Coaxial/Cylindrical Capacitors

This web page deals with the capacitance of Coaxial Cable and Cylindrical Capacitors because both consist of a circular conductor, inside another circular conductor. The difference is that Coaxial Cable usually has a center conductor that is made up of solid or stranded wire and a solid dielectric material. The Cylindrical Capacitor, on the other hand, could be a hollow center conductor and has air as a dielectric.

Knowing the capacitance of coaxial cable is useful when you are trying to make coaxial antenna traps, where the cable provides both the inductance and capacitance. Building a capacitor from two different sized copper pipes might be useful as a capacitor in a antenna matching section.

Below is a calculator to determine the effective capacitance for Cylindrical Capacitors and Coaxial Cable. It goes beyond just the capacitance and also calculates the effective Impedance, Inductance per unit length, and Velocity Factor. In the appropriate boxes, enter the required data and then click your mouse outside the entry area. Output is displayed in US/Imperial and Metric measurement units. The Init button will set the the calculator back to the default values (ID = #14 AWG, OD = 0.5", and Air dielectric).

Below the calculator is a table that lists the dimensions for a few selected types of coaxial cable. The information came from a Wiki web site on coaxial cable (see bottom of table for link). I chose these cables because they are commonly used in Amateur Radio operations. Clicking on one of the coaxial cable buttons will transfer the physical characteristics of the cable to the calculator.

Type Imp. Core Diel. VF Dielectric OD Type Imp. Core Diel. VF Dielectric OD
Ohms mm Type in mm in mm Ohms mm Type in mm in mm
501.0PF0.750.1854.70.2426.1 751.630PE0.660.2857.20.41210.5
502.170PE0.660.2857.20.40510.3 502.05232PE0.660.2857.20.40510.3
500.810PE0.660.1162.90.1955.0 500.46PE0.660.04171.060.1102.79
750.640PE0.660.1463.70.2426.1 501.12PF0.830.1162.951.954.95
750.762PF0.780.1463.70.2426.1 501.42PF0.840.1503.810.2406.1
751.0PF0.750.1854.70.2706.86 502.74PF0.850.2857.240.40510.29
PE = Polyethylene, PF = Polyethylene Foam, Source: Coaxial Cable Wiki