General Class Question of the Week G9D02 from the General License Course, Section 5.1 Antenna Theory & Principles p.135:
Which of the following is an advantage of an NVIS antenna?
A. Low vertical angle radiation for working stations out to ranges of several thousand kilometers
B. High vertical angle radiation for working stations within a radius of a few hundred kilometers
C. High forward gain
D. All of these choices are correct
There is quite a bit of conceptual information behind this question about an NVIS antenna and its related questions listed at the end of this Question of the Week article. Let’s first define NVIS [en’- vis] and then explore a little about this unique antenna configuration.
The unique characteristic of an NVIS antenna is embodied in the full name: Near Vertical Incidence Skywave (NVIS) antenna. An NVIS antenna will direct the strongest portion of its radiated signals in a “near vertical” or upward direction. As a result, the signal’s angle of incidence with the ionosphere is a steep one, hence “near vertical incidence.”
In contrast, other typical antennas for DX or long distance contacts tend to be arranged so that the greatest portion of the radiated signal is directed low to the ground, heading toward the horizon. The low angles of propagation of the DX antenna provide the greatest skip distances. The angle of incidence with the ionosphere is shallow, and signals are refracted back toward the earth in similarly shallow angles, resulting in great skip distances. However, stations in the skip zone – the area “under the skip” of this propagation technique – usually cannot be contacted.
The NVIS antenna is not intended for long distance communications. The near vertical signals of the NVIS antenna are refracted back toward the earth in similarly steep angles, resulting in shorter skip distances. This is the advantage of the NVIS antenna, and it reveals the correct response option for this question – the high vertical angle radiation provides communications within a radius of a few hundred kilometers. An NVIS antenna can reach stations that would be in the skip zone of distantly propagating antennas.
The 40-meter, 60-meter, and 80-meter bands tend to work best for NVIS propagation, even during the daytime. These lower HF frequencies are very effectively refracted by the ionosphere, providing the short skip with the small angles involved. Higher HF will usually not be refracted sufficiently to affect good NVIS propagation.
But you may question the daytime performance due to that pesky D-layer of the ionosphere that is known for its daytime absorption of low frequency HF in these bands! In typical daytime operation the 40-meter and lower bands are considered poor skip bands because the D-layer attenuates these signals severely, and only once the D-layer dissipates at night do these bands open for long distance propagation. However, with the steep propagation angles with NVIS, the transit distance through the D-layer is minimized, and the attenuation is much less than with low-angle propagation. As a result, NVIS communications tend to work even in daylight hours on these low bands, given sufficient activation of the higher ionosphere layers.
An NVIS antenna is a horizontally polarized antenna, such as a horizontal wire dipole, positioned above the ground less than ¼ wavelength high. Heights of 1/8 to ¼ wave tend to provide good performance, although some operators prefer heights as low as 1/10 wavelength. For the 40-meter band that is only 4 meters above the ground, or roughly 13 feet.
The answer to General Class question G9D02, “Which of the following is an advantage of an NVIS antenna?” is “B. High vertical angle radiation for working stations within a radius of a few hundred kilometers.”
Related Questions: G3C05, G3C11, G3C13, G9D01, G9D03