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.


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

Note the different colors; blue is current day (UTC time), red is previous day, and the green traces show the previous 5 days.  In this trace, the blue trace FoF2 bottoms out below 3 MHz, suggesting that 80 meter NVIS may not have been possible this morning before about 1030 UTC, but that the trace never went below 2 MHz, showing that the F layer will allow NVIS 24-7 on 160 meters.   It also shows that 40 meter NVIS may have been possible at various times during the past week, right around sunset at Wallops Island and at least one spike mid-morning. This is a good proxy for the Mid-Atlantic US, but there are other ionosondes located around the world- here is a link to the complete list. (Note that not all of these stations are guaranteed to be in operation):
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:
There is obviously variance in how the computer program that abstracts these data does its analysis; I do not pay as much attention to the spikes, but rather the average height. If you look at the relationship between time of day and average F layer height, you see considerable consistent daily variance. This has considerable bearing on the area which NVIS can cover, and thus on operational security. During the day, the height of the F2 layer averages around 225 kilometers AGL. At night, the F2 layer average effective height is 325 km. Because of the D layer, during daytime, low-angle low frequency RF (80 or 160 meters, for amateur bands) is attenuated severely by the D layer; the lower the frequency and the lower the angle, the more attenuation. This explains why daytime NVIS range is limited to about twice the average height for the ‘bread and butter’ NVIS band of 80 meters, about 450 km or about 300 miles radius; more on 40, if the FoF2 is high enough, and less on 160. At night, with essentially no D layer attenuation, and a higher reflective layer, the area in which HF signals can be received expands dramatically. Nighttime HF on 160, 80 or 40 meters can easily cover 10,000 miles or more, from sunrise line to sunset line, as long as noise levels are low enough.
Both of the graphs above highlight the daily cycle of changes in the ionosphere in a specific location as the Earth rotates, but there are other cycles, too. We’ve already touched on the ~11 year cycle of sunspot formation, but just as the Earth rotates, so does the Sun. As the sun rotates, of course surface solar features such as sunspots and coronal holes rotate too, along with other movement, (mostly toward the solar equator) and it is common for these features to persist for an extended period, sometimes for two or more solar rotations. This means that a large sunspot group that raised the SFI may reappear, and again raise the SFI, in turn changing ionospheric conditions. This effect will be most pronounced when that group faces the Earth, so about 27 days after a peak in the SFI due to a sunspot group, you might see another SFI peak from that same, but older large sunspot group.
Those who want to really dive into the weeds on this may wish to look at individual ionograms, which are available online. Here is a link to how to look at an ionogram and do your own analysis: If you browse the NOAA website in detail, you will find a wealth of information on the status of the Sun, solar weather, and Earth’s ionosphere, as measured by earth-side instruments and by satellites, helping you understand how to pick a frequency, one of the key factors in successful NVIS communication.
While 160, 80 and 40 can all be viable frequency options for NVIS, at present, most NVIS communication on the amateur bands is carried out on 80 meters. In part, this is due to physical constraints; while 160 is NVIS capable almost 100% of the time, even during solar minimum, a 160 antenna is a big antenna, about 260 feet long, give or take, while an 80 meter antenna is about 130 feet long or about half the size. Especially in the woods, threading an almost 100 yard long antenna through the trees and getting it 25 to 50 feet off the ground is a non-trivial activity. Keeping it up in the air can likewise be a challenge and take significant time effort and chain saw work. 40 meters does not have the physical constraint 160 has, and can be useful for NVIS, but until solar emissions increase, NVIS openings on 40 meters will be sporadic and of short duration.
In general, 80 is NVIS capable from about an hour after sunrise until an hour after sunset, in the mid-latitudes of North America; further south, it may be open longer, while further North, especially in wintertime, 80 will open later and close sooner for NVIS. Time of day, sunspot cycle/SFI, and physical constraints all will affect your choice, but my first choice on the amateur bands is 80 meters. If I were using military frequencies, for practical reasons, the lowest I’d probably go would be 2.5 MHz, past that the antenna becomes a multi-person job. The highest I’d go right now would be around 5 MHz; as you can see in the above graphs, the foF2 usually gets to MHz even during solar minimum.
Moving on, we touched earlier on noise levels as being an issue during night-time HF communication; the reason that this is less of an issue during the day is that both atmospheric and man-made noise is absorbed by the D layer during the day, so that there is less noise being received, and weaker signals propagated vertically can be more readily heard; the signal to noise ratio is higher. Lower height antennas also improve NVIS signal/noise ratios, as they have less gain for signals arriving from far away, independent of D layer attenuation, day or night, as you can see in the graph below-
25' high 80M dipole
This graph is a computer model of the radiation pattern of a flat-top dipole erected over perfectly flat, level, average ground. It is well to remember that your ground is not likely to be flat, level or average, and that your antenna will not be either. The reality will not match the model exactly, but unless these differences are dramatic, the model will give you a reasonable idea of the reality. One can see that the simple NVIS dipole at 25’ AGL on 80 meters provides reasonably good gain and a broad pattern overhead, and reduces low-angle noise, improving S/N ratios on reasonably flat terrain.
With respect to power, 80 meter NVIS using sideband is entirely workable using 5 watts or less, especially if you use a radio that allows signal compression. I have made CW and digital contacts both NVIS and long-haul, using less than a watt of RF output.
Lower antenna heights reduce the antenna gain; here is a comparison between a 25 foot high dipole and a 5’ high dipole:

You can see that dropping the antenna by 20 feet reduces the signal strength by ~10 dB, or about 2 S units on a calibrated receiver. Most amateurs interested in DX would think this was a bad thing, but for NVIS this can be very useful.
For example: Say you are communicating with another station and your signal is being received at, say, S7 with your antenna at 25’ high, using 5 watts, but you are having trouble hearing the other station because of noise; your radio does not have advanced signal filtration, so you are at the mercy of what your antenna and radio pull in. You decide to lower the antenna to 5’ AGL to reduce the lower angle noise, to improve your S/N ratio. The model says that this will reduce both your transmit and receive signals by about 10 dB, but you, being a knowledgeable and trained NVIS operator, know that because of local terrain ( you are in a wooded valley,) that this will improve your S/N ratio on receive more than it will reduce your transmit signal and make it easier to hear the other station. (In fact this is common, in reality, especially if you are operating in a heavily wooded area. Trees attenuate RF, especially when the sap is up.) So far, so good. Let’s take this another step.
You have made this change, but now the other station is having trouble copying your signal; his antenna is located in a wide open plain 50’ AGL close to a big city, and his ambient noise level is higher. You cannot control his antenna height or location, but you do have some options.

  • You might elect to increase your transmit power, if you can. This option is solely within your control;
  • You might deploy a separate listening antenna for receive, which you can keep low to the ground, while still still transmitting from your higher antenna. This option is likewise solely within your control.
  • You might ask the other station to switch modes to digital or CW, if you both can. This requires both stations to adopt a change.

We’ll get further into this and discuss how to improve your S/N ratio in both Part 2 and Part 3, but for now, know that the essence of successful NVIS operation is Signal to Noise ratio, and that the successful NVIS operator knows how to manage that. Power is only one factor; as noted above, the Terrain, (ground characteristics, topography, and vegetation) plays a big role too.
Terrain encompasses ground characteristics, vegetation, and topography.
Ground characteristics can vary considerably over a very short distance, especially in the mountains, or it can be boringly uniform for many miles; large river valleys like the Mississippi Valley where alluvial soils are hundreds of feet thick are one example of the latter. If you can do so, it is worth your while to measure your ground characteristics. Rudy Severns, N6LF has written different procedures on how to do this and published them on his website-  These methods are also in the most recent ARRL Antenna Book, an invaluable resource for everything antenna related. If your soil is more or less uniform, one of the point measurement techniques might be better, but if you are located where the soils change quickly, using the low dipole technique might be preferred. The type of area in which you are setting up makes a big difference too.
Operators setting up in urban or suburban areas, even those built on former agricultural land, usually good to excellent soil, find that their overall ground conditions have been reduced by the process of development itself; the ARRL Antenna book talks about this. Salty soils and some sorts of clay soils can be very conductive, while rocky and sandy soils away from salt water are much less so.  If you don’t have time or equipment to measure the ground, experiment.  As I noted in my old articles, if your receive and transmit signals are strong, try lowering your antenna.  This may improve your S/N ratio.  If weak, try raising it some.  You can’t change the soil easily, and under a down-grid situation, other factors will drive your location, but antenna height is something you can change, most times relatively easily.
Vegetation absorbs RF, and the higher the frequency the greater the effect. Even at HF, operators are cautioned to avoid close proximity of the antenna to large branches, and to avoid having an antenna parallel to a large tree trunk or tree limb. As noted above, densely wooded areas, especially those covered in evergreens, can degrade low angle vertically polarized signals passing through them. Most operators find this loss to be a problem, but for the NVIS operator, it is a benefit, reducing the noise received by his low antenna and thus improving his S/N ratio. It also reduces the vertically polarized RF transmitted by that antenna, reducing the likelihood of being DFed, a potential plus in a down-grid situation or in other non-permissive environments.
Similarly, topography can significantly affect NVIS. HF operators focused on long haul communication and wanting to work DX often opt for locations with a long even slope away from the antenna. Such locations reduce the departure angle by the amount of slope and improve long haul communication, as does increasing antenna height. NVIS operators, however, want to decrease their low angle reception and transmission. Sites in small valleys or canyons with steep walls provide significant attenuation of vertically polarized RF, reducing the low-angle noise and improving the signal to noise ratio. Such locations also reduce the potential for being DFed.
Before I close this article, I want to touch on one more topic.
At present the Internet allows easy online access to massive amounts of information on solar conditions, propagation, and a myriad of other topics at the click of a mouse. It is well to understand and remember these basics, however, as these systems may not be accessible or available in the future during a disaster or other event, or may attract highly unwelcome attention, especially if an emergency is used to revoke or suspend all amateur radio licenses, as happened during the Second World War. The US Navy, which drew most of its radio technicians and operators from the ranks of amateur operators, was a major supporter of re-establishing amateur radio after World War 2.
If the federal government decides that amateur radio is more of a threat than a help, then amateur radio may become a memory, and the current benign neglect of the amateur service and civilian communication could then be replaced by ruthless suppression, outlawing ownership of any radio equipment at all, even receivers or scanners, as is the case in many other places. One may discount this possibility, but the recent multiple warnings from the FCC about amateur involvement in ‘illegal’ activity argue powerfully that there are people in the Federal government who are concerned about some amateur operators, and by extension, the amateur service. More on this shortly, but the point is that if you wish to be capable of communication during emergencies and disasters, it is essential to understand and remember at least the basics of the factors that affect sky wave propagation and not make yourself and those who depend on you to provide communication dependent on our fragile electronic infrastructure.
I hope you find Part One of interest. I plan on sending either NC Scout or Historian the next installment within a week, in which we’ll review and discuss interception and DF concerns, rigs and perhaps some other topics.