A radio wave propagates at the light speed only in a vacuum. In any other medium, the propagation speed is slower and at the same frequency, the wavelength becomes shorter. In RF cables, this property is reflected by the value of the so-called velocity factor, which depends on the material of the coax internal insulation. With this calculator, you can calculate the length of a coaxial cable with the most common internal insulation materials, as well as the most commonly used quarter, half, or three-quarter wavelengths.
Cable length calculation:
The shortening factor is numerically equal to the square root of the relative dielectric constant of the internal insulation material. The velocity factor is the inverse value of the shortening coefficient.
- PE is solid polyethylene;
- FPE is foamed polyethylene (or PF is polyethylene foam);
- PTFE is polytetrafluoroethylene (or ST is solid Teflon);
- FPTFE is foamed PTFE;
The velocity factor of a foam insulated cable depends on the production technology and varies from manufacturer to manufacturer. Therefore, in this case, one cannot do without measurements, the calculator shows a certain average value. For a cable with solid or “air” insulation, the velocity factor is uniquely dependent on the material of the inner insulation, and the wavelength in the cable can be calculated more accurately. The characteristic impedance of the cable does not affect the deceleration coefficient and, accordingly, the length of the cable.
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