Dear LF Group,
I have been doing some
investigation into the design of tapped loading coils for LF antennas –
specifically, where to put the tap for the TX connection to achieve matching to
50 ohms. As far as I can see, there is no simple formula that gives a realistic
result, and generally the tapping point is selected by trial and error. But it
would be nice to know at least approximately where the taps should be, to take
the guesswork out of making a coil.
It appears that the tapping
point can be calculated reasonably accurately (within about 10% resistance, or
+/- 1 turn or so – assuming the antenna loss resistance is known), but it
is quite a complicated and tedious process. So I have made an Excel spreadsheet
that does all the coil design calculations, the user just has to enter the coil
diameter, winding pitch and some other details, and the antenna capacitance,
and gets a graph of matched antenna resistance vs. tapping point turns (and
some other info, like the overall number of turns needed). I have tested the
spreadsheet results against measurements of 3 real loading coils with different
geometries, with the kind of results mentioned above. It is also quite easy to “reverse
engineer” an existing loading coil.
Some interesting results arose
that I was not aware of previously. There is a definite upper limit to the antenna
resistance that can be matched, which depends on the winding pitch and diameter
of the coil. “Short, fat” loading coils, with large diameter and
closely spaced turns, can match higher antenna resistances than “long,
thin” coils. Any tapped coil can match resistances below the TX output
resistance, if it has enough turns. But for lower antenna resistance, a “long,
thin” coil offers the advantage of a smaller change in resistance between
each tap, leading to a more accurate match.
I have written a short article
as a pdf file that explains how to fill in the spreadsheet, and showing the
measured coil data, to go with the spreadsheet itself. These files are too big
for the reflector, but let me know if you would like me to email them direct. I
would be interested to know if others get reasonable agreement between the
spreadsheet results and their loading coils – if you don’t have
Excel on your PC, let me know details of your coil (diameter, length, number of
turns or winding pitch), and I can run the calculations for you.
Cheers, Jim Moritz
73 de M0BMU
