There has been quite a bit of interest lately in what is commonly called the NVIS antenna or Near-Vertical-Incidence Skywave antenna.
A couple of people have asked “where do you buy one?” as if it was a brand or specific design. Fact is, any horizontal antenna can be more or less an NVIS antenna if it is installed right. (or wrong depending on your intentions)
These things used to be called “Cloud warmers” and really are nothing more than a dipole, flat-top, or other horizontally polarized antenna that isn’t installed high enough. Many times a ham will start out to install a dipole, hoping to work long distances and after hanging his wire finds that he or she can’t seem to reach much farther than a few hundred miles or even less in some cases. This is usually caused by the antenna having too high an angle of radiation, which is the result of it not being high enough off the ground. Every antenna, except the isotropic antenna is affected by it’s distance from the earth. Normal installations call for a minimum of 1/4 wave with 1/2 wave above ground being preferred. Often we are constrained by the availability of supports and often end up with the antenna only a few feet high, perhaps along a fence for example.
This in not necessarily a bad thing. Often, particularly in EmComm this is exactly what we want. HF operation in a disaster situation is usually limited to 100 miles or less; too far for ground wave and too close for skywave. By directing your signal nearly straight up, what is not absorbed by the Ionosphere is refracted almost directly back to the source, covering a circle as small as 100 to 200 miles. The signal will suffer severe attenuation from absorption but can often be quite strong. Years ago I worked a regular traffic net and the strongest signal on the net came out of Los Altos on the San Francisco peninsula. I found out after working this fellow for a couple of years that he was running 75 Watts and his antenna was strung on the same supports as his clothesline in the backyard.
For a very interesting discussion on the effects of antenna height on propagation, see the on-line version of WorldRadio at World Radio Online and see the excellent article by Carl Luetzelschwab, K9LA, Using Antenna Height as an aid to Propagation. February, 2011, page 33.
The WorldRadio also has some informative links to the ARRL where a wealth of antenna information can be found.
*Note: The WorldRadio magazine is in PDF format so it has a very high quality appearance but the file is huge (close to 20 megs) so check your bandwidth before you try to download it.