At what height above ground is an NVIS antenna typically installed? [G9D03]
A. As close to 1/2 wavelength as possible
B. As close to one wavelength as possible
C. Height is not critical as long as it is significantly more than 1/2 wavelength
D. Between 1/10 and 1/4 wavelength
First off, what the heck is an NVIS antenna?
NVIS is an acronym for Near Vertical Incident Skywave. The military uses HF antennas of this design in order to facilitate short-range communications. As the name suggests, the idea is to send the signal almost nearly straight up to the sky, where it will be reflected down again.
The answer is D. Between 1/10 and 1/4 wavelength. At HF frequencies, this is pretty low. Low enough to make it easily deployable. There’s a lot of math and physics involved, and rather than really try and dive into it without a lifejacket, lets just say that the other answers are incorrect. The reason is actually pretty obvious when you think about the wavelengths involved. To mount a horizontal dipole, which is how this technique is normally used, at such distances above ground is often impractical, if not impossible.