Hi
Recently I wound a cylindrical air-coil of about 15 uH for a loaded antenna. I used stranded electrical wire with plastic insulation. Clearly, the insulation helps in making all windings nice and tidy, but the downside is an increased coil capacitance.
Coil capacitance can be derived with ease from the coil's resonant frequency, however accurate measurements of coil resonance are not so easy to perform.
What is easy, is measuring the coil inductance with the FA-VA5 on a range of frequencies and deriving the coil capacitance from the measurement data. An ideal inductance has no capacitance at all and therefore the inductance L is frequency-independent. Real inductances (coils)? always have a more or less noticeable coil capacitance which makes the "effective inductance" (what the FA-VA5 measures) frequency dependent.
The valid equivalent circuit of a practical "real-world" coil is simply that of a parallel-tuned resonant circuit. Validity exists at least up to the resonant frequency which is naturally quite far above the frequency of operation of the coil (at which the coil is used). If we go higher in frequency the equivalent circuit loses validity due to "higher order effects" which are beyond the scope of this message.
I have prepared a Google-sheet for finding the coil capacitance from FA-VA5 inductance (LCR) measurements. Below is a link to the sheet.?
The idea is to calculate the admittance (1/reactance) of both the measured L value and the L value derived from the equivalent circuit. We start with a guess-value (small C) for the coil capacitance and observe the resulting two curves of L versus frequency. When both curves coincide, the value of the parallel capacitance is correct and the equivalent circuit is valid.?
I should mention here that the L-measurement with the FA-VA5 should be done with the coil suspended in air. Maybe a bit of foam under the coil or pieces of dry wood. Any material with low loss and low permittivity is OK. Especially the L-measurements closer to the resonant frequency of the coil will be strongly affected by the table or desk surface. This is because of the high electric fieldstrength around the coil at resonance.
Note: We neglect the coil quality factor because it is relatively high in the given case and for reasons of simplification.
with best 73s/regards
Dieter, VK3FFB
Dieter VK3FFB