Near Vertical Incident Skywave

Near Vertical Incident Skywave antennas (better known as NVIS antennas) are one of the most viable antenna possibilities that survivalists might use. They are quick to deploy, easily maintained, and are high performance antennas as well. For simplicity the remainder of this article will use NVIS to denote the information is about Near Vertical Incident Skywave antenna systems.

While the term NVIS might be new to most it it highly likely that a survivalist or prepper might already have a NVIS antenna in their communications arsenal.

As for the application of the NVIS antenna it would pay us to realize that NVIS antennas are generally lightweight, high performance antennas, and quickly deployed. Just these three attributes make NVIS antennas highly desirable for both survivalists and preppers.

One other feature of NVIS antennas is that the always “handy with their hands” survivalist and prepper can make their own NVIS antennas. All that is needed to construct a NVIS antenna is a vertical support and wire. For the wire you can use wire gauge as small as 24 gauge as long as you will not be using any type of large amplifier.

If you want a really great portable antenna mount then you might want to take a look at the Tilt-N-Raise® portable antenna mount. This unique mount slides into the receiver hitch on the back of your vehicle and allows you to quickly and safely to tilt the antenna up into place. For more information about ordering the Tilt-N-Raise you can visit their website at: http://www.tilt-n-raise.com.

Here is part of an article that goes into the nitty gritty of NVIS antenna technology by Harold Melton, KV5R who is the copyright owner of the full article. The full text of the article is available in its entirety at the following link:

Understanding Amateur Radio NVIS Antennas and Propagation

What is NVIS?

Near-Vertical Incident Skywave is a combination of radio hardware, skywave radio propagation, operating procedures, cooperation, and knowledge used by a group of radio operators who need reliable regional communications. It fills the gap between line-of-sight groundwave and long-distance “skip” skywave communications.

German ground forces first documented NVIS techniques in WW-II. NVIS was more fully documented, studied, and used by US forces in Vietnam. Radiomen in military vehicles discovered that their HF whips would sometimes work much better when tied down horizontally. Amateur radio operators have been studying NVIS propagation and operating techniques for at least fifteen years. In tactical military use, NVIS allows communications around the region while providing very little groundwave signal for the enemy to home in on. Any radio operator that has used a horizontal antenna well under a half-wave high has used NVIS.

NVIS propagation is generally considered to be F-layer ionospheric reflection at angles of 70-90 degrees. It is skywave propagation without the usual skip zone. The purpose of NVIS is to communicate locally and regionally, out to a few hundred miles, with moderate power, simple antennas, and no skip zone. NVIS is typically used on 160, 80/75, 60, and 40-meter bands by Amateur radio operators using relatively low horizontal wire dipole antennas.

NVIS operations are optimized by understanding and controlling two major factors: (1) Proper antenna design and placement, and (2) proper training of the operators. The antenna is designed and placed to provide the maximum possible gain straight up, on two or three frequency bands. Operator training includes an understanding of antennas, ionospheric propagation, and operational procedures.

Why Do It?

First and foremost, to completely eliminate the skip zone. This enhances all forms of local and regional HF communications, for all practical and experimental purposes.

Emergency groups such as ARES and RACES are studying NVIS propagation, techniques, and equipment deployment for emergency preparedness. NVIS is the tactical communication system of choice in mountainous areas, any areas without complete repeater coverage, and all situations where repeater-based systems have failed or might fail. With the recent release of manufactured mobile and even portable HF radios, HF, and antennas employing NVIS propagation, should become much more popular and useful for disaster tactical communications.

Researchers and users have observed that NVIS antennas work considerably better in the valley than on the mountain top. This is due to much better ground conductivity in the valley than on the dry, rocky mountain top. This happy fact eliminates a lot of unnecessary climbing, and allows the antenna to utilize trees for both support and cover.

NVIS-equipped Amateur fixed stations enjoy regional nets and rag-chews without the annoying skip zone. It is particularly useful to net controllers and emergency practice groups. All fixed stations should take steps to immediately supplement their antenna farms with at least a dual-band NVIS antenna (described herein).

Antenna and propagation experimentation is FUN! Building and deploying antennas is as close as many hams get to home brewing. NVIS is as easy as antenna experimentation can get. The antennas are simple, and are installed very low. Light-gauge wire and nylon string may be nailed to trees at extension-ladder heights. Dropping a dipole and making a change to it takes only minutes and may easily be done by one person without the need to obtain helpers or plan a big event.

NVIS antennas are stealthy. Communism-by-contract property owner’s associations have restricted the placement of visible antennas and severely stifled Amateurs’ pleasure, emergency preparedness, experimentation, and innovation. With NVIS, a fine wire may be brought through the trees, or routed along the top of a privacy fence. The Ham thusly equipped may never win any low-band DX awards, but will still have ample opportunities for QSOs and nets within the regional circle provided by an NVIS antenna in the daytime, in addition to some low-band DX at night, particularly in the winter when the storms are gone.

If you could only have one antenna, it should be an NVIS with ladder-line feed and a tuner, as this may be operated on all bands. The “best” multiband antenna is probably the 260-foot dipole, or 520-foot loop, with 76 feet of windowed ladder line and a tuner.

How to Make a Good NVIS Antenna

The best NVIS antenna is one which is simple and effective. One favorite is the dual-band dipole. This antenna uses two dipoles, one for 75 meters (about 122 feet), and one for 40 meters (about 65 feet), both connected directly to 50-ohm coax and supported at 5 points by trees at 10-12 feet. The two dipoles should be well separated at the ends, or they will interact. They may be strung up in an “X” or a “+” shape. The bandwidth of the 75-meter dipole will be quite narrow (<100kc), so it will benefit from using two sets of stagger-tuned wires. Some researchers recommend that the ends of the wires should be a few feet higher than the middle. This will increase gain and raise the feedpoint impedance a bit. If the feedpoint impedance is too low to match, the antenna should be a folded dipole, which will raise the feedpoint impedance by a factor of four. Stringing the antenna over a highly conductive surface, such as salt water or a wet, acidic marsh, will substantially improve the antenna’s performance, compared to stringing it over dry rock or sand.

Since the support points are typically 10-12 feet high, the wires must be both light and pulled tight to remove annoying sag. Appropriate wire ranges from #17 aluminum electric fence wire, ($14 for 1/4-mile at farm-supply stores), to #14 insulated stranded copper THHN, ($15 for 500 feet rolls at electrical suppliers, and available in green). The #17 aluminum isn’t very strong, but is almost invisible. The wire may be supported with green nylon string, available at garden centers. The center feedpoint and coax may be built around a simple insulator, waterproofed, and nailed to a tree trunk at 10 feet. Insulators and coax may be sprayed dark green or brown as needed. Antennas below 8 feet should use insulated wire to avoid RF burns. Insulation does not affect the performance of antenna wire, except (1) reduced wind and rain static, (2) lowers the velocity factor a tiny bit, and (3) prevents corrosion.

It is better to use a broadband current balun at the feedpoint when using coax. A simple choke balun made of coiled coax may be used if needed to remove common-mode currents from the line. Try to design the installation so the feedline extends away from the antenna at a 90-degree angle, for at least one-quarter wave. Also, the line should be detuned — that is, it’s length should fall between resonance points. If these are done, feedline RF pickup and re-radiation will be minimized, and a balun should not be needed. Detuning the feedline is also the cure for “RF in the shack” problems. Suitable lengths will depend on how the antenna is fed and whether one side is grounded or not. See the Antenna Book for determining appropriate lengths.

For resonant dipoles, avoid using twin-lead or ladder line — the feedpoint of these low dipoles will be well under 50 ohms and attaching 300-600 ohm parallel feedline will present a severe mismatch at the feedpoint. However, if the antenna is to be used non-resonant, with a tuner, ladder line should be used because coax is very lossy when operated at high VSWR.

There is a long-standing myth that dipoles must be resonant to be efficient. Non-resonant dipoles of similar size are just as efficient as resonant dipoles, assuming that (1) impedance mismatches are matched, (2) the matching devices are designed so that losses are insignificant, and (3) feedline losses are minimized (use ladder line when the SWR is high). It is also important to remember that baluns and matching transformers are quite lossy when operated with a mismatch on either or both ends. The ARRL Antenna Book shows how to make baluns for any ratio of impedance transformation. The myth come from the fact of severe losses in mismatched coaxial line. In the author’s experience, a 160-meter dipole fed with ladder line will outperform a 75-meter resonant dipole fed with coax, both at the same height, and both operated on 75 meters. This is because the larger antenna, even though not resonant on 75, has an “aperture” twice as large as the smaller one and thus captures, and radiates, more signal. However, it does have 4 partial nulls, while the half-wave dipole has only two.

To connect aluminum or steel wire to copper, make a couple of short #14 solid copper pigtails, twist them tightly into the aluminum or steel elements at the feedpoint, then solder an SO-239, or direct coax feed, to the copper tails. Waterproof the dissimilar metals connections with waterproof grease and Coax-Seal, or silicone caulk. If any moisture gets into the connection, the metals will corrode one another and make a nasty rectification point. Mechanical connectors (split-bolts or set-screw lugs) may be used but they also should be waterproofed.

Fancier (more expensive) NVIS installations include full-wave loops with automatic antenna tuners at the feedpoint. These antennas, if installed at 15-20 feet or more, will provide both excellent NVIS performance on the low bands and DX on the higher bands, where the height of the loop is over 1/2-wave. However, the pattern of the antenna will have several peaks and nulls on frequencies where it is several waves long.

Two things about loops are worth mentioning: (1) Loops are resonant on every harmonic, not just odd harmonics like dipoles, and (2) the lower the frequency (greater the length) of the loop, the more harmonic points it will have. For example, an 75-meter loop will resonate at about 3.8, 7.6, 11.4, 15.2, etc. But a 160-meter loop will resonate at about 1.8, 3.6, 5.4, 7.2, 9.0, 10.8, 12.6, 14.4, etc. — and the peak SWR arising from imbalanced reactances will be lower between all these points. Therefore, a big loop should be strung up, even if it cannot be used on its fundamental frequency because of low feedpoint impedance.

Carrying this idea further, an operator with acreage might run a really big loop (like 1100 – 2200 feet) atop a perimeter fence and it would have so many resonant points as to be useful as a broadband antenna — although the fundamental and all harmonics below about 3-4 MHz might be unusable for transmitting due to extremely low feedpoint impedance, unless feedpoint matching is used.

Another good antenna is the 3-wire folded dipole. This design may be used on all HF bands, with a tuner. The rules are pretty simple: Make a 2- or 3-wire folded dipole as long as possible (preferably 260 feet). Feed it directly with ladder line, and match it to the radio with a balanced line tuner. Use a 1:1 current balun at the tuner’s input. The reasons: (1) Feedpoint resistance of low antennas will be very low, typically 15 ohms or so, and the 3-wire folded dipole will raise it by a factor of 9. (2) Ladder line does not suffer any significant loss when operated at high line SWR (unlike coax). (3) Balanced tuners with the balun on the input (the matched side) are considerably more efficient than unbalanced tuners with the balun on the output, because baluns are only efficient when both ends are matched.

Some emergency groups are successfully experimenting with mobile antennas mounted horizontally. For example, pairs of 75 and 40 meter Hamsticks make excellent shortened, portable NVIS dipoles. The mobile antennas are mounted back-to-back and fed in the center just like a dipole. These are oriented horizontally and placed a couple feet above the roof of a vehicle on a short mast.

Other operators carry (1) an autocoupler, (2) a 125′ roll of wire, and (3) traffic cones or fiberglass stakes in the trunk, for rapid roadside NVIS deployment. NVIS antennas have been used as low as 18 inches high. Surprisingly, S9 signals have been received from an antenna mounted 10-1/2 inches high.

Preparing for Portable NVIS Operation

Emergency communication groups should create and test an “NVIS kit” which contains a sturdy NVIS antenna, feedline, tuner, and sundry tools, hardware, and accessories. The radio should be a small, “all-band” rig like the Icom IC-706, powered by a deep-cycle battery. Hardware should include a 20-30 foot telescopic pole, #18 nylon line, stakes, throwing weights, hammer and nails, extra feedline and connectors, etc. Tools should include the usual electronics hand tools, including a small butane soldering torch and extra butane fuel. Accessories should include a folding table and chair, a rain tarp with it’s lines, and an ice chest with food and drink. Another piece of hardware worth having is the notebook computer, with appropriate software and cables, that may be used to provide radio teletype traffic. The station should also include a 2-meter transceiver and antenna, and a scanner.

The entire station may be packed in a medium-sized ice chest, using custom-cut foam rubber for the sensitive parts. The serious portable operator will also have a tent and various other camping equipment and supplies. Some clubs even purchase and equip a small travel trailer for this purpose. This is the best solution, since the trailer will contain all the needed equipment and supplies, at the ready, and will also provide a measure of security and protection from the elements.

Don’t have just one NVIS antenna. Have one at home (a dual-dipole, or multiband nonresonant, or a loop), and have another for fast portable deployment. The portable antenna and its feedline may be rolled up on an extension cord reel. You never know when you may be needed to quickly deploy a portable station. The goal should be to prepare to provide reliable regional tactical communications services without power mains, in the midst of large-scale emergency events. It’s also a good idea to have the radio “clipped” so that it may be operated outside the ham bands by emergency officials who are authorized to do so. The station will usually need to be located at the incident command post — however, it is very important to make prior arrangements with the authorities.

Tuners: The best tuner for barefoot NVIS is probably something like the MFJ 949E. It has a wide tuning range, internal balun with balanced output, three-position antenna switch, internal dummy load, and a large cross-needle meter. Of course, full power tuners must be used with linear amplifiers. Autocouplers by SGC and others work very well at the feedpoint, provided the impedance isn’t too low. The internal autotuners in most radios usually do not have sufficient range to match low antennas on 160. The 75/40 dual-dipole described above does not need a tuner, as the elements may easily be adjusted to resonance. If the SWR at resonance is still too high, raise the antenna a few feet, because the feedpoint radiation resistance is probably too low. Modeling over average ground shows a feedpoint resistance of 50 ohms at around 41 feet high. Short stubs with alligator clips may be clipped onto the elements at various places to provide multiple resonant points, and if bare wire is used for the antenna elements, these may be moved around to match the antenna (don’t burn your fingers).

Power Supplies: The portable station should use a deep-cycle marine battery and a portable generator. Small “camping” generators in the 900 to 1800-watt range, having both 13.8 VDC and 120 VAC outputs, are the most preferable. Connections to batteries should be made using ring lugs soldered to the wire, attached to the battery with stainless steel bolts, washers, and wing nuts. All connections should be greased. The battery should be connected to a power distribution box, of the type with several sets of 5-way binding posts.

{NVIS antenna article is Copyright © 2002, 2006 by Harold Melton, KV5R}