NVIS N.V.I.S antenna
NVIS N.V.I.S antenna
There are claims circulating that placing
antenna at very low heights
operation, or that an antenna has to be very low to be an NVIS antenna. This is
a misconception at best, and an untruth at worst. These claims are based
on unsubstantiated rumors, or assuming that because a low antenna was deployed
and made contacts, it must be the best way to install a NVIS antenna. For
example, if someone digs up some claim the military uses a very low antenna for
communication, it must be the best or only way to do things.
N6BV published at least one QST article trying to dispel myths about NVIS.
Dean Straw’s (N6BV)
2005 QST article, on page 38, is a good source. Dean is meticulous with his
I’ve decided to
post some actual
measured data from a
VietNam-era NVIS antenna study by Hagn-baler.
The little circles
that appear above or
below the smooth
curves are actual
measurements made in
1970 by Hagn and
Barker. The data is from actual antenna measurement taken from helicopter
flights over the antenna. This data is actual field strength measurements as
they compare to NEC-3 predictions. To the best of my recollection, I
obtained this data many years ago from my dear friend Walter Maxell.
measurement points plotted as small circles to NEC-3
curves (the unmarked curved
line) that use a slightly different conductivity.
The measured data, represented by circles, is over
very good SE Asia
rich black jungle
soil. This soil is better
than soil found in most
US locations, thus having less loss.
Notice between .02 wavelength and .12 wavelengths the measured data (circles)
matches the computer model closely. At about .02WL the model and measured data
depart, with the model being over optimistic by about 6 dB at very low heights.
The optimum height for NVIS antennas is something over 1/8th wavelength, or
about 30-35 feet on 75 or 80 meters. Signal level decreases rapidly as height is
lower than about .05 wavelength, or approximately 14-feet.
MEASURED DATA ANALYSIS
From 1/4 wave high to 1/8th wave high, there is no difference in NVIS
field strength. This would be about 35 feet high on 40 meters or 70 feet high on
NVIS antenna height for 80 through 40 meters is about 30-feet!!
Please note the following trends as we decrease antenna height
from ~16 feet on 40 meters, or ~32 feet on 75 meters:
wavelengths high (16
feet on 80M, 8 ft on 40M) field
strength is down 3dB. This is about 50% TX signal reduction.
At .04 wavelength
(10 feet high on 80M or 5 ft on 40M) field
strength is down 5dB. This is about 2/3 reduction in TX signal level!
At .02 wavelength
(5 feet high on 80M, 2.5 ft on 40M) field
strength is -5dBi,
or -13dB from 0.125
wavelength height. This is 1/20th the optimum transmit signal level!!
wavelength (1.35 ft high on 80M, or 0.66 ft on 40M ) field
strength is down by 25dB. This is about 1/300 optimum transmit
signal height signal level!!!
We can reasonably
expect, over very
good soil, a 5-foot high 75-meter antenna
would be down 13dB
from an antenna 30-ft high. This
would be true at any
wavelength (31 feet)
Compare this to
+8dBi measured in
Thailand. Very good
agreement. Now let’s
move the antenna
Compare this to
-5dBi for the real
antenna and we see
it is still very
good agreement. This
shows both the
EZNEC+ model and a
EZNEC+ tells us we
lost 11.6dB while
measurements tell us
we lost 13dB.
We now see EZNEC+
shows -11.4dBi which
-19.4dB from our 31
foot high antenna.
Thailand shows the
signal was down
25dB from the higher dipole. In this case EZNEC+ is
signal strength by
Now here is
signals get weaker
and short skip
signals get stronger
when the antenna is
lowered to very low
that just isn’t
true. We have
helicopters to fly
over the antenna
area and plot
as the antenna is
lowered below .05
wavelength (about 12
feet). We have
models that closely
agree (except at
very low heights).
We also see the
changes. If the
ratio on receive
will not change
either. The only
difference will be a
weaker signal and
weaker noise, just
like adding an
problem is a low
antenna kills your
turning a 100W rig
into the equivalent
of a 1 watt
transmitter in the
case of a very low
I’m afraid I have
to side with R. Dean
Straw’s QST article,
EZNEC+, and my own
and using antennas
for the past 40+
Please, let’s not
give silly advice
like 5-foot high
antennas are good
ideas for emergency
NVIS operation. Very
low antennas produce
very low signal
levels at any
compared to antennas
of modest height.
Ground Screens or Reflectors placed below NVIS Antennas
Some claim a screen, counterpoise, or reflector wire grid below a low dipole
will not improve signal levels. My own direct experience shows the idea a screen
will not improve signal levels is NOT true, and that a counterpoise system can
actually improve signal levels. This is true even when the screen or
counterpoise is not tuned. On 75-meters, back in the days of 3830 and 3895 radio
wars, there was always a definite advantage to stations using low dipole with
good screens or counterpoises below the antenna. this was a trick I first earned
from W8PSX in Westlake Ohio , where Jim had a counterpoise system below his
Let’s look at losses in a low 80-meter dipole over soil like mine:
This antenna with no ground screen has 4.81 dBi gain
One counterpoise wire below antenna.
This antenna has almost 2 dB more signal level.
This antenna, with three untuned ground counterpoise wires, has 3
dB more signal level.
With many counterpoise wires, the model improves 3.7 dB.
It’s my belief the actual improvement I saw was around 5 dB in my
own systems, although this varies greatly with antenna height and soil
Copyright W8JI 2006