Terminations, Reflections and Cable Input Impedance This note presents results of calculation for the Zo dependence on frequency of common RG58U "50 ohm" coaxial using high and low frequency approximate expressions.ĭetailed field plots based on more comprehensive modelling for a typical coaxial cable quantify the field penetration outside of coax at low frequencies.Ī 200 kHz reference example provides accurate reference data for all field components.Ī script calculator accurately computes coaxial cable parameters discussed below and transmission line parameters for any frequency. In addition, it is not difficult to calculate the frequency dependence of Zo at lower frequency, given the material properties and dimensions of the cable. High frequency approximations are readily available (e.g. Instead, it is instructive to consider approximate solutions which are valid at either high or low frequency.
Schelkunoff, 1934), the general solutions involve complex Bessel functions which are difficult to analyze for frequency trends. Although complete solutions for Zo are available for any frequency
This is due to the frequency-dependent relative values of series resistance and inductance, combined with the skin-depth effect. While this description is usually sufficient for frequencies above a few MHz, Zo is in general complex (reactive) and at lower frequency, the reactive nature of Zo becomes more apparent. The nominal Zo value refers to the high frequency RF value, appropriate to typical usage of this type of cable. To the current phasor, which may be phase shifted, for a single outbound travelling wave (with no reflection from the cable end). Zo generally is defined as the ratio of a sinusoidal AC voltage phasor Coaxial cable is characterized by a "nominal" real characteristic impedance Zo such as 50 or 75 ohm.