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Introduction
DX Engineering’s Near Vertical Incidence Skywave (NVIS) Antennas for both 80 and 40 meters
are available in two models. The difference between the two models is the foot print size area
needed. The larger version the
DXE-NVIS-8040
is a full size 80 and 40 meter NVIS antenna that
uses no coils. The footprint for this model is approximately 158 feet by 75 feet. The second version
-
DXE-NVIS-8040S
uses two coils on the 80 meter legs which shrinks the footprint to
approximately 75 feet x 75 feet.
Both versions operate virtually identical to each other and provide the operator with a complete kit
to get on the air in an emergency or other temporary communications event. The NVIS antenna can
also be permanently installed for every day use.
For those that have their own non-metallic 15 foot mast, you can purchase the NVIS antenna kits
with all the parts except the mast assembly. Part numbers for those antenna kits are:
DXE-NVIS-
8040NM
and
DXE-NVIS-8040SNM.
NVIS
Near Vertical Incidence Skywave (NVIS) is a
propagation mode which uses high angle
radiation to send signals almost straight up to be
reflected back to Earth for very effective short to
medium distance communications. This mode of
operation makes it ideal for in-state
communications during disasters or other
emergency situations. The military has used
NVIS techniques for decades to provide short
haul communication with other units on the
ground.
NVIS only works at frequencies from 2 MHz to
10 MHz. The signal must penetrate the D layer of the ionosphere, and bounce off the F layer.
Lower-frequency signals will not penetrate the D layer; higher frequencies will not bounce off the F
layer at these sharp angles and just goes out into space. Remember the Maximum Useable
Frequency (MUF) For amateur radio operators, we're looking at 40 and 80 meters primarily for
NVIS use.
A good NVIS antenna will not work well at DX distances. Antenna gain is a zero sum game. There
is a fixed amount of energy radiating. If we push it all out in one direction (the near-vertical angles),
we have to take it away from another direction (the low DX angles).
Regular height dipoles or vertical antennas have a lower take off angle and your signal may be
heard three states away, but not in your state due to the skip zone. This skip zone is the area
between the maximum ground wave distance and the shortest sky wave distance where no
communications are possible. Depending on operating frequencies, antennas, and propagation
conditions, this skip zone can start at roughly 10 to 20 miles and extend out to several hundred
