The radio frequency characteristics of coaxial antenna cables are determined by properties of the materials used to create the cable, along with its dimensions. If you know some of these properties it may be possible to determine other properties of a cable, which might be useful:

- Inner diameter of shield (outer conductor)
- Outer diameter of center (inner conductor)
- Characteristic impedance (typically 50 or 75 Ohms)
- Velocity factor
- Dielectric constant/relative permittivity

For symbol names, please see the end of this post.

The **characteristic impedance** is determined by the** diameter of the cable’s conductors**, and can be found using this formula:

Note that precisely measuring the needed thicknesses may be difficult, but you should easily be able to tell if it’s a 50 or 75 Ohm cable you’re dealing with.

The **velocity factor** is determined by the **dielectric constant** (relative permittivity) of the dielectric material making up the **insulation** inside the cable:

The **velocity factor** can be found if you know the **characteristic impedance** of the cable and the **diameter of its conductors**:

Note that this assumes an insulating material with relative permeability = 1, which is a reasonable assumption in most cases.

The **dielectric constant** of the insulation inside the cable can be found if you know the cable’s **velocity factor**:

If you’re rolling your own, you can find the **ratio of conductor diameters** for a cable with **impedance** using dielectric insulation of **relative permittivity**:

If you’re making an *air-insulated* hardline/cable ( = 1), the ratios are 3.490 for 75 Ohm cable and 2.301 for 50 Ohm cable.

The **ratio of conductor diameters** for a cable with **impedance** and a **velocity factor** of :

The **resistive loss** at a given **frequency** per length of cable is determined by the **permeability** and the conductivity/**resistivity** of the cable’s conductors, plus their **diameters**, and can be found using this formula:

This is usually the main contributor to loss of signal strength in a normal antenna cable.

The **dielectric loss** at a given **frequency** per length of cable is determined by the **dielectric constant** and the **loss tangent**, and can be found using this formula:

This is usually small enough that you do not need to worry about it. A cheap Polyethylene-insulated cable with a loss tangent of 0.00031 at 3 GHz, works out to a loss of less than 0.01 dB per 100 meter at a frequency of 1 GHz.

= characteristic impedance

= permeability

= vacuum permeability (1.2566370614…×10^{−6} H/m)

= relative permeability

= permittivity

= vacuum permittivity (8.854 187 817… x 10^{−12} F/m)

= relative permittivity (dielectric constant)

= velocity factor

= natural logarithm

= inner diameter of shield (outer conductor)

= outer diameter of center (inner conductor)

= frequency (Hz)

= frequency (gigahertz)

= resistivity

= resistive loss

= dielectric loss

= loss tangent

= decibel