NVIS for EMCOMM and Survival Communications.

Not clear on the concept.

If you or someone you know has an HF station with a goal of DX communications in disaster situations, I have a question: What are you really trying to do? What is the need for very long distance communications in a SHTF scenario? Save for a few narrow, exceptional circumstances, you probably cannot give a solid answer to the question. A more practical HF communications plan for disasters is NVIS, or near vertical incidence skywave.

Reality check.

In a disaster situation, talking to another continent will not have much utility. That’s why I don’t understand these survival and EMCOMM guys who invest time, money, and effort into HF stations expecting to run DX. A more useful and productive area of interest will be your immediate locale, extending out a few hundred miles or so. That’s where NVIS comes in.

VHF/UHF is good for about 25 miles/45 km, maybe fifty miles with a good antenna and favorable conditions. For intermediate ranges of 50-500 miles, VHF/UHF isn’t enough, and conventional HF is too much. NVIS is an underrated communications technique that fills the gap between local and DX.

How NVIS works.

High frequency DX works by “skipping” a signal across the ionosphere. For best results, the radio signal should hit the ionosphere at a sharp angle.

Imagine a rubber ball. When thrown straight forward (almost parallel to the ground), the ball will travel a pretty good linear distance and bounce several times. Now suppose we threw the ball again. This time, we throw the same ball with the same amount of force, but at a nearly right angle to the ground. The ball would bounce the same number of times but the linear horizontal distance it travels from where it was thrown would be relatively small.

GRAPHIC COURTESY RADIONERDS.COM

This is how NVIS works. By sending a signal out at a near vertical angle to the ionosphere, it will by nature of physics travel a shorter linear distance. In practical terms, this distance is 50-500 miles, a range that nicely fills the hole between local and DX communications.

Other benefits of NVIS.

If your antenna is set up to pitch a signal a limited distance, it naturally follows it will only receive from a limited distance as well. While this sounds like an undesirable trait, it actually works as an advantage. This “limitation” has a built-in ability to reduce noise and interference from distant signals.

Secondly, NVIS stations have no concern about common physical obstructions or antenna elevation. A DX operator will want their antenna as high as possible to get that acute angle. An NVIS operator is sending their signal nearly straight up, so all they need is a clear shot at the sky. Nearby mountains, buildings, trees, and similar barriers to radio coverage are not an issue.

Implementing NVIS.

Setting up an NVIS station is crazy-easy. An operator could do it with equipment they already own.

As far as radios go, there are no “special” NVIS rigs. Any HF radio will do. The action is all in the antenna. If you have a dipole or similar wire antenna, mount it so it is between 0.1 and 0.25 wavelengths above the ground. This translates into 13-23 feet (3.96-9.75 meters) on 40 meters. This should give your signal the near-vertical angle needed to make NVIS work.

Vertical antennas are not effective for NVIS. If you have no other option, mount your vertical antenna low to the ground. Beam antennas are also not a prime choice. If you’re giving it a go anyway, point it a few degrees off from straight up. Horizontally-oriented antennas such as dipoles are the most effective.

PHOTO 1. COURTESY HAMUNIVERSE.COM

On disadvantage of NVIS is that it generally works best under 10 mHz, so the choice of operating frequencies may be limited. It can be done at higher frequencies, but it’s less reliable and some trial and error may be needed.

NVIS is ideally suited for QRP operation. Since it’s not DX so you don’t need a big boomer powerhouse transmitter nor an elaborate antenna. And as we already mentioned, the nature of the antenna reduces noise and interference. That means weaker signals are easier to copy.

Sweating the details.

For SHTF purposes, ideally both you and the the station you are communicating with should be configured for NVIS. It’s not absolutely required, but it does give each side the best chance of hearing the other. Setting up a schedule on pre-determined frequencies is also a smart move.

If for some reason DX is essential to your survival communications, then the easy solution is to have one antenna for DX and one for near vertical incidence skywave. NVIS-specific antennas are simple, cheap, and take up very little weight and space for transportation. There aren’t very many good reasons not to have NVIS capability.

If you are part of an organized EMCOMM or disaster response group, there’s a strong chance that they do not even require members to have HF equipment. But if HF is in the mix, then it probably specifies NVIS.

When planning for disaster/survival situations, there will not be a pressing need to work DX. NVIS is more practical, can be done with minimal equipment, and lends itself well to QRP which will reduce the burden on your power source.

Resources.

Here is a lengthy but detailed technical document describing NVIS.

This in depth article has good info about NVIS-specific antennas.