Ted’s essays

NVIS antennas

Unless we are on a mountain peak or Kansas flatlands with line-of-sight views of everyone we want to talk with, there are barriers resisting our two-way communications.

With our Very High Frequency (VHF) and Ultra High Frequency (UHF) radios we compensate by installing specialized radio systems (repeaters) on high ground to repeat signals over a much wider area than we can reach from our own radios. These can extend our VHF and UHF ranges to 50 miles or so.

Repeaters are both expensive and could be a weak link in emergency comms – that is they may not be working in certain disaster scenarios. Amateurs with General or Amateur Extra class licenses can use their High Frequency (HF) radios to fill these gaps.

Near Vertical Incidence Skywave (NVIS) is a propagation mode which uses high angle radiation to send signals almost straight up to be reflected back to Earth for very effective short to medium distance communications. This mode of operation makes it ideal for in-state communications during disasters or other emergency situations.

The military has used NVIS techniques for decades to provide short haul communication with other units on the ground. NVIS nly works at frequencies from 2 MHz to 10 MHz. The signal must penetrate the D layer of the ionosphere, and bounce off the F layer. Lower-frequency signals will not penetrate the D layer; higher frequencies will not bounce off the F layer at these sharp angles and just goes out into space. So, for amateur radio operators, we’re looking at 40 and 80 meters primarily for NVIS use.

A good NVIS antenna will not work well at DX distances. Antenna gain is a zero sum game. There is a fixed amount of energy radiating. If we push it all out in one direction (the near-vertical angles), we have to take it away from another direction (the low DX angles).

Regular height dipoles or vertical antennas have a lower take off angle and your signal may be heard three states away, but not in your state due to the skip zone. This skip zone is the area between the maximum ground wave distance and the shortest sky wave distance where no communications are possible. Depending on operating frequencies, antennas, and propagation conditions, this skip zone can start at roughly 10 to 20 miles and extend out to several hundred miles, preventing communications with the desired station. The other term called ground wave is where your signal does reach someone closer. A ground wave signal can go up to approximately 50 miles if conditions, including terrain and obstacles, are favorable.

The military uses a dual band NVIS antenna known as the AS-2259/GR. It consists of two crossed inverted “V” dipoles positioned at right angles to each other and is supported at the center by a 15- foot mast. The dual dipole wires do the job of providing guying support for the mast.

There are many NVIS antenna designs available in books and on the Internet, plus ready-to-go kits and deployable packages.

DX Engineering is a good source for antennas, kits and components – not to mention information. Here is their page concerning NVIS:

Set your priorities. To save time buy a ready-to-go antenna system. To save money, scrounge and buy components to build your own. Both approaches have merit.

I leave you with three different .pdf documents covering NVIS function, operation and antennas in much greater detail than I did above.

DX Engineering WP-NVIS-Rev2