Velocity Factor

The conductive material of the radiating elements and the surrounding (insulating) materials around it immediately affects how well the RF propagates through the antenna. These materials actually cause the rf to slow down through the conductor. In free space we use the speed of light as the measure of how fast radio waves are travelling, but in a conductor such as a copper wire, the rf travels at much slower speeds.

The term we use to indicate how much a conductor slows down the propagation of rf is Velocity Factor, often written as VF.

If the radiating elements are made from bare copper copper wire, the velocity factor is around 0.95 (95% the speed of light). We need to take this into account when we cut the wires for our dipole:

Frequency½ λ in free space½ λ in bare copper wire
433MHz34.6cm34.6 x 0.95 = 32.9cm
868MHz17.3cm17.3 x 0.95 = 16.4cm
915MHz16.4cm16.4 x 0.95 = 15.6cm

 

As you can see the differences are not huge, but you would end up with a less than optimal dipole if you didn’t take into account the velocity factor.

Velocity factor also applies to any insulating material that goes over the conductor.

single-insulated solid core copper wire

 

The insulation material will further cause the VF to drop and result in radiating element lengths that need to be made even shorter for resonance on the frequency of interest. The VF for most common insulating materials is between 0.95 – 0.98 (PVC, Polyethylene, Teflon) so be sure to take that into account as well.

Frequency½ λ in free space½ λ in insulated copper wire
433MHz34.6cm34.6 x 0.95 x 0.95 = 31.3cm
868MHz17.3cm17.3 x 0.95 x 0.95 = 15.6cm
915MHz16.4cm16.4 x 0.95 x 0.95 = 14.8cm

 

Note that these figures are an approximation as the Velocity Factor will vary from material to material.