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 lower in frequency as you lower the antenna. At first the change is minor, but as you get down to 5 or 10 feet above the ground the decrease in resonant frequency becomes faster and faster. In some cases a dipole that is resonant at 7.2 MHz when up reasonably high will drop to around 4 MHz. This happens when the velocity factor drops from around 98 % to 50 %.

I did a test in several places in my yard using a 40 meter dipole. The resonant frequency went from about 7.1 MHz in the air to 5.7 MHz when flat on the ground in the woods. That equates to a velocity factor of about 78%. Other places in the yard such as gravel driveway and in the middle of my back lawn gave various velocity factors down to 65%. My gravel driveway gave 65%, my mowed lawn gave 72%, while the woods with the wire essentially on pine straw gave 78%.


H.H. Beverage published a chart in his original Beverage article in QST showing how the Velocity Factor of a wire varied from 90% at 14 feet high to 70 % at 2 feet high. So the velocity factor significantly decreases as the wire approaches the ground. 

The change in VF as the wire height decreases from 14 to 12 feet is small, maybe 1 or 2 percent whereas the change from 4 feet to 2 feet is more like 5 to 7 percent. 

For this reason I like to rough tune my wire antennas at least 6 to 8 feet in the air. Then the decrease in VF as I raise the antenna to 50 feet or so is rather insignificant and further trimming is rarely necessary. Even then it is probably due more to sloppy measurement on my part than velocity factor change! 


Low antennas tend to be less efficient that high antennas. While a NVIS dipole will work quite well on 40 meters at 4 to 6 feet above the ground, the optimum height for NVIS is about 1/4 wavelength high. Higher than 1/4 wave the pattern starts to develop an overhead null. Some gain can be realized by increasing the height of a NVIS dipole up to the 1/4 wave height. The gain may or may not be worth the effort. That frequently depends on band conditions and how much harder it is to elevate the dipole higher than just a few feet above ground. In general the difference in a 40 meter dipole at 6 feet and one at 33 feet (1/4 wave on 40 meters) is just over 6dB. That’s about 1 S unit.


Ballpark gains of a 40 meter dipole at a 90 degree elevation angle are as follows:

Gain at 33 ft straight up is about 5.8 dBi

Gain at 10 ft straight up is about 2.4 dBi

Gain at 6 feet straight up is about - 0.7 dBi

Gain at 4 feet straight up is about -3 dBi

Note that the gain of a 6 foot high 40 meter dipole is still at least 5 to 10 dB higher than a vertical when used for NVIS propagation! Even the 4 foot high dipole is still better than a vertical at high angles. 


So while 33 ft may be the best, a 10 foot high 40 meter dipole is a very good height. It should be easy to achieve in most cases and is only about 3 dB down from the ultimate height of 33 feet. 


Even the 6 foot high dipole should be significantly better than the best quarter wave vertical for most POTA contacts and all NVIS QSOs. 

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