NVIS N.V.I.S antenna

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There are claims circulating that placing
an
antenna at very low heights
enhances NVIS
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)
December
2005 QST article, on page 38, is a good source. Dean is meticulous with his
writing.

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
field strength
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.

Compare the
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.

NVIS Antenna gain vs Height

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
75/80 meters.
The OPTIMUM
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:

At .06
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!!

At .005
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
distance.

EZNEC MODELS

Height .12
wavelength (31 feet)

Compare this to
+8dBi measured in
Thailand. Very good
agreement. Now let’s
move the antenna
lower.

Height .02
wavelength
(5.2 feet)

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
real measurement
agree closely.
EZNEC+ tells us we
lost 11.6dB while
the actual
measurements tell us
we lost 13dB.

Height .005
wavelength (1.3
feet)

We now see EZNEC+
shows -11.4dBi which
is
-19.4dB from our 31
foot high antenna.
The actual
measurement from
Thailand shows the
signal was down
25dB from the higher dipole. In this case EZNEC+ is
overestimating
signal strength by
about 6dB.

Now here is
something to
consider. People
claim distant
signals get weaker
and short skip
signals get stronger
when the antenna is
lowered to very low
heights. Factually
that just isn’t
true. We have
measurements made
with sophisticated
equipment using
helicopters to fly
over the antenna
area and plot
radiation. Those
measurements prove
we lose
considerable signal
level on
transmitting
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
pattern hardly
changes. If the
pattern or
directivity hardly
changes, the
signal-to-noise
ratio on receive
will not change
either. The only
difference will be a
weaker signal and
weaker noise, just
like adding an
attenuator. The
problem is a low
antenna kills your
transmitting signal,
turning a 100W rig
into the equivalent
of a 1 watt
transmitter in the
case of a very low
antenna.

I’m afraid I have
to side with R. Dean
Straw’s QST article,
Hagn-Barker’s actual
field strength
measurements,
EZNEC+, and my own
experience measuring
and using antennas
for the past 40+
years.

Please, let’s not
give silly advice
like 5-foot high
antennas are good
ideas for emergency
communications or
NVIS operation. Very
low antennas produce
very low signal
levels at any
distance when
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
160-meter dipole.

Let’s look at losses in a low 80-meter dipole over soil like mine:

low dipole no screen

This antenna with no ground screen has 4.81 dBi gain

One counterpoise wire

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.

Low dipole 15 counterpoise wires

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
conductivity.

G5RV Antenna
Efficiency



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