ARLA/CLUSTER: O que é a Propagação NVIS.? por Onno (VK6FLAB)

[João Costa > CT1FBF]

Foundations of Amateur Radio #171


*Cloud Warming in style or what is NVIS?*
The term *NVIS*, or *Near Vertical Incidence Skywave* is in my short
experience as an amateur heaped with scorn and ridicule. Terms like
cloud-warmer come to mind when people discuss the principles associated
with NVIS, but that does happen in the context of where I live, that is,
one of the most isolated cities on the planet, Perth in Western Australia.

NVIS has several advantages over other forms of HF communication, it can be
done with low power, there is little or no signal fading, simple antennas
work well, it has low path loss, better signal to noise ratios and if
you're in a valley, you can still use it.

So what exactly is NVIS?

In the past I've talked about long distance HF communication. Your radio
signal bounces off the ionosphere, bounces back to earth and so-on. Like
skipping a stone on a pond, the angle at which your signal hits the
ionosphere determines what happens next. In general, shallow is good, steep
is bad, much like the plop you hear when you don't hit the pond just right,
a radio signal can go through the ionosphere, never to be heard again.

NVIS is about hitting the ionosphere at a steep angle, in such a way that
it reflects back to earth. Without going into detail, generally you can use
40m during the day and 80m at night with some variation depending on the
solar cycle and whom you want to talk to.

NVIS gives you communications less than 1000 km away, plenty to talk to
everyone in your city and surrounding area. In the case of an emergency
that's also likely enough to get out of any emergency affected area, so
plenty of excuses to set up and try for yourself.

I can start talking about angles, maximum usable frequencies and so-on, but
I won't. These all relate to specific circumstances, depend on what antenna
you're using, what the ground conductivity below you is and as is typical
in our hobby, many other variables.

What I can say is that NVIS to NVIS station works best, so if you're going
to test this with a friend, it will help if you both set up a similar
station while you learn the variation associated with this kind of
communications.

Now I did mention up to 1000 km, that isn't enough to leave Western
Australia, Perth to the border is about 1500 km, but if you live in the
Netherlands, you can get to 15 or so countries. Depending on where you are,
NVIS will give you different outcomes and what I'm talking about affects
each station differently.

For me, the attraction of NVIS is solid communications on 40m and 80m,
something that has eluded me so far. It also allows for a low simple wire
antenna, an inverted vee dipole, two bits of wire strung up on a pole, 6m
in the middle 2.5m at the end will get me up and running. Perfect for a
field-day, excellent for a local contest and brilliant if you're only using
low power as a beginner.

Because the antenna is close to the ground, it's pretty much
omni-directional. If you set-up an antenna for 40m and then cross that with
an 80m antenna and feed them both from the same point, you'll have a
configuration that will operate well for 24 hours without needing to move
antennas in the dark.

I have no illusions that an NVIS antenna will help me make contact between
Perth and Japan, but then it's not intended for that. I've spoken in the
past about finding the right tool for the job. NVIS is a tool, it has a job
and it's very good at doing that. It's not for everyone, all the time, but
it's a tool that you as an ambitious amateur should know about.


I'm *Onno VK6FLAB*

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