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AIM FOR THE CLOUDS AND
GET BETTER "LOCAL" SIGNALS!
WARMER NVIS BEAM
AN NVIS STYLE "BEAM" ANTENNA FOR
BETTER "LOCAL" AREA COVERAGE ON
Some of you may recognize this design as nothing more than a
half wave dipole, but upon closer examination, you will see that there is
a reflector at the bottom of the antenna spaced at about .15 wavelength or
less from the driven, (dipole), element.Hense the name "Cloud Warmer Beam".
This in fact, makes this
antenna a 2 element wire "beam" aimed straight up at the clouds.
style antennas work best below about 8mhz as confirmed by the U.S.
If you already have a half wave dipole up and running,
then you have been using this type of antenna to some extent without
knowing it, however, yours is not as effective in getting your signal to
the "local" area out to a few hundred miles due to the properties of the
ground underneath, your present dipole, and the nature of the dipole
This design gives you the ability to more closely match
the ideal situation for your dipole to perform much better in the close
in range, (a few hundred miles radius), from your station and
give you a little added"gain"!!!!
The military uses the NVIS
configeration while operating mobile for better "local" coverage on their
low bands by laying down their whips in a horizontal position on
their mobile units.THERE IS NOTHING SPECIAL ABOUT THIS ANTENNA
CONSTRUCTION OTHER THAN THE ADDED REFLECTOR AT THE BASE OF THE DIRECTOR
By adding the reflector, which is 5 percent longer
than the driven element, and spacing it .15 wavelength or less below
it, you turn your dipole into a beam type antenna projecting your signal
up to that big reflector in the sky where it is bounced back down into a
sort of upside down cone pattern extending out several hundred
miles! THIS IS NOT A DX
The standard formula can be used for calculating the
length of the director....468/freq mhz
Reflector length = director
length + 5 percent longer.
Spacing = aprox 140/freq
See further experimentation
concerning spacing below~
Design for middle of the General Phone Band
468 / 3.925 = 119.24
FEET FOR DIRECTOR (DIPOLE)
REFLECTOR = 5 percent LONGER THAN
DIRECTOR = 119.24 X .05 = 5.96 FEET ADDED TO 119.24 = 125.20
SPACING = 936 / 3.925MHZ = 238.47 FEET X .15 = 35.77 FEET FOR
(See further experimentation concerning
If your starting
this project from scratch, start with the director, (the dipole), a little
longer and prune to lowest swr for middle of band as with any other
If your dipole is already up with low swr, then just
add the reflector at the proper spacing distance.
The distance from the
reflector and the ground should not make any difference.
You will note
by the calculations above that the distance from the driven element and
reflector would require that the director be at least 35.77 feet from the
ground! If you can't get the formula spacing
for installation reasons, then just do the best you can. Some
experimenters state that even much lower overall dipole height above the
reflector work even better. See below.
More recent experimentation by Pat Lambert, W0IPL and
others conclude the distance from the antenna and the ground can be
lowered considerably with much better results.
Here is a teaser
comment made by him:
"While 1/8th wave works reasonably well,
better coverage is obtained if the antenna is mounted at about 1/20th
wavelength above ground. A second advantage of lowering the antenna to
near 1/20th wavelength is a lowering of the background noise level. At a
recent S.E.T. communication on 75 Meters was started with a dipole at
approximately 30 feet. We found communication with some of the other
participants to be difficult. A second 1//2 wave dipole was built and
mounted at 8 feet off of the ground. The background noise level went from
S7 to S3 and back when we switched back the antennas, plus communications
with stations in the twenty-five and over mile range were greatly
complete article here with lots more on NVIS NOTE:
ANTENNA SUPPORTS MUST
BE NON-CONCUCTIVEFOR BEST RELULTS!
GOOD HEAVY WIRE SIZE SUCH AS # 12 OR 14.
OTHER TYPES OF ANTENNAS CAN BE
USED NVIS STYLE BY JUST ADDING THE CORRECT LENGTH REFLECTOR AT THE BOTTOM
OF THE ANTENNA.
Edited from U.S.Military training
propagation is simply sky wave propagation that uses antennas with
high-angle radiation and low operating frequencies. Just as the proper
selection of antennas can increase the reliability of a
circuit, short-range communications also require proper antenna selection.
NVIS propagation is one more weapon in the communicator’s arsenal.
communicate over the horizon to an amphibious ship or mobile on the move,
or to a station 60-190 miles away, the operators should use NVIS
propagation. The ship’s low take-off angle antenna is designed for medium
and long-range communications. When the ship’s antenna is used, a skip
zone is formed. This skip zone is the area between the maximum ground wave
distance and the shortest sky wave distance where no communications are
Depending on operating frequencies, antennas, and propagation
conditions, this skip zone can start at roughly 12 to 18 miles and extend
out to several hundred miles, preventing communications with the desired
NVIS propagation uses high take-off angle (60 to 90
degrees) antennas to radiate the signal almost straight up. The signal is
then reflected from the ionosphere and returns to Earth in a circular
around the transmitter. Because of the near-vertical
radiation angle, there is no skip zone. Communications are continuous out
to several hundred miles from the transmitter. The nearly vertical angle
of radiation also means that lower frequencies must be used. Generally,
NVIS propagation uses frequencies up to 8 MHz.
The steep up and down
propagation of the signal gives the operator the ability to communicate
over nearby ridge lines, mountains, and dense vegetation. A valley
location may give the operator terrain
shielding from hostile intercept
and also protect the circuit from ground wave and long-range sky wave
interference. Antennas used for NVIS propagation need good high take-off
angle radiation with very little ground wave
techniques concentrate on the areas which are often in the skip zone. The
idea is to radiate a signal at a frequency which is below the critical
frequency, at a nearly vertical angle, and have that signal reflected from
the ionosphere at a very high angle of incidence, returning to the earth
at a relatively nearby location."
more about NVIS ANTENNAS
CLICK HERE /
AND HERE /
Gain NVIS Antenna Project
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