[HCARC] NVIS - Local 10 meter Propagation Test

[Kerry Sandstrom]

Gary and the rest,

A few quick words about NVIS.  NVIS is about as old as ham radio.  When I 
was a kid, most of my 80 and 40 meter contacts were NVIS although I didn't 
know it at the time and no one called it that anyway.  NVIS is simply using 
the the signal that goes up nearly vertically to the ionosphere and is 
reflected right bak down to the ground.  There is no skip zone so it is 
useful for short range communication.  If you have a low dipole, which most 
of us had in the 60's, you were working NVIS on 80 and 40 whether you wanted 
to or not.

There are ionosondes spread around the world in various unbelievable 
locations.  Ionosondes are automatic transceivers which send out a signal 
that increases in frequency from perhaps 2 MHz up to 15 or 20 MHz.  The time 
of arrival of the reflected signal and its signal strength is measured. 
>From these measurements you can determine the virtual height and the highest 
reflected frequency of the various layers in the ionosphere.  Thanks to the 
magic of the internet, you can get the data from probably 50 of these 
ionosondes in near real time.  The closest one I could find to the 
Fredericksburg/Kerrville area that had current data is in Boulder, Colorado. 
The highest vertical incidence critical frequency for the f-layer was 7.2 
MHz at 1545Z today, ~1200 local time.  At 1130Z just before sunrise (in 
Boulder) it was at a minimum of 3.5 MHz.  In order to use NVIS at 1145Z 
today, we would have to be at a frequency below 7.2 MHz, realistically, 
75/80 meters.  Before sunrise, we wouldn't even be able to use 80 meters for 
NVIS.  When the sun was more active a couple weeks ago, the vertical 
incidence critical frequency was in the 11-12 MHz range.  It is usually 
abbreviated as f0f2 although at night it is really f0f since the f2 and f1 
layers combine.

Right now if we wanted to use NVIS for a local daytime net, we would have to 
use 80/75 most of the time and once in a while we could use 40.  At night, 
we would have to use 160 meters most of the time with 80/75 occasionally. 
Don't ever expect to use 10 meters for NVIS unless you happen to be directly 
under an intense patch of sporadic E.

Typical NVIS antennas are very very low dipoles.  For 80 or 40 meters they 
would be 10 - 15 feet off the ground.  Needless to say, you need some kind 
of antenna tuner or have to be very lucky to feed these low dipoles. 
Equivalent height for a ten meter NVIS antenna would be 5 feet or less!  You 
can't use vertical antennas for NVIS because they have very little radiation 
at high elevation angles needed for NVIS.

Antennas for ground/surface wave are completely different than the antennas 
for NVIS.  For the former you need a vertical although good dipoles seem to 
be able to work while for the latter you need very low dipoles.

The comment on General class frequencies was driven by the table from the 
Antenna Book which showed 14 MHz and lower, 10, 7, 3.5 and 1.8 MHz, required 
for the distances we wanted to cover.  A General or higher is needed to 
operate SSB on these frequencies.  Yes, this is all SSB.  FM as used in ham 
radio is not particularly good for weaker signals which we will have to deal 
without at least some of the time.  PSK and CW could be used and I believe 
there are still some CW frequencies available for all to use on 80 and 40 
meters, but I'm not sure about that.

If anyone has any questions, ask away.  Propagation is not that difficult to 
understand.  It just takes some reading and you have to pay attention to who 
you hear on the air and when.

Kerry