Velocity Factor, What Is It?

Many hams do not understand what the term velocity factor means. Abbreviated as VF, it is way of expressing what the speed of a radio signal is in a transmission line and it is written in a percentage, such as 66%. This means the radio signal is travelling at .66 the speed of light in space. Ok, you say, so how does this affect me?

Well, if you want to build a phased array, in order to get the correct length of the phasing lines, you must figure the VF to get the correct length. This is important to ensure that the driven elements are phased correctly.

If the cable has a VF of .66, then you must take the figure calculated for a half wavelength at the frequency you are going to operate and then multiply it by the VF in this case, .66.

So, for a half wavelength phasing line on 40 meters, by dividing 486/7.00 MHz you will get 66.86 ft. Then multiply 66.86 by .66 and you get 44.13 feet as the correct length for this particular piece of cable. Different cables have different velocity factors but the manufacturers have these figures usually in their catalogs. TV twin lead has a velocity factor of .88 and open latter line is slightly higher at about .90. For the most part, VF is not important to most hams, but if you intend to phase antennas, it becomes very important, e.g.,

468/7.00x.66 = 44.13

There will be more articles on phased array antennas in antenneX and being aware of velocity factor is very important if you want to build a phased array. Phased arrays can give you more for your money than any other kind of array, if constructed properly.

Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF

Last Updated : 16th March 2024