Administrative note: The following was sent by Keypounder, the nom de plume that should be familiar to a few readers. A longtime radio amateur and communications specialist in his own right, he’s generously submitted this article to me, and in my opinion, is among the best I’ve read on the topic for public consumption. Due to length and volume the article is posted in three parts. Contained is a keen rundown of how Skywave propagation works, the requirements for NVIS on the HF bands, the history, antenna modelling with EZNEC, and fantastic examples of antennas in the field for this task. It is meant to be a much more in depth article than my previous work on the subject, which was intended for beginners.
NVIS is a highly misunderstood term parroted by many ‘preppers’ and militia-types with usually very little demonstration or explanation of said skill. It is for this reason the previous was written, and for this reason Keypounder has given his time to explain it further and give a very practical explanation of skywave propagation. I will re-iterate that these skills, along with Land Navigation, are among the most perishable and most difficult to learn- under duress, is 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.
NVIS 201, or peeling the onion on advanced HF techniques
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. His interests also include, in no particular order, emergency communications; rag-chews; HF contesting on both CW and SSB; and direction-finding techniques.
This article is not intended for the beginner; it assumes 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. A license these days is easy to get; you don’t even have to learn Morse code.
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. Finally, this material is presented with the thought that NVIS will be most useful in a grid-down emergency situation, where the current VHF and UHF repeater systems are not available.
With that out of the way, let’s get on to the meat of the matter!
I have studied NVIS quite a bit over the years, and I have had a fair amount of experience with it, too. It wasn’t always called that; the current name really came into vogue in the mid 1990’s, but the idea has been around a long time, as we’ll see. Many people have a variety of different, often conflicting, ideas about what and why NVIS is. I was told a long time ago by a very wise and very smart person (and no, they are not the same thing!) that the truth is a lot like an onion; every time that you peel away a layer to get to a deeper understanding of the truth, there is another layer underneath that. Well, folks, I can tell you that I’m not going to get all the layers peeled, but maybe we can shave a layer or two off and get at a better understanding of what NVIS is, why NVIS is, and how NVIS can be of benefit.
NVIS stands for Near Vertical Incidence Skywave, and what that means is that the radio operator bounces a radio signal off the ionosphere more or less directly overhead, and is able to send and receive signals to other stations out to perhaps 300 to 500 miles away, depending on the time of day and the state of the ionosphere.
Near Vertical Incidence Skywave is a specific operational mode for HF radio communication and the distinguishing characteristics are:
Intentional limitation of communication range;
Use of high angle lower HF radio emissions reflected off the F layer(s) of the ionosphere; NVIS is a type of skywave communication.
Generally, use of low power and reduced height antennas.
Use of frequencies from 1.8 to perhaps as high as 10 mHz in North America (possibly higher during solar maxima or close to the equator); frequency used depends on solar flux, time of day and other factors.
Let me digress for a moment to clarify some things about HF radio communications. There are three main types of radio propagation:
Ground wave, where the signal is usually vertically polarized (perpendicular to the surface of the Earth) and usually low frequency. Ground wave is the propagation mode for most daylight AM radio broadcast stations; the radio waves actually hug the surface of the Earth and travel along the ground. Geologic discontinuities like mountains, rivers, and deep gorges attenuate ground wave propagation, as does the absorption of the RF by the ground. This propagation is inversely proportional to frequency; the higher the frequency, the quicker it attenuates.
Sky wave, where the signal travels from the transmitting antenna to the receiving antenna by reflecting one or more times off any one of several layers of the ionosphere. It may include reflection from the ground in between as well. This includes NVIS as well as skip communication, auroral reflection, Sporadic Eskip, and tropospheric ducting, among others. This type of propagation is the most common propagation mode for HF radio communication. While the signals may be horizontally, vertically or circularly polarized by the transmitting antenna, the reflected signal can arrive at the receiving antenna with any polarization due to their reflections from the ionosphere and/or the earth. To reiterate, NVIS is a skywave propagation type, because the signal bounces off the ionosphere and comes back to Earth.
Line of sight, (LOS) where the transmitting antenna and the receiving antenna are in view of one another, and propagation is not by means of ionospheric reflections or by ground wave. This propagation can be horizontally, vertically or circularly polarized. LOS signals can be bounced off structures or geologic features like cliffs, too, or refracted off mountain tops, and while the purist might argue that these are not line of sight anymore, the generally accepted definition includes such reflections. Much VHF and UHF communication is LOS, although there are exceptions.
NVIS does not use VHF or UHF frequencies, does not use multiple hops or cover great distances, does not require high antennas, and does not typically use high power.
History of NVIS-
NVIS was first discovered or developed by the German Army in World War Two, while they were engaged with the Soviet Union on the Eastern Front. What they found was that while their excellent upper HF/lower VHF radios, developed during the late 1930s during the run-up to the solar peaks of Cycle 17 (1937 to 1939) worked very well for long haul HF communication, they had great difficulty using these radios for intermediate communication between groups or columns; their columns were often too far away for LOS communication and too close for HF skip. Amateur operators (Hams) today call that being in the “skip Zone.”
In modern technical terms, the F layer direct reflection frequency (FoF2) required for short range skip would have been too low for many of their radios. So the Germans started using a lower frequency radio originally intended for infantry use and created special horizontal dipole and loop antennas mounted on their vehicles to give their lower frequency signals maximum amplitude directly up. Here is a photograph of Heinz Guderian’s command vehicle showing a NVIS cage antenna. One can also see the extendable mast used for VHF in the center.
(photograph original Wehrmacht, h/t to http://www.tactical-link.com/WWII_NVIS.htm for posting it. Note that Patricia Gibbons WA6UBE, the author, who was a serious student of NVIS, died in 2010, so best to get any content from that site ASAP as hackers have penetrated the site, deleted some content and posted other material. I have found that this site http://www.raqi.ca/~ve2cvr/ve2cvr/sites/default/files/sura/surra_misc/hfradionvis.pdf has apparently copied much of Gibbon’s material and reposted it.)
During D Day and again during the Vietnam War, the US military also had issues keeping in touch with various elements of their forces, especially in rugged highlands terrain. The answer was, again, NVIS, using low frequency low elevation horizontal antennas to maintain communications. Today, NVIS communication is practiced and used by military organizations and their affiliates, as well as various members of the preparedness community, to provide reliable, fast, and secure communications with lower probability of DF location.