Near Vertical Incidence Skywave (NVIS)
Near Vertical Incidence Skywave (NVIS) is an ionospheric skip operating technique that directs the strongest signals from a station vertically, or upward, rather than toward the horizon. Signals propagating nearly vertically approach the ionosphere with steep incidence angles and may be bent back to earth with similarly small angles. The operational result is skip communications effective within a radius of a few hundred miles. The NVIS technique can help to bridge the communications gap between the local range of VHF/UHF repeater or simplex communications and the longer distance skip propagation of low-to-the-horizon HF signals.
The NVIS technique relies upon a combination of station factors, most importantly the frequency used, the power of transmissions, and the antenna configuration. Let’s consider each of these three factors in the context of the NVIS technique.
Frequency: The refractive effects of the ionosphere vary with frequency. The bending effect on signals is reduced as frequency increases. This is why the 2-meter band (144 – 148 MHz) and higher frequencies are almost never received via skip propagation. The HF bands of 10-meters (28 MHz) to 30-meters (10 MHz) are often effectively refracted back to earth’s surface when directed toward the horizon where incidence angles into the ionosphere are closer to the horizontal, and this propagation geometry provides long skip distances with single skips up to 2500 miles. However, the ionosphere usually does not have sufficient bending strength to return these upper HF band frequencies to earth with the steep take-off angles necessary for the NVIS technique.
The ionosphere’s bending effect is sufficient, even at steep “near vertical” angles of incidence, to bend back to earth the lower HF frequencies, particularly the 40-meter band (7 MHz), 60-meter band (5.3 MHz), and 80-meter band (3.5 MHz) signals. These bands are most suitable for the NVIS technique, even during daylight hours when more distant skip propagation on these bands is ineffective due to D-layer absorption.
Transmitting Power: Transmitting power with the NVIS technique does not need to be great. Very effective NVIS communication can be completed with the typical 100 watts of many HF transceivers “running barefoot.” In good ionospheric conditions much lower power may be quite sufficient for effective QSOs. When atmospheric conditions are less favorable, increasing transmitting power with the use of an RF power amplifier can help to keep NVIS communications reliable. It is common for amplifiers to be employed by NVIS operators in the high daylight part of the day when the D-layer absorption attenuates signals more severely.
The D-layer of the ionosphere normally absorbs skip signals below the 30-meter band during daylight hours, so long distance skip is not effective on the low bands during the day. These bands open for long distance skip at night when the D-layer dissipates and the F-layer refracts these frequencies. However, since NVIS signals travel through the D-layer at very steep angles, the transit distance through the layer is minimized, as compared to the long skip signals traveling low to the horizon. As a result, D-layer absorption of NVIS signals is minimized, and NVIS is usually a viable technique throughout the daylight hours, with performance variations for ionospheric conditions.
Antenna Configuration: Perhaps the most critical factor, and certainly the most controversial among ham discussions, is the antenna configuration for NVIS that produces the best vertically directed signals. Let’s consider the basics first, and then we will address some details that are not universally agreed upon.
A horizontally polarized antenna provides the best NVIS propagation. A wire half-wave dipole trimmed for the frequency of use is very effective and also the most common type of antenna used for NVIS. Horizontal full-wave loop antennas are also very effective. In the half-wave dipole case, a flattop configuration or mildly down-sloped inverted V configuration works well. But, regardless of the specific type of horizontally polarized antenna used, the key factor in configuration is the antenna’s height above ground.
To direct the greatest portion of the transmitted signal vertically, the antenna must be positioned relatively low to the ground. The interaction of directly radiated signals with ground reflections results in more signal strength radiated in the vertical direction when the horizontal antenna is much less than ½ wavelength above the ground. Height above ground is usually less than ¼ wavelength for the NVIS technique, and much lower heights are preferred by many operators due to reported performance improvement. A height of 1/8 to 1/10 wavelength is often used for effective NVIS. On the 40-meter band a dipole elevated just 4-meters (13 feet) above ground can provide very effective NVIS propagation in a radius of several hundred miles.
The precise height above ground for the very best NVIS performance is not a well-agreed value. Antenna models reported by Jack Swinden W5JCK (and based on work of L.B. Cebik W4RLN) seem to point to best performance on 40-meters at 0.175 wavelength (7 meters, ~21.7 feet) above ground, and on 80-meters a height of 0.165 wavelength (13 meters, ~41 feet). Pat Lambert WØIPL has conducted extensive objective data collection in Colorado and reports an experience of better coverage with a height of only 1/20 wavelength above ground. He notes that noise is significantly reduced as the antenna is lowered below 1/8 wavelength, and that communications with close stations (up to 300 miles away) was greatly enhanced with such low antenna height, particularly using the 80-meter band.
Other Factors: Beyond the antenna height, power, and frequency, other factors will impact performance. The height above ground effects the dipole feed point impedance. As the dipole is lowered below ¼ wavelength the feed point impedance will be significantly reduced in value, and SWR may rise. For best performance, trim the dipole antenna while at the height at which you intend to use it.
The local ground conductivity will impact performance, with the poor conductivity of rocky or sandy and dry soil reducing antenna gain. With a more conductive ground, such as richly conductive and moist soil, antenna gain will improve. This brings up another less-than-solidly-agreed factor, the use of a parallel ground wire under the horizontal dipole element. You may think of this arrangement as a vertically pointed, two-element Yagi directional, with the ground wire providing an enhanced “reflector” element.
A parasitic wire reflector is usually implemented 5% longer than the driven element, or 5% longer than the half-wave dipole, and positioned below the driven element. The distance below the driven element is usually recommended as 0.15 wavelength, although other values are also advocated. Various sources recommend the ground wire be elevated above the surface of the earth 0.01 to 0.06 wavelength (1.24 to 7 feet for 40-meters) for best effectiveness and least impact on the antenna’s SWR bandwidth. Implementing the wire reflector narrows the SWR bandwidth somewhat, and Jack W5JCK indicates a substantial narrowing of 25% to 50% with the reflector wire on or near the earth. Further, his data claim a transmit gain with such reflectors of only 0.2 dB to 0.7 dB in the best cases, putting into question the value of the ground reflector wire. On the other hand, Pat WØILP reports up to 6 dB improvement of the transmitted signal with some experimental ground wire configurations he has tried.
The upshot of these conflicting data and reports is that the arena of NVIS antenna configuration is ripe for experimentation! It is most likely that the variation among models, reports and claims is a result of uncontrolled factors that impact NVIS antenna performance. Soil conductivity, height above ground, reflector element implementation and configuration, other RF-coupling conductors in the vicinity, varying atmospheric conditions, transmitter power levels, transceiver and feed line quality, precision of signal strength measurements, and perhaps many other things can impact the measured performance of the NVIS antenna. So, perhaps the best policy is to familiarize yourself with some of the theory of these factors and then try a few things to see what seems to work best for your specific situation.
The Bottom Line on Antennas: If you are not aiming for the very optimal NVIS station by manipulating the somewhat controversial factors above, a horizontal wire positioned a fraction of a wavelength above the ground will likely provide you quite acceptable short radius communications via NVIS propagation paths. I often erect a 40-meter wire dipole in a gentle inverted V or flattop configuration at 1/10 wavelength (13 feet) above ground, with no reflector ground wire and above my absolutely terrible Colorado rocky, dry soil. With a 100 watt signal I frequently make clear contacts of 25 to 500 miles. The following describes my NVIS portable antenna solution, only one of many different ways to implement such an antenna.
A 40-meter NVIS Portable Dipole Concept: I constructed this 40-meter dipole to readily switch between wire elements and loaded hamstick elements. The center mast connecting component is the MFJ-347 Double T Pipe Mount. It mounts easily to any mast up to 1/25” diameter, and I use an extendable painter’s pole supported by a second-hand utility tripod.
This MFJ connector sports an SO-239 coaxial connector and two standard 3/8”– 24 thread antenna mounts. One threaded mount is electrically connected only to the coax center conductor, and the other only to the coax shield, as required for a dipole antenna. Simply connect the pair of dipole driven element conductors, one to each mount, and you have a simple dipole antenna. With the MFJ connector mounted to the painter’s pole extended up to 13 feet, and with the driven element properly extended and anchored at the end points, the NVIS dipole is ready to operate.
For the full-length wire dipole, I connect a pair of 3/8”-24 thread bolts to the MFJ-347, and ring connectors with the dipole wire soldered solidly into them are snugged down using star washers and nuts. Alternatively, I sometimes opt for the convenience of two 40-meter hamsticks, specifically MFJ-1640T HF Stick. The HF Stick each fit into the same 3/8”-24 mounts, replacing the wire elements. The SWR bandwidth is narrower and performance is somewhat reduced (roughly -3 dB) when using the loaded and shortened sticks rather than the full length wire, but the stick dipole antenna can be quickly deployed with my portable station and it provides acceptable NVIS communications in most instances.
NVIS is one of my favorite operating techniques. I really enjoy connecting with hams in my local region, and NVIS is terrific for emergency communications across the local area outside of repeater range, or in the case of repeater failure. Throw up your own low altitude wire dipole and give NVIS a shot!