Notes:
RF Inductor Calculator was developed to help users predict the RF characteristics of a large single-layer solenoid-style air-core inductor.
This version is suitable for larger inductors such as for calculating the choking impedance of a coax choke.

Inputs via the slider widgets:

• ⌀a : Outer conductor diameter in millimeters (mm). Equivalent coax cable size is also displayed.
• ⌀b : Coil diameter in millimeters (mm).
• c/a : 'c' is the inter-winding spacing, and 'a' is the conductor diameter, so 'c/a' is the spacing ratio. (Must be >= 1.1) A low-value will increase the resistance due to the proximity effect.
• N : Number of turns or windings.
• f : The frequency of interest (MHz) for some of the calculations. Frequency dependent results are shown on the top-right.

Characteristics on the left are independent of frequency, while the characteristics on the right are dependent on the selected frequency.

Each of the graphic representations attempt to keep the relative geometry correct, without exceeding the drawing boundary. The coil diameter relative to the conductor diameter are representative.

Calculated dimensions:

• ⌀o : Outer coil diameter (mm)
• ⌀i : Inner coil diameter (mm) - corresponds to the diameter of the winding former.
• c : Distance between windings, measured from the conductor centers (mm).
• ℓ : Length of the coil (mm). Equal to c x N.

Calculated parameters:

Frequency independent:[L]
• L : Inductance is calculated using Nagaoka's equation incorporating his coefficient.
• C : Capacitance is calculated using Knight's 2016 paper on self-resonance and self-capacitance of solenoid coils.
• Rdc : DC resistance is calculated using conductor length divided by the conductor cross-sectional area, assuming a copper conductor.
• SRF : Self-resonant frequency (MHz) for the unloaded coil.
• wire : Length of wire required to wind the inductor.

Frequency dependent:[R] (Text goes RED when selected frequency > SRF. Inductor model is not accurate once SRF is exceeded.)
• f : Selected frequency in MHz
• δ : Skin depth due to skin effect (μm)
• Rac : AC resistance is calculated using the skin effect and proximity resistance from empirical data collected by Medhurst using the spacing ratio, and length-to-diameter ratio.
• Xₗ : Inductive reactance at the given frequency. (Ω) - pure inductive component, ignoring parasitic capacitance
• Z : Complex impedance at the given frequency. (Ω) - includes losses due to series Rac and parallel parasitic C
• |Z| : Impedance magnitude at the given frequency. (Ω)
• Q : Effective Quality Factor of the inductor at the given frequency. - (|Z.im|/Z.re)