[KYHAM] KEN Training for July 2005: NVIS - How a "Cloud Warmer" Can Work for You

[Ron Dodson]

NVIS – How a “cloud warmer” can work for you
Contributed By Tony Whobrey, KY4SP

Most amateurs know how to respond if asked to provide regional
communications in an emergency situation; typically a few 2 meter or 440
radios are employed along with the nearest available repeater. Since
Kentucky has well over 100 repeaters on 2 meters, and nearly as many on
440, it is usually not too difficult to find a “machine” that will
support communications over a range of perhaps 40 to 50 miles. This
range is also easily attainable between fixed stations on these bands if
they can be located on a modest hill top. A more elaborate installation
can add a few miles to this range if equipment is available and time
permits the installation of these facilities. This kind of response is
relatively easy to implement and is the one familiar to most people
involved in emergency communications.

How would you respond if asked to provide expedient communications at a
range of perhaps 100 or 150 miles? While there are a few areas in our
state where a 100 mile range is attainable via linked repeater systems,
these are the exception rather than the rule. In most areas, an
alternative must be considered when point to point communications are
required at ranges of greater than 50 miles or so. Contacts at this
range are certainly possible between fixed stations using 2 meter SSB,
but such installations are usually impractical for field use. Tall
towers and long yagi antennas with low loss feed line are not generally
available for temporary field use, and would require considerable time
to implement.

 The Near Vertical Incident Skywave (NVIS) technique has long been
employed by military forces requiring greater range than that offered by
typical VHF FM systems. Use of NVIS methods in the 80 or 40 meter
amateur band offers communication at ranges of 40 to more than 200
miles, and requires only an HF radio and common dipole antenna which can
be erected in a few minutes by two people. Fortunately, a high angle of
radiation is desirable for NVIS work, so the antenna can be installed at
a modest height. In fact, if you can walk under your antenna safely, it
should work nicely for NVIS. Another factor which makes NVIS worth
consideration for emergency use is that remote support infrastructure is
not needed, since the stations communicate directly with each other.

 There are several sources of design information for NVIS antennas that
are loosely based on the military AS-2259/GR unit. Simply described the
amateur version consists of two resonant inverted “V” antennas at 90
degrees to each other and fed at a common point. The usual configuration
has one antenna for 80 meters, and one for 40, although there is no
reason why this antenna could not be made to cover 60 and 40 meters or
80 and 60 meters if desired. It is also possible to cover all three
bands by using 3 pairs of wires with careful tuning. This can be a very
robust and “storm proof” antenna if properly constructed, as it is
essentially a center mast guyed by the antenna wires themselves.
Expedient antennas can be easily fabricated using wood, PVC or
fiberglass mast material and common wire antenna methods. The
combination of stand-alone operation, significant range and simple
inexpensive hardware makes HF NVIS an attractive option for medium range
amateur communications. 


Some Useful NVIS Links

http://www.qsl.net/wb5ude/nvis/

http://www.vcars.org/tech/NVIS.html

http://www.emcomm.org/svares/projects/nvis.htm

http://www.hamuniverse.com/nvisbeam.html

http://www.athensarc.org/nvis.htm

http://www.w0ipl.com/ECom/NVIS/nvis.htm

NVIS Propagation Maps  - http://www.w0ipl.com/ECom/NVIS/NVISprop.htm

NVIS Yahoo Group - http://groups.yahoo.com/group/nvis/