Re: Spaced Coil measurements.
I took some more measurements of the highly spaced coil and
came up with more interesting results. However, I was unable to be as
comprehensive about it as I would have liked and I would much rather
repeat and extend these in a quiet period at work in an empty
classroom in a vacation break where interference is minimal and
isolation good. I may get the opportunity in a few week's time.
I measured coils with 38 turns, 28 turns, 20 turns, 15 turns
and 20 turns. Each had a diameter of 12.5" and height of 31.5".
Brief results are as follows:
(1) there seemed to be an error in Wheeler's inductance formula which
ranged from 20uH too low for the 38 turn coil to 9.5uH too low for
the 10 turn coil. This translated into a difference of almost 50%
too low for the 10 turn coil. Using an old favourite version of
Wheeler which has 8.85r rather than 9r in the denominator made
virtually no difference.
(2) Using the calculated value of Cs a la Medhurst and adding that to
each bare coil and a range of shunt capacitances gave consistent
inductance figures for each coil within about 2%.
(3) I calculated out the wirelengths and their corresponding
frequencies for each BARE coil. The measured and calculated values
started to converge noticeably as the number of turns in the coil
were reduced. A 1 turn coil *should* be a 1/4 wave antenna after all.
Meant by that is that actual wirelength in the coil started coming
closer to the calculated 1/4 wavelength at resonance.
Conclusions: Wheeler is far from reliable for greatly spaced coils.
Coils do look as if they are becoming antennas as turns are reduced
if height and diameter are held cconstant. Lumped calculations appear
to be breaking down badly for greatly spaced coils. Watch this space