NVIS techniques – Part one
Keypounder, a name a few folks may remember, has written an update to his articles on NVIS that NC Scout published almost 5 years ago at the Brushbeater site. He has continued his research and study of NVIS, and wants to update and expand upon his earlier articles on the subject. As NC Scout is out of town, Keypounder asked me to publish his update on NVIS.
As NC Scout stated 5 years ago-
“…. I will re-iterate that these skills, along with Land Navigation, are among the most perishable and most difficult to learn- under duress, near impossible. So for those of you who feel you’ll do it when ‘the time comes’, you’ll be sadly mistaken. Please folks, try this at home.”
About the author:
“Keypounder” is the pen name of an amateur radio operator first licensed in the 1970s. He is a long-time student of radio propagation, and antenna design and construction, having written an article on low band listening antennas for Signal-3, and a number of articles published on the Brushbeater website. His interests include, in no particular order, emergency communications; rag-chews; HF contesting on both CW and SSB; and direction-finding techniques.
Caveat:
These articles are not intended for the beginner; they assume that the reader has basic knowledge of radio electronics and is a licensed amateur operator with an FCC General Class license, or the foreign equivalent. It is NOT possible to gain skill in NVIS operation, the subject of this article, without actually operating. I could spend several pages detailing all the reasons I think unlicensed operation is a bad idea, but if you are thinking about operating without a license, please don’t. The amateur radio Service is threatened as it never has been before, and many amateurs are becoming aware of this threat and increasingly militant about defending their avocation, me among them.
Technician Class licensees who do not operate CW, don’t have the frequency privileges to operate NVIS, but there are a lot of new General Class operators and even some old-time Advanced and Extra Licensees who can benefit from this information. This material is presented thinking that NVIS will be extremely useful in a grid-down emergency situation, where the current VHF and UHF repeater systems are not available.
After almost 5 years, I have revisited my older article on NVIS, and while I am satisfied with what I wrote then, I think it is worth revisiting and expanding on what has been published to date. Right now, I plan at least three articles. This first part is a recap of and expansion of what was written in my first three-part article. The second part will look at NVIS operation and operational imperatives in detail. The third part will look specifically at antennas for NVIS in detail, compiling my research and modelling efforts over the last five years.
Let’s get started by looking at what NVIS operation is and how it works:
- NVIS uses sky wave communication, typically in the lower HF spectrum.
- NVIS is not Line-Of-Sight, nor is it groundwave.
- Amateur frequencies that are suitable here in North America currently include 160 meters, 80 meters, and (sometimes)40 meters; in general, frequencies from the upper AM broadcast band (1 MHz) to perhaps as high as 8 MHz have been successfully used for NVIS communication.
- NVIS is a operating method distinct from long-haul HF communication, and it is specifically intended to allow radio communication for locations beyond line of sight from each other, but within relatively close range, inside the “skip zone” encountered on higher HF frequencies.
- NVIS provides highly reliable local and regional communications at distances from just a few miles to as much as 300 miles away.
- NVIS can be used with any operating mode: Voice, CW and all sorts of digital modes can be employed.
- Unlike satcomm, NVIS uses easy to set up low-tech equipment. Antennas can be easy-to-erect low height field expedients and required equipment can be very simple. No infrastructure is required to support NVIS; operation is possible 24/7/365, solar storms excepted.
Unlike long-distance HF communication, as the name indicates, NVIS uses HIGH ANGLE reflection of MF and lower HF from the ionosphere to accomplish its mission. NVIS antennas are deployed to maximize vertical RF, to minimize noise pickup from both natural and man-made sources and to constrain transmission to the regional area. Working DX is not an intended part of NVIS operation.
A number of factors affect NVIS operation, all of which were touched upon in previous articles. These include but are not limited to:
- Antennas
- Sunspot cycles and solar flux
- Solar emissions and space weather
- Ionospheric conditions
- 27 day solar rotation and time of day
- Frequency choices
- Signal to Noise Ratio- why power is not always the answer.
- Terrain- vegetation, topography and ground type.
A brief discussion on these factors follows.
Antennas are one of the most important factors in any radio system, but in NVIS they are especially important and a major factor in successful NVIS communication. This article will explore other more advanced options for antennas for NVIS communication in detail in Part 3 of this series. To briefly recap the basics, a good NVIS antenna is: horizontally polarized and relatively low in height, about 1/10 of a wavelength or less to maybe ¼ wavelength. Examples of simple NVIS antennas include dipoles of various sorts (horizontal, vee, inverted vee; both compact and full size) and horizontal loops.
Sunspots and solar flux. As previously discussed elsewhere, the Solar Flux Index, (SFI) measures 10.7 cm radio emissions and is largely a function of the status of the sunspot cycle; when there are more sunspots, the solar flux is higher, and the ionosphere is more heavily ionized and able to reflect higher frequency radio waves. During solar minimum, which is coming to an end at the time this is being written (May of 2021) SFI is usually low, in the high 60s to low 70s, and frequencies useful for NVIS are likewise lower than they would be at solar maximum.
Solar flux is one easy measurement that is a reasonable proxy for over-all solar activity, but keep in mind that it is only one of a spectrum of factors that drive what solar physicists and radio operators call “space weather.” Sunspots, together with Solar features like coronal holes and events such as various types of solar flares, emit a huge spectrum of various sorts of particles and radiation ranging from radio waves to ionizing radiation. The Solar Flux index tells you a lot about the status of the Sun; in general when the SFI is higher, the ionosphere is more ionized and will reflect higher frequencies better, but SFI does not tell the whole story.
Solar emissions create space weather. Space weather in turn affects what the Earth’s ionosphere does. The ionospheric conditions determine what time of day communication is possible on a given frequency, or whether communication is possible at all. While NVIS is highly reliable, nothing is 100.0%, and NVIS is no exception. Solar emissions may or may not be geo-effective; “Space weather” is a loosely defined term that primarily refers to solar emissions that have, or potentially could have, an effect on the Earth’s ionosphere. Most of the Sun’s emissions do not directly affect the ionosphere; but some can cause, and have caused, geomagnetic storms that can interfere with HF sky wave propagation, and in extreme cases can shut down ALL HF communications on the sunward side of the Earth.
Space weather affects the ionosphere, and ionospheric conditions rule HF sky wave radio communications. Absent extreme interventions like the HAARP, there is nothing the average person can do to change the ionosphere; just as a ship captain during the days of sail could not change the wind, or the weather system that made it, so radio operators cannot change the ionosphere or the space weather that drives it. But we can observe and measure it and the space weather that drives it, and to some extent, predict what factors affect radio communications, and modify our operation to take advantage of it. Keep in mind that it is not just what the space weather is, but when it affects the ionosphere that matters. Anyone who has been following ionospheric conditions knows that these conditions are affected not only by space weather, but by time of day. As the Earth rotates, the effects of solar emissions, including but not limited to solar wind, X-rays, visible light, and UV light change the ionosphere on an ongoing basis.
One of the basic methods for measuring the state of the ionosphere, and tracking these changes, uses a special transceiver and antenna system called an ionosonde; ionosondes measure the time between transmission of a radio pulse and the received reflection. An ionosonde transmits a series of pulses on increasing frequencies and measures the return time (or lack thereof.) This data is compiled into an ionogram; these are usually taken every so many minutes from a given location. The ionogram data are abstracted into graphs of various important features; usually I look at these compilations rather than the individual reports, but those are available too.
Following are two graphs created from compilations of ionosonde data, the first of which shows the maximum frequency which the F layer reflects ordinary RF straight back down, (foF2) at Wallops Island Virginia.
foF2 graph 5/9/2021
Here is the second graph showing the height of the F layer (hmF2):
The above graph is also taken from the NOAA site; here is the link to the NOAA list for real time hmF2 data: https://www.ngdc.noa