[TCARC-NTx] Learn NVIS

[David Johnson]

I hope you had a great weekend hamming it up.

I had a great time.  I attended a MARS meeting and learned alot and met a
lot of really dedicated, good hearted people.

I also participated in a multi-state MARS emergency communications
simulation and test with a scenario of a Terrorist Attack with multiple
attack modes and multiple cities in harms way.

During the test I learned something pretty important that you might benefit
from, too.

During one of the Texas nets, there were multiple observations about how bad
the propogation was
really poor on 40 meters (around noon today).

However, after the net closed, a net from another state came on and all
their signals were coming in perfectly clear and readable.

What was the difference?  Proximity.

The Texas stations were not close enough to each other to work the ground
wave propogation.  But they were not far enough apart for skywave
propogation to work well either.

The solution?  NVIS.

Near Vertical Incidence Signal operations.  Sounds technical, but it's easy.

Most amateur radio antennas are designed and positioned to radiate their
signals as far to the horizon as possible, to extend the distance of the
skywave propogation. That's the "low angle of radiation" desired in
antennas, meaning more energy is radiated out to the horizon. The result can
leave a dead zone between the end of the groundwave region, and the
beginning of the skywave returning to earth.

To improve signals into the dead zone, you have to chance the incident angle
from nearly horizontal to more vertical.

The easiest way to do this?

Lower your antenna.  That's it.

As it turns out, the closer an antenna is to the ground, the more of it's
energy is directed up (nearly vertical), instead of out.  I have read
several articles about Near Vertical Incident Signal operations, but now I
know why it is so important.

Not all communication is directed to the other side of the planet.
Sometimes we want to talk to people just 100 miles away, for example.

What height should the antenna be for NVIS?

According to a study done by the military, about .15 wavelength off the
ground is the best mix of suppressing ground noise, and getting good near
vertical radiation.

For 40 meters, that's about 20 feet, give or take a few feet.  Works fine
for dipole or vee configuration.

I added a 40M and 80M dipole to my tower today, at about 25 feet, and got
instant good signal reports from stations that earlier could hardly make out
my signal.

What if I want to talk to stations that are both near (in the NVIS zone) and
far (Skywave zone) during the nets?

Well you can try to find the best height for your antenna that will give you
propogation to the areas you want to work.  Or just use two antennas and
switch between them using an antenna switch.  Any station complaining they
can't hear you may hear you quite well if you switch to your alternative
antenna position.

This NVIS mode is also good for getting signals over mountains or other
obstructions, too.

David KB5YLG







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