The NVIS Illusion

I don’t think there is any subject that is so misunderstood by hams as NVIS. I hope I can help by sharing some of what I have learned that may dispel some of the myths.

NVIS is an abbreviation of Near Vertical Incidence Skywave.

There is no such thing as a NVIS antenna. The term NVIS describes a signal, not an antenna.

Various types of antenna can be oriented to produce a NVIS signal, usually by erecting them close to the ground.

NVIS signals are propagated at a high elevation (greater than 60 degrees) and are reflected back down from the ionosphere (principally the F2 layer) over an area of a few hundred kilometers from the source.

Some antennas that produce NVIS signals may also propagate low angle signals suitable for DX only 1 S-unit weaker!

NVIS propagation is only possible on the lower bands at mid-latitudes. A low-to-the-ground antenna tuned to the higher bands will produce high angle propagation that will pass right through the ionosphere and go off into space (in several million years you may get a reply from an extra-terrestrial civilization hi hi).

Let’s Illustrate this using a 40m End-Fed Half Wave antenna in inverted-V configuration with the apex at 18 feet and each end 3 feet above the ground.

I modeled this antenna using EZNEC Pro 2, version 6. Here is what the antenna looks like in the model. NB: I have used a EFHW antenna but the results would be the same for a dipole.

Now let’s look at the predicted propagation pattern for the 40-meter band. As you can see, the maximum gain is at an elevation of 90 degrees. This will result in the signal being returned from the ionosphere to ground in the immediate vicinity of the transmitter. But you will also note the -3dB points are at an elevation of 50 degrees. This will result in the signal also being received over a wider idea.

Now, in the second chart we can see the -6dB points at an elevation of only 25 degrees. Remember -6dB is only 1 S-unit! So our “NVIS antenna” is also a “DX antenna”. No wonder some folks believe they are sending and receiving NVIS signals over a much wider area.

It could be argued that this is an inefficient antenna arrangement for NVIS operation. But that depends on your objectives. During the Vietnam war US troops used NVIS to communicate between forward operating positions and command posts. The intention was to minimize opportunities for the enemy to intercept messages. Only receiving stations physically close to the transmitter would be able to receive the traffic. Low angle propagation had to be avoided at all costs.

For amateur radio traffic, on the other hand, an antenna like the one used in this example could be very useful. Suppose band conditions are very poor and regular skywave propagation is unreliable. This antenna will still allow quite strong signals to be sent and received between stations within a few hundred kilometers of each other using NVIS propagation. With a bit of luck continent-wide signals and even DX are also possible but will involve regular skywave instead of NVIS propagation.

How far does a NVIS signal reach?

You don’t have to be a mathematical genius to calculate this for yourself, although you do have to take out temporary membership in the flat Earth society. We are going to assume that a small section (a few hundred kilometers) of our planet’s surface is flat. The ionosphere follows the contour of the Earth’s surface so it’s a reasonable assumption. The math gets more complicated if we don’t make that assumption. We know the ionosphere’s F2 layer is at a height of at least 215km and may be as much as 300km or more high (it varies). Let’s use 300km for our calculation because that will give us an idea of the maximum range for NVIS signal propagation. We know that NVIS, by definition, describes signals propagated at an elevation of at least 60 degrees. Now the math is very simple.

Using trigonometry we can see that the NVIS signal range is approximately 350 kilometers or about 220 miles. The diagram is 2-dimensional so we must remember that the NVIS signal range is a radius around the transmitting station. If we were to draw a circle around the transmitter, any receiving station within that radius should be able to receive the NVIS signal.

So clearly, any signal sent or received much beyond that range has been propagated by regular skywave – NOT NVIS.

Can any frequency be used for NVIS?

Unfortunately no. Higher frequencies will pass straight through the ionosphere and off into space. Special propagation analysis stations called ionosondes transmit NVIS signals up to the ionosphere to determine what frequencies will be returned to Earth. The critical frequency, labeled foF2, is the highest frequency at which the ionosphere will reflect the signal back down. The critical frequency varies according to latitude, time-of-day and the 11-year solar cycle. The accompanying diagram shows a sample chart from the ionosonde at Alpena, Michigan.

The diagram shows that stations at or near the same latitude as Alpena MI, at the date shown, have an foF2 in the 80-meter band at night and the 40-meter band by day. Reliable NVIS communication occurs at frequencies below the foF2.

What about groundwave?

Ground waves are transmitted by just about every antenna and extend to the local horizon. In the area close to the transmitting station it is likely that the ground wave will interfere with the received NVIS signal to produce fading (QSB) or other effects. It is a good idea to keep transmitted power low to mitigate this effect.

What about the receiving antenna?

While the transmitter should be using a NVIS optimized antenna to direct most of its signal energy straight up to the ionosphere, the same is also true for the receiving station. If both stations are using the same kind of NVIS optimized antenna the received signal strength will be higher. And, of course, for 2-way communications it is essential!

A regular receive antenna (i.e. a high wire antenna) will usually still be able to receive NVIS signals but may have less success in being heard by the other station. A vertical antenna is usually deaf to signals coming from high elevations angles so it is highly unsuitable for NVIS communications.

Summary

Okay, go right ahead and refer to “NVIS antennas” if you wish, but remember this is just common usage of the term and is not strictly accurate.

If you receive a signal from a station more than a few hundred kilometers away you are probably not receiving a NVIS signal. The exact range of NVIS is unpredictable unless you know the exact height of the F2 layer at the time of the contact. Instead you are probably receiving a regular skywave signal.

If you receive a signal from a station in very close proximity to you, it will most likely be a ground wave signal.

For best reception of NVIS signals, both transmitting and receiving stations should be using low antennas optimized for NVIS. But note that using a NVIS optimized antenna for receiving may reduce your antenna’s ability to receive weak DX signals.

Thanks for reading, your comments, constructive criticism and questions are always welcome.