NVIS antennas, velocity factor and height above ground
NVIS and velocity factor Lots of Hams build “NVIS” dipoles and place them just a few feet above ground. I want to shed some light on some of the issues a lot of them run into. First is the length of the antenna. In free space a halfwave is 492/frequency in MHz. So for a 40 meter frequency of 7.2 MHz that gives the halfwave as 68.3 feet. When using wire to build a dipole, the wire has a velocity factor that needs to be considered. The wire also has two ends where it’s connected to some kind on insulator. To take into consideration the effects of both velocity factor and end effects on the wire a factor of 95% is applied to the free space constant of 492. That results in the familiar formula of length in feet being 468/frequency in MHz. The velocity factor of a wire changes as it gets closer to the ground. You can measure the velocity factor change by cutting a dipole for 7.2 MHz when it is reasonably high. You can observe the resonant frequency move lower and low...