The author, a professional technical writer and training developer, has
written adult vocational training materials for the petroleum refining,
chemical, and aviation industries; and various technical articles for
amateur radio. He has also been studying, building, and testing antennas
for about 30 years as a hobby.
The material in this article is
derived from The ARRL Antenna Book, 1957, 1974, and 1999 editions; Army
Field Manual 24-18 and others, several QST articles, and various web
articles written by other NVIS experimenters and HF antenna and propagation
experts such as W4RNL. Various specifications given as examples are rough
estimates, some with wide variability. They do not represent the absolute
limits of performance, but practical applications where a high percentage
of messages may be passed accurately, under average conditions.
article is not intended to be a complete primer on HF radio propagation or
emergency communications. Users needing background in HF radio propagation
should see several of the excellent links at
the many links at the bottom of this article, and the ARRL Antenna Book.
The author assumes that the audience for this article has a general
understanding of HF antennas and ionospheric propagation. Users needing
background in emergency communications should see the various web sites of
ARES and RACES groups, as well as FEMA and MARS sites.
wishes to thank all that have contributed useful information on this
subject, and encourages comments, in writing, that may be used to improve
this article. To quote a great writer from the 1830s, "Before you can
convince me of error, you must first convince me you understand what I
say." Please read carefully.
What is NVIS?
Near-Vertical Incident Skywave is a combination of
radio hardware, skywave radio propagation, operating procedures,
cooperation, and knowledge used by a group of radio operators who need
reliable regional communications. It fills the gap between line-of-sight
groundwave and long-distance "skip" skywave communications.
ground forces first documented NVIS techniques in WW-II. NVIS was more
fully documented, studied, and used by US forces in Vietnam. Radiomen in
military vehicles discovered that their HF whips would sometimes work much
better when tied down horizontally. Amateur radio operators have been
studying NVIS propagation and operating techniques for at least fifteen
years. In tactical military use, NVIS allows communications around the
region while providing very little groundwave signal for the enemy to home
in on. Any radio operator that has used a horizontal antenna well under a
half-wave high has used NVIS.
NVIS propagation is generally
considered to be F-layer ionospheric reflection at angles of 70-90 degrees.
It is skywave propagation without the usual skip zone. The purpose of NVIS
is to communicate locally and regionally, out to a few hundred miles, with
moderate power, simple antennas, and no skip zone. NVIS is typically used
on 160, 80/75, 60, and 40-meter bands by Amateur radio operators using
relatively low horizontal wire dipole antennas.
NVIS operations are
optimized by understanding and controlling two major factors: (1) Proper
antenna design and placement, and (2) proper training of the operators. The
antenna is designed and placed to provide the maximum possible gain
straight up, on two or three frequency bands. Operator training includes an
understanding of antennas, ionospheric propagation, and operational
When a horizontal dipole is 1/2-wave high, it has a wide null overhead,
and a main signal radiation pattern shaped like an inverted cone. The
reflected wave from the ground is out of phase with the antenna and so
causes partial phase cancellation overhead. This makes a good "DX" antenna,
with gain at relatively low angles, and a wide skip zone. Problems arise,
however, on regional nets and rag chews, because of the skip zone.
the dipole is lowered below a half-wave high, this inverted cone closes up,
the overhead null disappears, and most of the power is radiated upward in a
wide lobe shaped like an egg. The reflected wave from the ground is closer
to being in-phase with the antenna, increasing the amplitude of the
vertical-angle RF power. The effect is somewhat like a 2-element yagi
pointing straight up. At a height of .15 to .2 wave, over excellent ground,
gain can approach 7 dbi, straight up. Imagine pointing a powerful
searchlight straight up at a cloud: The resulting bright spot would provide
indirect lighting for miles around! With a horizontal antenna suspended
well under a half-wavelength high, we achieve the same effect. We
deliberately illuminate the F-layer (which varies from about 100 to 300
miles up) with a wide RF flood, which causes indirect RF illumination of
the whole region.
The following graphs show typical elevation
profiles for a 75 meter horizontal dipole at various heights. It is
apparent that the best NVIS coverage may be obtained at about 3/8ths-wave,
or 90 feet high. However, this is impractical for most installations, and
antennas much lower will perform almost as well, with the main difference
being in the fringe area of coverage. Please note the amounts of power
available at various angles, for each height represented.
1a: 75-meter NVIS antenna at 20 feet high
The -10db ray is at about 38 degrees.
The -20db ray is at about 20 degrees.
1b: 75-meter NVIS antenna at 67 feet high (quarter-wave).
The -10db ray is at about 28 degrees.
The -20db ray is at about 6 degrees.
1c: 75-meter NVIS antenna at 90 feet high (3/8ths-wave). The -10db
ray is at about 22 degrees, -20db at about 4 degrees, and
considerably more power is now available at 30-60 degrees.
1d: 75-meter antenna at 125 feet high (half-wave)
No longer NVIS, but now a "skip" antenna,
with most of the power at about 42 degrees.
Figures 1a - 1d: Elevation profiles of the 75-meter horizontal
dipole, over average ground.
(These graphs compare closely with the ARRL Antenna Book)
Thus we can
see that raising the 75 meter NVIS antenna from 20 feet to 90 feet will add
about 8db to the signal ray at 30 degrees, which is considerable, but
usually not a sufficient justification for adding two 90-foot supports to
the antenna farm. The rule here is pretty simple: If you want a reliable
range of, say, 300 miles, use a real low antenna. If you want a little
better morning/evening coverage, go to 90 feet as the optimum height.
Raising the antenna from 20' to 90' simply gives you a little more power at
lower angles. Part of this extra power comes in part from the top of the
lobe, and in part from reduced ground absorption. Best vertical gain (about
7dbi) is achieved at .15 to .2 wave high, but the 20-foot high antenna will
still have a gain of about 5. The best possible SWR may be achieved at
about 41 feet, over average ground.
The graphs in Figure 1 do not
reflect the whole mechanism involved in daytime 75-meter path length
reduction. The next major factor is D-layer absorption, which gradually
builds up in the morning after sunrise, and gradually fads away in the late
afternoon. Since most of the signal power is at high angles, it continues
to penetrate the absorptive D-layer on the way up, reflect off the F2
layer, then penetrate the D-layer again on the way down. At lower angles,
the available power is much less, due to ground interaction with the
antenna, and the low-angle path suffers additional losses by passing
through the D-layer (twice), at lower angles. For example, if the D-layer
is 30 miles thick, the high-angle ray will pass through about 30 miles of
absorptive D-layer (twice), but the 30-degree ray will pass, at that angle,
through 60 miles (twice). (See Figure 2.)
Figure 2: Daytime Path Losses for 70 and 30 Degree Rays, 75 meter band
In Figure 2, we observe two of the three mechanisms that combine to
attenuate low-angle daytime signals: (1) Compare the radiated power, which
is about 2db below peak at 70 degrees, with the 30 degree angle, which is
down about 14db. (2) Compare the distance the rays must travel through the
absorptive D-Layer (twice) at various angles: the 30 degree ray has about
twice as much loss as a very high angle ray. (3) Add the normal attenuation
due to path length (not shown). These three factors, plus a little loss in
the troposphere, all combine to attenuate low-angle signals in the daytime.
As the sun gets higher, D-layer ionization intensifies, and the effective
range decreases further.
From late evening to early morning, 75
meters may spread out to 1500 miles or more, as the D-layer disappears and
absorption is no longer a factor in path losses. The typical NVIS antenna
pattern shows the signal power at 20 degrees is down 20db, so it is more
likely that 1000-2000 mile contacts are made not by the -20db single-hop
ray, but two or even multiple hops from the much more powerful rays
available at the higher angles. Another factor in "stretching out the band"
is the Pedersen ray hop, a mechanism that may be roughly described as
ducting in the F-region of the ionosphere. One unfortunate effect of the
band "going long" is that many thunderstorms will exist during the warm
months somewhere within that giant coverage area. This leads to a great
deal of static noise that tends to render the low bands rather useless in
the summer, at night -- particularly for weak-signal contacts.
morning, the sun gradually reestablishes D-layer ionization. It starts
absorbing signal power like a giant blanket of attenuation, and its effect
increases steadily (more or less) as the sun gets higher. With the NVIS
antenna, the low angles where RF power is lowest will become useless out at
some point, and the practical signal path distance will draw in to two to
three hundred miles radius by mid-to-late morning. This is because most of
the signal power is at high angles, and only the main lobe of the antenna
is powerful enough to penetrate the D-layer twice. By late-morning
(typically), signals beyond 150-200 miles usually become very weak, then
inaudible. Raising both antennas substantially (to 125 feet) would provide
more power at lower angles and thus increase the range and/or time
available, but this is usually quite impractical. The practical solution is
to switch to 40 meters, where D-layer absorption is much less and the
antenna is twice as high in terms of wavelength. Switching to 20 meters
will give nationwide coverage, but with a wide skip zone, assuming the
F2-layer is undisturbed. Amateurs, short-wave broadcasters, military,
maritime, and aviation stations regularly switch frequencies with the
day-night cycle, to maximize reliable signal levels at a given distance.
Of course, the "numbers" are widely variable, depending on prevailing
ionospheric conditions and the environment of the antenna, particularly
ground conductivity. The "numbers" are roughly based on carefully logged US
Army tests, where they compiled the percentage of messages accurately
passed, for various frequencies at various times of the day and night, and
generated frequency-time charts There are occasionally exceptions, but this
article is concerned with reliable tactical communications, not occasional
exceptions caused by unusual, atypical band conditions.
All of this
explains why the reliable, effective path length on 75 meters contracts and
expands with the day-night cycle.
Another consideration is ground
quality, consisting of conductivity and dielectric constant. The power
lobes and nulls of an antenna's radiation pattern are created by the
distant mixing of the direct and ground-reflected rays, which may be in
phase for a lobe, out of phase for a null, and everywhere between for the
intermediate levels of power. The ground's conductivity in the immediate
vicinity of the antenna affects the feedpoint resistance and degree of
power absorption. The ground quality out to several miles will affect the
radiation pattern. Excellent quality ground, such as sea water, will
provide the best results, while poor quality ground will absorb more power
and substantially shift the phase of the reflected ray, greatly reducing
power at low angles. Variations in ground quality from one location to the
other explains why one station with a relatively poor antenna may enjoy
much better signals, over very good ground, while another station with a
highly-optimized antenna may have a relatively poor signal, over poor
ground. Since ground quality is what it is, the operator with very good
ground need not go to extra trouble to optimize his installation, while the
operator with poor ground should optimize his antenna to the greatest
practical extent. Average ground is considered to be 5 milliSiemens per
meter (mS/m) and a dielectric constant of 13. The topography of the land in
the vicinity of the antenna also affects the pattern. The height above
average ground affects the feedpoint resistance as follows:
Table 1: Feedpoint resistance of a center-fed, resonant
half-wave horizontal wire dipole, over average ground.
In Table 1,
the dipole was modeled over average ground, at various heights. For each
height, the dipole length was optimized to resonance. It is apparent that
regular dipoles should be mounted about 41 feet high, if the lowest
possible SWR is to be achieved, when feeding with 50-ohm coax. Alternately,
a folded dipole may be placed at about 16 feet. Since there is a 4:1
transformation with folded dipoles, the feedpoint resistance will be around
50 ohms. The folded dipole will also have considerably broader bandwidth.
A desirable effect of the NVIS vertical lobe is the reduction of
received atmospheric static from distant storms (in the daytime), since
most atmospheric static comes in from angles below about 15 degrees.
Narrowing the vertical beam width of the antenna reduces the noise further.
This is accomplished by lowering the antenna until the best signal-to-noise
ratio is achieved. Alternately, the antenna may be raised to 0.2-wave and a
reflector element installed underneath, on the ground, to create a
2-element yagi thereby narrowing the beam width and increasing the gain.
This will improve the s/n ratio further, on sites with poor soil.
Experience shows that storms may be heard when they are within the
antenna's effective area of coverage, which is much greater at night than
in the daytime. When the storms are nearby, no amount of antenna fiddling
will improve the static -- but if your contact is at, say, 150 miles, and
the storm is at 300 miles, lowering the antenna will certainly improve the
signal-to-noise ratio by reducing the effective signal radius to exclude
The frequency to be used at any particular time is
selected to fall between the vertical MUF (the highest frequency that will
reflect back down from nearly straight up), and the upper frequency end of
severe D-layer absorption. Typically, this is 40 meters in the daytime and
75/80 meters at night. 160 meters may be used in the wee hours, if RF
begins to break through the F2 on 75. In actual practice, 75 may be used
for regional contacts from about 5 PM to about 9 AM, and 40 meters will
carry you through the day from 9 to 5. There are several exceptions
worth noting: (1) 75 may be useless at night, in the stormy months, due to
high static -- and 160 certainly is. (2) 75 may drop completely out in the
wee morning hours due to failure of F-layer reflection. This is when 160
may be employed, assuming the static will allow it. (3) 40 may drop to
unusable levels during the day due to solar activity. Other bands will be
affected as well.
Another factor to consider is groundwave. If the
stations are close enough, the groundwave and skywave will mix in the
receiver, and cause multipath distortion, due to the considerable
difference in path lengths. For example, for stations 10 miles apart, the
groundwave will travel 10 miles, but the skywave will travel 200-300 miles.
For this reason, groundwave must be reduced as much as possible. This is
done by both stations lowering their antennas to the practical minimum
height. This is usually 10-15 feet across open spaces, and 4-6 feet on
Some people operating mobile have noted a small "dead zone"
that extends several miles outside of groundwave range. This may have one
of two causes, or both. First, the fixed station is running NVIS, but the
mobile is running a vertical whip. RF power arriving from directly above
the whip cannot induce current into it. This is why military vehicles are
instructed in their Field Manuals to tie the whip down. Second, some
researchers theorize that the vertical NVIS lobe has a tiny "hole" right at
90 degrees. The reasoning for this is that the up-going wave and the
down-coming wave cannot occupy the same space without phase cancellation --
so we might say that there is a "skip zone," albeit a tiny one. The author
believes that both of these mechanisms may come into play in certain
Why Do It?
First and foremost, to completely eliminate the skip zone. This enhances
all forms of local and regional HF communications, for all practical and
Emergency groups such as ARES and RACES are
studying NVIS propagation, techniques, and equipment deployment for
emergency preparedness. NVIS is the tactical communication system of choice
in mountainous areas, any areas without complete repeater coverage, and all
situations where repeater-based systems have failed or might fail. With the
recent release of manufactured mobile and even portable HF radios, HF, and
antennas employing NVIS propagation, should become much more popular and
useful for disaster tactical communications.
Researchers and users
have observed that NVIS antennas work considerably better in the valley
than on the mountain top. This is due to much better ground conductivity in
the valley than on the dry, rocky mountain top. This happy fact eliminates
a lot of unnecessary climbing, and allows the antenna to utilize trees for
both support and cover.
NVIS-equipped Amateur fixed stations enjoy
regional nets and rag-chews without the annoying skip zone. It is
particularly useful to net controllers and emergency practice groups. All
fixed stations should take steps to immediately supplement their antenna
farms with at least a dual-band NVIS antenna (described herein).
Antenna and propagation experimentation is FUN! Building and deploying
antennas is as close as many hams get to home brewing. NVIS is as easy as
antenna experimentation can get. The antennas are simple, and are installed
very low. Light-gauge wire and nylon string may be nailed to trees at
extension-ladder heights. Dropping a dipole and making a change to it takes
only minutes and may easily be done by one person without the need to
obtain helpers or plan a big event.
NVIS antennas are stealthy.
Communism-by-contract property owner's associations have restricted the
placement of visible antennas and severely stifled Amateurs' pleasure,
emergency preparedness, experimentation, and innovation. With NVIS, a fine
wire may be brought through the trees, or routed along the top of a privacy
fence. The Ham thusly equipped may never win any low-band DX awards, but
will still have ample opportunities for QSOs and nets within the regional
circle provided by an NVIS antenna in the daytime, in addition to some
low-band DX at night, particularly in the winter when the storms are gone.
If you could only have one antenna, it should be an NVIS with
ladder-line feed and a tuner, as this may be operated on all bands. The
"best" multiband antenna is probably the 260-foot dipole, or 520-foot loop,
with 76 feet of windowed ladder line and a tuner.
How to Make a Good NVIS Antenna
The best NVIS antenna is one which is simple and effective. One favorite
is the dual-band dipole. This antenna uses two dipoles, one for 75 meters
(about 122 feet), and one for 40 meters (about 65 feet), both connected
directly to 50-ohm coax and supported at 5 points by trees at 10-12 feet.
The two dipoles should be well separated at the ends, or they will
interact. They may be strung up in an "X" or a "+" shape. The bandwidth of
the 75-meter dipole will be quite narrow (<100kc), so it will benefit from
using two sets of stagger-tuned wires. Some researchers recommend that the
ends of the wires should be a few feet higher than the middle. This will
increase gain and raise the feedpoint impedance a bit. If the feedpoint
impedance is too low to match, the antenna should be a folded dipole, which
will raise the feedpoint impedance by a factor of four. Stringing the
antenna over a highly conductive surface, such as salt water or a wet,
acidic marsh, will substantially improve the antenna's performance,
compared to stringing it over dry rock or sand.
Since the support
points are typically 10-12 feet high, the wires must be both light and
pulled tight to remove annoying sag. Appropriate wire ranges from #17
aluminum electric fence wire, ($14 for 1/4-mile at farm-supply stores), to
#14 insulated stranded copper THHN, ($15 for 500 feet rolls at electrical
suppliers, and available in green). The #17 aluminum isn't very strong, but
is almost invisible. The wire may be supported with green nylon string,
available at garden centers. The center feedpoint and coax may be built
around a simple insulator, waterproofed, and nailed to a tree trunk at 10
feet. Insulators and coax may be sprayed dark green or brown as needed.
Antennas below 8 feet should use insulated wire to avoid RF burns.
Insulation does not affect the performance of antenna wire, except (1)
reduced wind and rain static, (2) lowers the velocity factor a tiny bit,
and (3) prevents corrosion.
It is better to use a broadband current
balun at the feedpoint when using coax. A simple choke balun made of coiled
coax may be used if needed to remove common-mode currents from the line.
Try to design the installation so the feedline extends away from the
antenna at a 90-degree angle, for at least one-quarter wave. Also, the line
should be detuned -- that is, it's length should fall between resonance
points. If these are done, feedline RF pickup and re-radiation will
be minimized, and a balun should not be needed. Detuning the feedline is
also the cure for "RF in the shack" problems. Suitable lengths will depend
on how the antenna is fed and whether one side is grounded or not. See the
Antenna Book for determining appropriate lengths.
dipoles, avoid using twin-lead or ladder line -- the feedpoint of these low
dipoles will be well under 50 ohms and attaching 300-600 ohm parallel
feedline will present a severe mismatch at the feedpoint. However, if the
antenna is to be used nonresonant, with a tuner, ladder line should be used
because coax is very lossy when operated at high VSWR.
There is a
long-standing myth that dipoles must be resonant to be efficient.
Non-resonant dipoles of similar size are just as efficient as resonant
dipoles, assuming that (1) impedance mismatches are matched, (2) the
matching devices are designed so that losses are insignificant, and (3)
feedline losses are minimized (use ladder line when the SWR is high). It is
also important to remember that baluns and matching transformers are quite
lossy when operated with a mismatch on either or both ends. The ARRL
Antenna Book shows how to make baluns for any ratio of impedance
transformation. The myth come from the fact of severe losses in mismatched
coaxial line. In the author's experience, a 160-meter dipole fed with
ladder line will outperform a 75-meter resonant dipole fed with coax, both
at the same height, and both operated on 75 meters. This is because the
larger antenna, even though not resonant on 75, has an "aperture" twice as
large as the smaller one and thus captures, and radiates, more signal.
However, it does have 4 partial nulls, while the half-wave dipole has only
To connect aluminum or steel wire to copper, make a couple of
short #14 solid copper pigtails, twist them tightly into the aluminum or
steel elements at the feedpoint, then solder an SO-239, or direct coax
feed, to the copper tails. Waterproof the dissimilar metals connections
with waterproof grease and Coax-Seal, or silicone caulk. If any moisture
gets into the connection, the metals will corrode one another and make a
nasty rectification point. Mechanical connectors (split-bolts or set-screw
lugs) may be used but they also should be waterproofed.
expensive) NVIS installations include full-wave loops with automatic
antenna tuners at the feedpoint. These antennas, if installed at 15-20 feet
or more, will provide both excellent NVIS performance on the low bands and
DX on the higher bands, where the height of the loop is over 1/2-wave.
However, the pattern of the antenna will have several peaks and nulls on
frequencies where it is several waves long.
Two things about loops
are worth mentioning: (1) Loops are resonant on every harmonic, not just
odd harmonics like dipoles, and (2) the lower the frequency (greater the
length) of the loop, the more harmonic points it will have. For example, an
75-meter loop will resonate at about 3.8, 7.6, 11.4, 15.2, etc. But a
160-meter loop will resonate at about 1.8, 3.6, 5.4, 7.2, 9.0, 10.8, 12.6,
14.4, etc. -- and the peak SWR arising from imbalanced reactances will be
lower between all these points. Therefore, a big loop should be strung up,
even if it cannot be used on its fundamental frequency because of low
Carrying this idea further, an operator with
acreage might run a really big loop (like 1100 - 2200 feet) atop a
perimeter fence and it would have so many resonant points as to be useful
as a broadband antenna -- although the fundamental and all harmonics below
about 3-4 MHz might be unusable for transmitting due to extremely low
feedpoint impedance, unless feedpoint matching is used.
antenna is the 3-wire folded dipole. This design may be used on all HF
bands, with a tuner. The rules are pretty simple: Make a 2- or 3-wire
folded dipole as long as possible (preferably 260 feet). Feed it directly
with ladder line, and match it to the radio with a balanced line tuner. Use
a 1:1 current balun at the tuner's input. The reasons: (1) Feedpoint
resistance of low antennas will be very low, typically 15 ohms or so, and
the 3-wire folded dipole will raise it by a factor of 9. (2) Ladder line
does not suffer any significant loss when operated at high line SWR (unlike
coax). (3) Balanced tuners with the balun on the input (the matched side)
are considerably more efficient than unbalanced tuners with the balun on
the output, because baluns are only efficient when both ends are matched.
Some emergency groups are successfully experimenting with mobile
antennas mounted horizontally. For example, pairs of 75 and 40 meter
Hamsticks make excellent shortened, portable NVIS dipoles. The mobile
antennas are mounted back-to-back and fed in the center just like a dipole.
These are oriented horizontally and placed a couple feet above the roof of
a vehicle on a short mast.
Other operators carry (1) an autocoupler,
(2) a 125' roll of wire, and (3) traffic cones or fiberglass stakes in the
trunk, for rapid roadside NVIS deployment. NVIS antennas have been used as
low as 18 inches high. Surprisingly, S9 signals have been received from an
antenna mounted 10-1/2 inches high.
Preparing for Portable NVIS Operation
Emergency communication groups should create and test an "NVIS kit"
which contains a sturdy NVIS antenna, feedline, tuner, and sundry tools,
hardware, and accessories. The radio should be a small, "all-band" rig like
the Icom IC-706, powered by a deep-cycle battery. Hardware should include a
20-30 foot telescopic pole, #18 nylon line, stakes, throwing weights,
hammer and nails, extra feedline and connectors, etc. Tools should include
the usual electronics hand tools, including a small butane soldering torch
and extra butane fuel. Accessories should include a folding table and
chair, a rain tarp with it's lines, and an ice chest with food and drink.
Another piece of hardware worth having is the notebook computer, with
appropriate software and cables, that may be used to provide radio teletype
traffic. The station should also include a 2-meter transceiver and antenna,
and a scanner.
The entire station may be packed in a medium-sized ice
chest, using custom-cut foam rubber for the sensitive parts. The serious
portable operator will also have a tent and various other camping equipment
and supplies. Some clubs even purchase and equip a small travel trailer for
this purpose. This is the best solution, since the trailer will contain all
the needed equipment and supplies, at the ready, and will also provide a
measure of security and protection from the elements.
Don't have just
one NVIS antenna. Have one at home (a dual-dipole, or multiband nonresonant,
or a loop), and have another for fast portable deployment. The portable
antenna and its feedline may be rolled up on an extension cord reel. You
never know when you may be needed to quickly deploy a portable station. The
goal should be to prepare to provide reliable regional tactical
communications services without power mains, in the midst of large-scale
emergency events. It's also a good idea to have the radio "clipped" so that
it may be operated outside the ham bands by emergency officials who are
authorized to do so. The station will usually need to be located at the
incident command post -- however, it is very important to make prior
arrangements with the authorities.
Tuners: The best tuner for
barefoot NVIS is probably something like the MFJ 949E. It has a wide tuning
range, internal balun with balanced output, three-position antenna switch,
internal dummy load, and a large cross-needle meter. Of course, full power
tuners must be used with linear amplifiers. Autocouplers by SGC and others
work very well at the feedpoint, provided the impedance isn't too low. The
internal autotuners in most radios usually do not have sufficient range to
match low antennas on 160. The 75/40 dual-dipole described above does not
need a tuner, as the elements may easily be adjusted to resonance. If the
SWR at resonance is still too high, raise the antenna a few feet, because
the feedpoint radiation resistance is probably too low. Modeling over
average ground shows a feedpoint resistance of 50 ohms at around 41 feet
high. Short stubs with alligator clips may be clipped onto the elements at
various places to provide multiple resonant points, and if bare wire is
used for the antenna elements, these may be moved around to match the
antenna (don't burn your fingers).
Power Supplies: The portable
station should use a deep-cycle marine battery and a portable generator.
Small "camping" generators in the 900 to 1800-watt range, having both 13.8
VDC and 120 VAC outputs, are the most preferable. Connections to batteries
should be made using ring lugs soldered to the wire, attached to the
battery with stainless steel bolts, washers, and wing nuts. All connections
should be greased. The battery should be connected to a power distribution
box, of the type with several sets of 5-way binding posts.
How to Work NVIS into Tactical Emergency Communications
NVIS is not just an antenna type or a propagation mode -- it is a
tactical communications system that was designed by military radio
operators in the field. The NVIS antenna is only part of that system. The
other part is the knowledge and cooperation of the operators, which must be
accurately applied to achieve the best results -- particularly when results
are a life-and-death matter. Emergency communications should be driven by
clearly written procedures that have been well-designed and tested. The
procedures should be drilled on a regular schedule, and the drills should
be followed by debriefings attended by everyone, so that all can learn to
avoid mistakes. Suitable procedures are available in books, Field Manuals,
and on the web. Look for ARES and RACES web sites and capture their
procedural documents. Other excellent sources are FEMA and MARS sites.
Groups of operators using NVIS must understand and cooperate on the
basics. (1) All must be using NVIS antennas (defined as any horizontal
antenna well under a quarter-wave high), as well as the radio hardware and
propagation theory. (2) All must understand that the frequencies used must
stay between the total absorption and vertical MUF ranges. (3) The group
must decide whether it will equip itself to use 160. (4) Calling
frequencies and other procedures should be established, in writing, with
contingencies clearly stated.
1. Meet at 7228 before 8 PM summer, 6 PM winter.
2. Meet at 3853 after 8 PM summer, 6 PM winter.
3. If the frequency is occupied: Move UP 2 kc and listen or call, for
4. If occupied, move UP 2 kc MORE and listen or call two minutes, etc.
5. When QSYing to another band, if no contact made in 6 minutes, return to
This procedure helps to keep people from getting
lost and scattered on the dial. It's a good idea to keep assigned
frequencies in VFO A and B, or use radio memories.
In the evening, 40
will spread out and suffer interference from foreign broadcast stations,
and later, show signs of fading as the vertical signal starts breaking
through the diminishing F layer. Before the operator is lost in the noise,
QSY to 75 meters. In the mid-morning, as the absorption rises and kills off
75, QSY to 40. The 60-meter band should provide a much-needed transition
frequency -- but, alas, the government has limited it to 50 watts EIRP and
five discreet channels...
The signals for every NVIS operator within
200-300 miles, running 100 watts, should be well over S9. If you hear a
very faint station and want to work it, switch to a higher dipole or a
Running high power is usually not needed. QRO will
greatly increase your groundwave radius, and thus, the number of possible
stations which will receive multipath distortion. High power may be needed
to overcome QRM or QRN. Otherwise, keep the power down, keep the groundwave
close in, and let F2 do the work. If already getting out an S9+10, why QRO
and make it a +20? It you can't get above the static, lower your antenna
and tell your field contacts to do the same. If you can't get the distance
you need, switch to a much higher antenna with a lower angle of radiation,
or QSY to a higher band.
Interesting "Rules of Thumb" based on tests on the 40 meter band (from
Returning briefly to the "What's the best NVIS height?" question,
observe the excellent research of Patricia Gibbons:
half-wave dipole at 1/4 wavelength above ground as a reference for
comparison: A half-wave dipole at 6 to 7 feet off the ground will have an
attenuation of approximately -4 dB. A half-wave dipole 10-1/2 inches over
lossy ground will have a worst-case attenuation of approximately 20 dB."
"Assuming correct choice of frequency and a 10.7 cm solar flux value in
the 200 range, a half-wave dipole at 1/4 wavelength above the ground would
provide a 20 dB over S9 signal reading at the distant station when the
transmitter has a power output of 100 Watts. If the transmitting station
uses an antenna at 6-feet above ground-level, the resultant signal strength
would be: 16 dB over S9. If the transmitting station uses an antenna at
10-1/2 inches above the ground, the resultant signal strength would still
Links to More NVIS Study
(and see also other articles at
A great introductory article:
The excellent site of
NVIS guru Patricia Gibbons, WA6UBE:
http://www.tactical-link.com/field_deployed_nvis.htm -- lots of tests
Field Manual 24-18 Appendix M -- Lots of info and graphics
(now on this site!)
Army Manual TM
11-5985-379-14&P for the AS2259/GR Military NVIS Antenna -
See esp. Section 3 (p.25) for lots of easy NVIS theory. Note: 78 pages,
takes a while to load!
Northern California RACES NVIS experiments:
Antenna Guru L.B. Cebik's Notes on "Cloud Burners:"
Search Usenet groups for NVIS
(hundreds of postings by NVIS experimenters):
The NVIS discussion group on Yahoo Groups (350+ members):
TELEX military NVIS antenna
The MINIBAC antenna system and the neat web site of Bonnie Crystal,
The HF Portable Group:
Space Weather -- Learn what all those solar predictors mean and how to use