The True Inductance and Distributed Capacity of a Coil

The true inductance and distributed capacity of a coil can be obtained by observing the capacity that must be added to tune the coil to resonance at several frequencies, and then plotting the added capacity as a function of 1/f² as shown in Fig.1.

This will result in a straight line that will intercept the capacity axis at a negative value C0 that equals the distributed capacity. 

The slope of the line  gives the true inductance according to the equation:

Coil inductance in henrys = 0.0253m

 Where m is the slope of the curve of added capacity plotted against 1/f² where f is in megacycles and capacity is in picofarads. 

The distributed capacity can be determined without plotting a curve by adjusting the coil successively to resonance with the fundamental frequency and then the second harmonic of an oscillator.

If C1 and C2 are the respective tuning capacities then:

Distributed Capacity =( C1+4C2)/3