Near Vertical Incidence Skywave- Simplified

I’ve brought this up enough times already; let’s de-mystify this beast. Communications fall into one of two categories: Line of Sight(LOS) and Beyond Line of Sight.

LOS

If you can see it, in theory at least, you should be able to communicate with it. Low-band VHF(10M/11M/CB) and above(UHF, Microwave) works in this manner. VHF can have some characteristics of HF; but that’s beyond the scope of this article. Squad level communications work in this manner. This would be your mobile rigs and HTs. Keeping it simple, if there’s something big in between you and the person your talking to(like a mountain or a bunch of buildings) or long distance, you need a repeater to compensate. Line of Sight(plus repeater) looks like this:

Beyond Line of Sight

So what if you’re outside the range of repeaters? Eventually the energy from your radio or repeater will fizzle out. At some point it hits what’s known as the radio horizon, where the signal keeps going straight and the Earth curves downward. Say, maybe I want to communicate halfway across the country. This is where HF comes in. Long distance communications relying upon radio waves bouncing off of the ionosphere. Kinda sorta like this:

There’s a problem though. Inside the skip zone you may or may not make contact with the intended receiver. From a hobbyist perspective, it may give a thrill to make random contacts, but from a reliability standpoint, it leaves something to be desired. Skip Zones look like this:

Many seasoned Radio Amateurs will tell you that the most difficult challenge to communications is that area inside that skip zone. How do we solve this? Something called Near Vertical Incidence Skywave.

NVIS

HF radio wave propagation can be shot nearly straight into the Ionosphere, hence the name Near Vertical Incidence. What goes up, must come down. Knowing that all things have equal and opposite reactions, the angle coming down is also nearly vertical. Then it goes back up, and down again, and over and over. Like this:

And it gives you a range Beyond Line of Sight, at the tactical level, like this(ignore the TACSAT in the photo):

With a regional picture looking something like this(remember Planning your Footprint?):

So what does this give us? We now have Beyond Line of Sight Communications that do not rely upon repeaters. Once operators are decently trained and have a good amount of time working in this method under their belt, it can be extremely reliable.

Important to note is that not all HF bands work well for this. Generally speaking, 80-40M work best. Experience as a Radio Operator should tell you when to use which band based on noise level, the amount of heard traffic, and beacon propagation near your operating frequency will give you a good idea if your traffic will be successfully transmitted or not.

Implementation at the Tactical Level

Just like any tool, it has a use. For units spread far and wide, such as perhaps a statewide area, NVIS is just the ticket to communicate between units. Digital means of message delivery such as RMS Express work well. And with some of the great QRP options at hand these days(such as a Raspberry Pi and an 817) such a system is lightweight and quickly deployable. In addition, it works well in Mountains or other areas where a large number of repeaters would be needed to facilitate communications.

Antenna Options

While there’s literally TONS of antenna options out there for HF, the simplest option I’ve found is the wire dipole. Remember 936? Yeah, it’s still important. Keeping it simple, a resonant dipole can be quickly constructed and erected. Each leg of the dipole is 1/4 wave long…so…

234/freq= antenna length in feet.

I use a 4:1 balun as a center to make using coax easy and keeping the impedance matched at 50 Ohms. Insulate the ends, hoist the center, and presto…you have an antenna.  The antenna itself should be no more than 1/4 wavelength high. Some sources say the lower the better, some say it makes no difference. For me, in my experience, 1/4 wave works just fine. Less works fine too. Just do what you can; field expedience rules the day. What enhances the NVIS setup more than anything is the use of a counterpoise. This is simply a wire used as a reflector, just like the “legs” of our Jungle Antenna. It comes out like this:

When using resonant antennas, since they’re cut to length for a specific frequency, a tuner is not normally required. It’s a good idea to use it anyway; it protects your radio. Something important to know is that once the antenna wire is cut to length, depending on many factors it may need to be shortened more to attain a usable SWR(2:1 or less). A tuner makes this unnecessary.

So with some simple, inexpensive parts(such as this Jetstream 4:1 balun, an excellent piece of kit for $30 and some cheap speaker wire):

And this:

An NVIS station can be put up almost anywhere, run from a couple SLA batteries also pictured connected to the 897, and sustained with a solar panel. And I can take it down and move out just as fast as I put it up.

Get to it Ladies and Gentlemen.