I have ever seen before about mounting a loading coil to the vertical antenna. "It was found that mounting the center-mounted loading coil at RIGHT ANGLES to the whip greatly improves the Q (efficiency)."

Now, I have to find the translation between US inches and feets to Euro-centimeters. Thank you !

Bertrand