[Ccarc] NVIS

[Tom Murray]

Jerry Reimer, KK5CA of Spring, Texas, sent in some interesting
comments about NVIS (Near Vertical Incidence Skywave) propagation
and antennas, and ionospheric data available on the internet.  An
automated ionospheric sounder, or isosonde, beams energy straight up
while sweeping the signal up in frequency, thereby determining the
MUF or Maximum Usable Frequency of that area by measuring the
reflected signal.  Jerry says that NVIS communication (which is used
to communicate with stations out to about 200 miles maximum) is best
at a frequency 50 to 80 percent below the MUF from the isosonde.  So
if the MUF of the patch of ionosphere overhead is 10 MHz, then NVIS
is best between 2-5 MHz.  With NVIS, users are trying to get high
angle radiation instead of low angle, which is usually the goal with
other modes of HF communication.  A page explaining Vertical
Incidence Soundings is linked from

http://www.ngdc.noaa.gov/stp/IONO/ by clicking on the Vertical
Soundings link on the left of the page.

He also pointed out some interesting real-time maps showing
continent-wide communication between various points at

http://www.ips.gov.au/HF_Systems/4/1.

For instance, if you select Hourly HAP Charts, then select Kansas City, what 
you will
see is the best frequencies for communications with Kansas City from
across the continent at that time.  So you can look at the color
region over any point on the map, and this is keyed to the best
frequency for communicating with Kansas City from that point.

More about NVIS and ionospheric soundings in next week's bulletin.

If you would like to make a comment or have a tip for our readers,
email the author at, k7ra at arrl.net.

For more information concerning radio propagation, see the ARRL
Technical Information Service at
http://www.arrl.org/tis/info/propagation.html.  For a detailed
explanation of the numbers used in this bulletin, see
http://www.arrl.org/tis/info/k9la-prop.html.  An archive of past
propagation bulletins is at http://www.arrl.org/w1aw/prop/ .
Monthly propagation charts between four USA regions and twelve
overseas locations are at http://www.arrl.org/qst/propcharts/.

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