NVIS Test
- 07 October 2024
- West Georgia Amateur Radio Society
On October 5th, 2024 Carroll County ARES hosted a Near Vertical Incidence Skywave (NVIS) test, and WGARS was well represented at the event. A little over a dozen members joined in to test our ability to engage in local and regional communications using HF in the event local repeaters are unavailable.
From this article written by WX4BK: The rule of thumb for HF is that you'll get better DX the higher your antenna. The reason for this is that as a typical dipole antenna is raised higher, the take-off angle of MOST of the radio signals become shallower, closer to the horizon. This shallower angle provides for greater skip of the radio signal off the atmosphere, just like skipping a rock on water. Remember, however, that not all the signal is on one narrow angle, just the largest single component. It's like a Bell Curve, where angles above and below the take-off angle will have decreasing levels of signal.
What many don't think about is the converse, that as the antenna moves closer to the ground the take-off angle becomes steeper, until at some point it is almost straight up. This is a result of Ground being close enough to interact with the dipole and act as a sort of director as in a Yagi antenna.
Operators from all around the West Georgia area, and even a couple over 100 miles away, participated in the exercise. We discovered that, at least in the morning hours, 40 meters worked very well for local and regional communications, even with operators using QRP (20W or less). While not always as clear as a repeater, stations were reporting 5-7 and 5-9 in most cases.
NVIS typically only works for 80-40 meters, sometimes 20M, but not reliably. We found 80 did not work as well, but that may have had to do with the time of day. We attempted communicating on 10M since a portion of the band is available to Technician class operators; while many contacts were made, it wasn't as impressive as 40M. More experiments are planned, particularly geared towards QRP and improving our ability to utilize 10M ground-wave!
