Near Vertical Incident Skywave (NVIS) Antennas
Hams have been making local contacts on 80m and 40m long before this mode was given such a fancy name to sell it to the military in the 1960s. But as part of that process we also have more complete studies on how to optimize antennas for this purpose.
NVIS isn’t just about antennas: it is a communications system that permits coverage up to 500 km (300 miles) or so using relatively low power equipment. To do so, the operating channels must be below the Critical Frequency, the highest frequency where signals radiated straight up will be returned to Earth by the ionosphere. Above that frequency, signals pass off into space, even though they may be reflected back when striking the ionosphere at flatter angles. Generally the critical frequency may get as high as 12 MHz in the tropics or 9 MHz at higher latitudes, depending on the current ionospheric conditions, so 160m, 80m, and 40m (and 60m where it is available) are the most likely bands to use. It varies throughout the day, with the seasons, and with the 11-year sunspot cycle. Some hams are used to thinking “40m during the day, 80m at night”, but here at 45 degrees latitude we have needed to use 80m during the day and 160m at night more often due to poor conditions (even in summer). And when the Critical Frequency drops below 2 MHz, even 160m may not work for NVIS (although ground wave might get through).
Successful NVIS operation requires being able to change frequency to suit current conditions, rather than making assumptions and hoping the ionosphere will cooperate.
Click here for current critical frequency map. (Opens in a new tab.)
I find the Local Area Mobile Prediction (LAMP) forecast from the Australian Space Weather Services to be handy for planning NVIS frequencies. It shows the expected best band vs. time of day for communicating over distances out to 1000 km (600 miles). To run it, click the link above, click HF Prediction, and choose LAMP. Then click on your location on the map (or enter the data in the “Base” tab), choose your frequencies (the amateur bands presets are along the bottom buttons), select an appropriate T index, and hit PREDICT. My current plot as I am typing this looks like this:
This is run for a specific latitude, longitude, and date, based on the current conditions. The vertical axis is time of day (UTC). The horizontal axis is distance in km. Looking at the color key at the top (which will vary depending on conditions and the bands selected), for this plot the red area shows the times and distances where 160m is the best (and likely the only) band that will provide NVIS coverage. The yellow portion shows where 80m will work best. The dark olive portion shows that 40m will only be better than 80m for distances over 500 km (300 miles), and only in the middle of the day. The green spot in the lower right corner shows that 30m would open for 1000km (600 mile) paths around noon local time. Obviously, this is not a good time to expect to use 40m! (We are in winter at the bottom of the sunspot cycle, so this is not a big surprise.)
To use NVIS, in addition to choosing an appropriate frequency where the ionosphere is cooperative, we want to radiate our signal straight up (or close to it). One advantage with this approach is that mountains between the two stations don’t obstruct the path.
The required elevation angle depends on the desired distance (and the height of the ionosphere). In the drawing above, for a nominal F-layer height, signals at 80 degrees above the horizon (10 degrees from vertical) will reflect back to Earth at a distance of about 150km (90 miles), while those at 60 degrees above the horizon (30 degrees from vertical) cover about 500km ( 300 miles).
Fortunately, installing an antenna with maximum radiation straight up isn’t difficult: a dipole or other horizontally polarized antenna at heights below about 3/8 wavelength will have this type of pattern. Many simple NVIS antennas will have a broad upwards beamwidth, so there is significant radiation even at 60 degrees. Here are some typical dipole radiation patterns at different heights:
The gain and radiation pattern varies with the height above ground. Heights of 0.1 (blue), 0.25 (fuscia), and 0.375 (turquoise) wavelengths are most suitable, though at the latter the gain straight up is starting to drop as maximum radiation shifts to lower angles. At 0.5 wavelength (red) there is a significant overhead null, while at 0.03 wavelengths (green) the gain is lower due to ground losses.
The gain also depends on the ground conditions. This plot shows gain vs. height for an 80m dipole over 4 types of ground, to give a sense of the variation:
Notice that the peak of the gain curves are very broad, somewhere between 0.13 wavelengths (10m, 35 feet) and 0.25 wavelengths (20m, 65 feet) depending on soil characteristics. However, for any of the soil conditions, a dipole at 0.07 wavelengths (6m or 20 feet), as shown by the red arrow, is within about 3 dB of the peak, and, especially for portable operation, this may be more practical height. With an inverted vee, try to keep the ends at least half that height above the ground. At lower heights, ground losses increase, although contacts are often still possible with the antenna 1m (3 feet) above the ground, or even less.
My recommendation for a maximum height for an NVIS antenna is about 0.3 wavelengths. So, for 80m this would be 24 m (80 feet: conveniently, in Imperial units the maximum height in feet is equal to the wavelength in meters.) At this height, overhead gain is down about 1 dB from the maximum. As shown by the 0.375 wavelength height plot in Figure 3, antennas at greater heights may still work, but interference from more distant sources may become more of a problem.
Choice of Antenna
Note that a vertical antenna often has a null straight up, which makes it less effective for paths shorter than about 300km (200 miles). The following plot shows the radiation pattern of a vertical (red) compared to a broadside dipole (blue) and a dipole off the ends (green):
Some of the simplest antennas, like the half wave dipole or inverted vee, and full wavelength loops (either horizontal, or fed for horizontal polarization) work very well for this purpose when they aren’t too high above ground. If you don’t have enough distance available, loading coils or folding the antenna can make it fit. For very limited space applications, a Small Transmitting Loop (mounted in the vertical plane) can also work well, as long as it is reasonably efficient (which is more difficult on 80m and 160m).
Maintaining an appropriate pattern from a single antenna on multiple bands, however, isn’t always an easy task, especially over a 4 : 1 range from 160m through 40m. For example, an OCFD or end-fed half-wave horizonal wire for 80m will have an overhead null in the pattern on the second harmonic on shorter paths:
(But note that lobe at around 60 degrees can be quite effective at distances around 500m / 300 miles off the end of the wire.) Horizontal full wave loops tend to have the same problem, though the null might not be as significant for some triangular shapes as it is for squares.
An easy solution is to use multiple dipoles on a common feedpoint for the desired bands, such as my portable dipole kit. This can be simplified, with some savings of space, by using a single loaded element for 40m and 160m that is about the same length as the 80m dipole, although it narrows the operating bandwidth somewhat, and the recommended minimum height for 160m is near the maximum for 40m, so it isn’t perfect. The Bow Tie Loop was designed to provide dual-band operation on 160m/80m, and the performance on 40m is often adequate. (Of course, it can also be built for 80m/40m.) An 80m doublet will have added overhead gain on 40m, but a the pattern of a 160m doublet has an overhead null on 40m unless the wires are bent. OCFD antennas will often work well enough on the second harmonic if the antenna is bent 90 degrees at the feedpoint, though it may shift the SWR curve somewhat.
For those using an end-fed wire with a remote tuner, I have a separate article on NVIS designs.
Further NVIS discussions:
- What about the US Military AS-2259 NVIS Antenna?
- Does lowering the antenna reduce interference?
- Can I use a single wire fed against ground with a remote tuner?
- What if I want more gain than just a dipole?