Steve Wolf, W8IZ@W8IZ

(This text from the W8IZ packet radio bulletin

board. It's formatted to fit a 80 character screen.)


Picture a fire hose aimed at the ceiling. The water leaves the nozzle and
begins to expand outward - some goes straight up, some doesn't. When it hits
the ceiling, it splashes back in a much bigger conically shaped form, wetting
an area much bigger than the hose's original outlet. Hills, buildings and so
on don't impede much, because the "signal" is coming DOWN, not ACROSS. It has
a limited size, but the size is most adequate for our purposes.

Antenna concepts which have been obscured by "normal" ham radio operations can
improve public service tactical communications. Since 1983, these concepts
have been labeled "NVIS" and championed in the military press.

At first blush, horizontal mobile and head-high dipole antennas may seem
ridiculous. They fly in the face of convention. Disaster antenna systems might
consist of NVIS for local area tactical work and verticals for skipping out of
the disaster zone. Where it is possible to raise the dipole to a half-
wavelength (only 30+ feet on 20M, 22 feet on 15M) it might be simpler to set
up a dipole than a vertical. The dipole might be raised and lowered depending
upon the use. Deciding which is best requires a complete understanding of
antenna fundamentals.

The NVIS system has been advocated since 1983 by a Lt. Colonel in the New
Jersey National Guard. Empirical research has been carried out by both the
NJNG and the U.S. Marines.

NVIS systems allow one vehicle to cover a disaster zone at distances where
repeater systems might not be operational or tactically useful. A mobile
station can cover a great amount of territory, reporting back to tbile/portable
repeatersÓus of 150 or
more miles, each NVIS station can represent major tactical units at site
EOC's, government centers and off-site resources. The military refer to "corps
size areas."

NTS operators may find the NVIS antenna to be the ideal solution for meeting
statewide or even regional nets where "regular" dipoles skip over desired
stations. In this case, two antennas are probably necessary, as well as other
operators also having NVIS capability.

end 1/8

For those unfamiliar with antenna theory, here are some fundamentals.


When a voltage is applied to something, its electrons move around, then try to
get back to their natural locations. Every time an electron moves, it creates
an ELECTROMAGNETIC WAVE. Applying an AC signal at radio frequency to an
ANTENNA makes useful EM waves.

emanate FROM the antenna.

2 - When EM WAVES strike ANYTHING, a current "like" the signal which spawned
it is "induced."

3.- EM WAVES consist of magnetic and electric components; how they line up
relative to the earth determines polarization.

Picture the radio station:
a. an AC generator with two wire output
b. the feedline/antenna junction as a two-wire connector plugged into the
generator output
c. the antenna as a series connection of the feedpoint, antenna, air and the
other feedpoint.

Maximize efficiency by matching impedances at the generator/feedline and
feedline/antenna junctions. Use the best mechanical and electrical engineering
available. This gets harder when the trees have been blown down and it's
raining cats and dogs!


GROUND can mean a number of things.

Use the term EARTH to distinguish dirt from chassis "ground." Electrical
values can be measured across dirt. EARTH has lousier efficiency than a metal
path of the same distance.

GROUND refers to the chassis side of the electrical system, usually negative
or minus in DC polarity, but MOST importantly the RETURN PATH for AC signals.
In these bulletins it means the ground RELATIVE to the antenna being

Picture a "vertical" antenna a quarter wavelength long. No matter how high you
attach it's base, the antenna must have a RELATIVE and/or EARTH "GROUND." The
"nominal" feedpoint impedance of a vertical antenna is 30 someodd ohms IF
everything is "textbook." Fed with 50 ohm coax, the MINIMUM SWR is 50:30 - 1.7
to 1, with PERFECT RELATIVE GROUND and mounted in FREE SPACE.

The AIR between the "antenna" and the feedpoint which is attached to the
RELATIVE GROUND is part of the system.

EM waves radiate from the antenna, striking the EARTH and RELATIVE GROUND.
Recall that as a wave passes through anything, it induces a current. Figure
out the most efficient way to get as much of the induced current back to the
feedpoint connection as possibl, and you have designed the whole path:
generator (transmitter) to feedline, feedline to antenna, air (EM
waves),RELATIVE GROUND, other side of feedline, back to the generator.


A lot of research has been done on short vertical antennas. I distilled the
following from a batch of articles in QST and elsewhere.
1. EARTH is not as efficient as metal. A metal GROUND system will enhance
performance. This is absolutely necessary if the antenna high in the air.
2. There is real cost involved so planning is necessary.
3. There is a point where the cost of a bigger GROUND system exceeds the
benefit. At some distance from the feedpoint, the current induced in the
GROUND will be wasted in heat caused by resistance from the point of
induction to the feedpoint.
4. 30 wires a tenth-wavelength long spread evenly around the feedpoint are
adequate. More wires are better. Longer wires are even better. Start at
the beginning of this paragraph, add as the budget permits.

end 2/8


Horizontal dipoles, vertical dipoles and other antenna types have a RELATIVE
GROUND, EARTH and RETURN PATH consideration.

A lead from your radio chassis to some EARTH connection (cold water pipes,
stake driven into the earth, etc.) gives your dipole 2 GROUND connections.

On one half of the cycle, one side of the dipole radiates and the EM wave
induces current in the other side; on the other half cycle, the opposite
happens. This is one RETURN PATH, through the feedline. The EARTH connection
is the other.

If everything is "textbook" (it is at least a half-wavelength above EXCELLENT
ground and in the open) a dipole has a feedpoint impedance of 70 someodd ohms.
The MINIMUM SWR, using 50 ohm coax, is 70:50 - 1.4 to 1 and obviously better
when using 75 ohm coax.

It is beneficial to put a metal GROUND system under a dipole. A dipole's
operation is directly related to its RELATIVE GROUND system, which determines
the feedpoint impedance and radiation lobe size and direction.

The RETURN PATH part of an antenna system deserves as much effort, thought
and money as the other parts. The relationships among the parts of the
standard quarterwave vertical system are used as an illustration, but the
same thing can be said for all antenna types.


The best way to transfer energy at the station/feedline and antenna/feedline
junctions is to use matching devices at both junctions. If you MUST choose
just one, try to c°oose the antenna/feedline junction. Remember that, relative
to the length of antenna and frequency in use, SHORT antennas need inductance
and LONG ones need capacitance to "match" (cancel the reactance) and make the
system resonant. A matching device may be fixed or adjustable.

The best device is an electronic/automatic "tuner" with a single wire output
and which can match a wide range of impedances, end feeding a wire antenna,
with a suitable ground system attached.

Tuners that only match the low impedance associated with coax cables are
insufficient except where the antenna/feedline junction is well-matched. They
only match the transmitter/feedline and have no effect on the antenna.

If you can't match the feedline/antenna junction for desired frequencies
(cannot ensure a resonant antenna), use open wire feedline, end-feed, a
folded-dipole or other tricks suitable for multi-frequency or multi-band

If you don't have a perfect RELATIVE GROUND, MATCHED FEEDLINE and ANTENNA, and
FREE SPACE to hang it in, you are not going to have a handbook value of
feedpoint impedance. Feedline comes in more or less fixed impedance values.
Something has to give and without proper matching, it is your transmit and
receive efficiency.

1. We are dealing with a system which includes two energy types and many
possible sources of compromise.
2. It is important to pay attention to the details.
3. It is of greater importance to plan for detailing in tactical communica-
tions because it will likely be even more necessary to make compromises and
more difficult to build efficient antenna systems in the field.

end 3/8


Beams work by using the effect of induced currents discussed above. Most beams
are horizontal or vertical, but point parallel to the earth. Signals leave
them at low radiation angles.

The EM wave leaves the "driven" element, just as it does a dipole. But here,
it strikes another nearby, slightly longer element, called a REFLECTOR.

A current is induced causing its OWN EM wave. This wave enhances the waves
going TOWARD THE DRIVEN ELEMENT and cancels waves coming FROM it.

Waves from the driven and reflector elements induce current in "directors,"
slightly shorter elements in the other direction, which in turn cause more EM
waves. The net effect is a high concentration of EM waves going in a specific
direction, adding to each other so as to give a stronger signal than
originally produced by the transmitter power.

Optimal distances between elements are .2 wavelength or less. This is worth




Let me define the terms:

GROUND - This can ee upwards of 50 miles or so depending on band and
EFFECT conditions. Signals follow the earth's surface or may be reflected
by it or other things.

SKIP - This is after the ground effect area and before the point where
ZONE skywaves are reflected back to earth. The lower the angle of radiation,
the greater the area of the skip zone. Signals "skip" over this area.


NEAR - Refers to almost vertical - the signal is radiated at an angle of
80 to 90 degrees from horizontal.
VERTICAL - Refers to the direction of the signal - straight up - and not to
signal polarization or style of antenna.
INCIDENCE - Refers to the point of impact of signals with the ionosphere;
nearly directly overhead of the transmitting antenna.
SKYWAVE - Refers to the fact that we are dealing with a signal which goes up
and comes down, as opposed to following the surface of the earth.
This overcomes the problems of the Ground Effect (absorption by
trees, hills, buildings, etc.) and the Skip Zone by providing a
reliable signal where there might not have been one.

Put it all together and you have a description of a signal which strikes the
ionosphere virtually overhead in a conical shape and comes back down in a much
larger cone. Depending on frequency and band conditions, the return cone may
be 300 to 400 miles (or more) in diameter about the antenna. Within this area
the signal is "omnidirectional."


The idealized image of a dipoles radiation patterns and feedpoint impedance
only holds true if the dipole is in the clear a half-wave above a GOOD
RELATIVE GROUND. When you lower it, you increase the angle of radiation and
lower the feedpoint impedance.

For DX, these facts can be a nuisance, and having a feedpoint out of control
can be a real problem. If you want DX, stick to one frequency and put the
dipole up a half wavelength high.

For NVIS, we can take advantage of the situation and explain to those hams
with 80M dipoles up 30 feet or so why they get out so well for a few hundred
miles, but don't seem to work the DX stations. My guess is that there are more
80M NVIS antennas out there than one would have thought, simply because it is
so hard to get an 80M antenna up 125 feet into the air! And since 62.5 feet is
a quarterwave, it goes without saying that most antennas will fall into the
NVIS category unless one has 70 foot trees or towers handy.

end 4/8


The Normandy Invasion communications were NVIS, designed to cover from the
center of Britain, across the Channel, to the landing area in France, and to
be effective from ships, shore stations and man-pack radios. The principal
problem concerned advance lookouts directing bombing runs, which were
initiated from airfields in England. Ground wave was too short and skywave
reflection was not dependable. Dr. H. H. Beverage redesigned the
communications shortly before the landing.

U.S. Marines used NVIS techniques in Saudi.

We all know that there are problems with HF radio: static, sun storms, size
and weight of the radios, the size of the antenna needed. NVIS doesn't solve
these, except that it is easier to erect low antennas and there is some
received noise reduction achieved.


2 to 12 Mhz. There is a frequency, called the CRITICAL FREQUENCY, ABOVE which
signals are not reflected by the ionosphere.░However, we can be practical. 20M
is not good for local area contacts. That leaves us with 30M, 40M, 80M and
160M. Or 1.8 to 10 Mhz., ham band specific.

160M antennas are quite large, and 80M is producing quite long ranges at the
moment. There may be a time deep in the coming sun cycle when we will need

30M is a good choice for data, but it is not the only one.

40M, in the day, and 80M, at night, are the primary choices for voice. There
is no reason to avoid testing these bands for digital use as well.

This means the dipole is some 130 feet long. With good matching it can be much
less. The electronic end-fed tuners can get excellent performance from wires a
quarter-wavelength or even shorter. The longer the better still prevails.

As a classic dipole is raised above the earth, its feedpoint impedance RISES
and it's radiation angle LOWERS. Since the goal is to have the highest
radiation angle possible, it pays to LOWER the antenna and it figures that the
feedpoint impedance will be lower than nominal for dipoles. If the antenna is
kept around a quarter-wavelength or lower above the earth, a fairly good match
for 50 ohm coax can be made. Check the diagrams in the Handbook.

There are a practical points for not using coax and for end or center feeding
the antenna with a tuner.

We are interested in 24 hour communications in a tactical situation. Unless
we want to put up two antennas, we should provide for handling any frequency
(and feed impedance) that we might encounter.

Frequencies OUTSIDE the usual ham band might be important. MARS, CAP and
others might need be within range of our antenna. New electronic tuners with
memories and tuning cycles of fewer then a couple of seconds can accommodate
split operations on really weird, wild frequency separations.

Emergency antenna mounts can mean unpredictable results. The earth itself,
as well as nearby "stuff" and makeshift ounts, make the feedpoint impedance
highly unpredictable.

end 5/8


A vertical power gain of about 1.5 Db can be obtained by drooping the center
of the dipole (5 feet or so for 80M). This cancels the low angle lobes. That's
about a 50% signal increase for drooping the wire - well worth the effort.

A 40M dipole at 26 feet with one or more 5% longer wires parallel to it on the
earth makes a 2 element 40 meter beam, spaced .2 wavelength, pointed straight
up. A good reflector might consist of several such wires spaced about 5 feet
apart (horizontally) and mounted just above the earth surface.

The optimum height above ground might be considered to be one tenth to one
quarter wavelength - 26 to 66 feet on 80M, 13 to 33 feet on 40M
(approximately). A wire over 30 feet above the earth should have a
counterpoise (wires under it) to constitute the RELATIVE GROUND.

A dipole configured as an inverted VEE can be used if the angle at the apex is
kept to 120-140 degrees instead of 90. I purchased a military NVIS antenna in
the flea market at Dayton. It consists of a 15 foot tall mast and two unequal
dipoles intended to be set up in an "x" pattern and looking very much like the
described inverted V.

Folded dipoles offer bandwidth and feedpoint control advantages (balanced
feedline and larger antenna size). Multi-wire (cage) dipoles offer increased

Ideas being tried for portable and fixed mobile use and found workable:
1. putting the antenna as low as 6 feet, with and without a parallel ground.
2. mount the wire on supports a couple feet high. The antenna tested was 110
feet long, center fed with a tuner at the feedpoint and mounted on traffic
cones. A slit in the top of the cone held the wire. A better rendition of
this might be to use slotted masonite for supports - they fold flatter for
storage and carrying. (I suggest painting them orange and white and using
the biggest wire in a red or white insulation that you can get - this is a
real person-hazard).

Since folded dipoles give greater bandwidth and physically larger antenna
elements give better efficiency, the experimental options abound. Consider
that in a disaster area, antenna parts might consist of aluminum siding, down
spouts, salvaged ac wiring, window screens - you name it. Properly configured,
a working NVIS antenna can be constructed of many things. Getting it up in the
air ceases to be a problem!


We're gonna get weird now. We covered all the basics about verticals. Mobile
antennas are usually vertical because that is easiest. Mounted well, in the
center of the vehicle, a mobile vertical works. The major obstacles are
mounting it in the center of the roof, getting a good connection to the roof
and body for RF and matching the antenna to the feedline.

All of this is still necessary, but verticals have a low angle of radiation
and that is not what we want for tactical communications.

We achieve high angle radiation by SWINGING THE MOBILE ANTENNA HORIZONTAL.
This means that the center mounting is no longer desirable. The highest
current will be in the bottom, near the base, so get the bottom of the antenna
out over the ground, away from the vehicle.

A van or station wagon can mount the antenna on the rear anywhere that allows
for the antenna to be swung out horizontally, behind the vehicle. The
"horizontal" angle can be anywhere between 45 degrees and 90 degrees. For
practical mobile operation, it may be rotated to the lowest SAFE angle. For
fixed operation, dropping it to horizontal will yield best results. Also, for
fixed operation, a wire can be attached to the tip of the antenna to lengthen

end 6/8

For cars, a permanent or removable roof rack can be used and the antenna swung
out sideways, or a trunk mounting might work if the antenna can be made safe
for mobile operation. Bumper mounted antennae are person hazards and
definitely cannot be used mobile, so they offer the least practicality.
However, a bumper mounted post could be used to mount the antenna base up

In any case, the GROUND side of the feedline must be well attached, at RF, not
just DC, to the main body of the vehicle. This implies that all the body and
other pieces are attached by straps or otherwise to each other. Ground
strapping should be used, not round wires, to effect the RF GROUND
connections. Good ideas for mobile HF: both DC power leads direct to the
battery and fused; ground the RF section to the chassis (I use hose clamps on
the coax connector(s) to attach the braid to the radio).

end 7/8


Now, pick up your HT. Hold it upright. Vertical antenna, right? Lousy relative
ground system, right? Right. Hold it sideways. What is it? It's a DIPOLE! The
ducky is one side, the metal body of the HT attached to the outside of the BNC
connector is the other side. Yep.

Now, pick up your vehicle. Hold it up so you can picture the mobile antenna
now horizontal. Dipole? You betcha. Wanaa LOOP? Bend the antenna so it loops
over the vehicle and attach the tip to the front bumper (it's all attached to
the body, etc., right?). With your mobile dipole so low, you get great

The dipole configuration is vertical, more or less 80-90 degree angle. The
vehicle can be made into a more resonant dipole by attaching a loading coil to
the front bumper/frame and tuning it for maximum RF current. This "loads" both
sides of the "dipole."

The loop configuration is an 80-60 degree angle, aimed in the direction away
from the base of the antenna (or toward front of vehicle). The loop is easiest
to live with but least efficient.

Why is a loop easier? Because the mobile antenna length should be 16 feet! or
even longer!! Military research has been based on HUMMV vehicles and 16 foot
rear mounted antennas. Both dipole and loop configurations work, but where
it's possible to drive with a 16 foot long whip, that is best! (Remember
seeing the Desert Storm vehicles with the 45 degree whips? NVIS. MOST of the
antennas WERE vertical, so my guess is they were close enough to use ground
wave). We are going to be using 80M thru 30M and 16 feet is VERY short - this
means a LOT of inductance will be required to match the whip, the radiation
resistance will be quite low and bandwidth very narrow.


I am testing some LOADED mobile antennas as well as a long whip:
1. a Hustler 4.5 foot shaft attached to a 22 inch short shaft, the resonator
some six feet from the top rear of wagon, horizontal;
2. both shafts attached to a fiberglass cb whip. (a steel cb whip was not
stiff enough). 6.5 feet of shaft and 9 feet of whip makes it long enough
for my electronic tuner to work on 40M and maybe 80M. I think it is
impractical to drive with this arrangement. I will try the loaded
antenna(s) while mobile. Mounted on the rear roof of a station wagon,
I can orient it mostly horizontal and still keep it from being a hazard;
3. the short shaft attached to the base of a 4 foot Outbacker antenna, in
place of the Hustler.

My first test will consist of the NVIS antenna and my Outbacker vertical in
the normal center mount with a coax switch to change antennas. The electronic
tuner will be attached to the NVIS antenna. The test will be primarily daytime
so 40M will be used most. The 80M resonator will be along just in case.

At TOSRV, over Mother's Day weekend, during the bicycle tour, mobile and fixed
stations will be trying out the concept. Frequencies will be promulgated
later, and your assistance in compiling signal reports will be appreciated.
The Tour of the Scioto River Valley begins Saturday morning (11th) in
Columbus, Ohio and runs to Portsmouth on the Ohio River, then returns on
Sunday (12th). Stations in the 150 mile radius of South Central/East Ohio will
be needed to ascertain how well it all works out.


These bulletins were prompted by successful experiments by California RACES
operators, a New Jersey National Guard unit and the U. S. Marines, as well as
antenna theory and practice set forth in the ARRL Handbook and other antenna

Tony Dacres, W8MDK @ KC8TW.OH


_________ The tip is NOT ___________ The tip IS
/ electrically / \ electrically
! iQQi connected to the ! iQQi ! connected to the
^iQI IQi front of the ^i-I IQi ! front of the
-oQQQ--Qo- vehicle. -oQQ--QQo- ^ vehicle.

More effective is a fore-and-aft loop over the vehicle. The rear is connected
normally, the front is attached to the front bumper securely with a bolt. The
loop is 1/8 wavelength (34 feet, 80M) long or more - not too hard with a two 9
foot whips bent over a long vehicle, the interconnection between whip tips
made with wire making up the difference. Remember that the one whip is normal
(connected to the feedline) and the other is bolted to the vehicle frame
(assuming that the body of the vehicle is solidly shorted together).

The U.S. Marine HUMMWV installation is a 100/400 watt HF sb/digital radio, a
base-loading electronic matching device, a fiberglass whip of BIG diameter and
TALL size. An adapter is used to tilt the whip back about 45 degrees.

Responders with mobile HF stations may not be prepared for NVIS operation.
Such stations can be a LONG-RANGE resource using a regular whip or, using
wires from the antenna mount to a nearby support or by hanging a dipole, a
fixed-site NVIS station.


Flexibility demands that low-angle radiators be considered. They may be
necessary. It is possible to have the best of both worlds with a little
careful planning.

Respondents may not be prepared for HF mobile NVIS operation. Such mobiles can
be a long-range resource on appropriate frequencies or fixed-site NVIS
systems. A matching device and some wire can make up an antenna.

A vertical monopole 1/4 wave long makes a good low-angle, long-range antenna.
Some characteristics are either not well known or misunderstood, although any
V/UHF antenna mounted in a hole in the vehicle roof is using all of them

An efficient ground system is essential. The ground and the "other feedline
conductor" connected to it are often relegated to the position of incidentals.
In fact, BOTH conductors of the feedline route EQUAL and OPPOSITE polarity
currents and the antenna SYSTEM consists of the monopole, the air and the
ground system.

Pictured as a cloeed system, the current flows from the transmitter via one
conductor into the antenna vertical element; electromagnetic waves generated
by the current radiate out into the world, but also strike the ground system;
current induced by the strong electromagnetic field flows from the antenna to
the transmitter via the other conductor on the equal but opposite side of the
AC at RF cycle.

The flow reverses on the next half cycle. Picture the ground system as the
radiator, the antenna element as the intervening source of induced current.
Viewed as a closed AC at RF system, it makes sense to make both the vertical
element and the ground element as efficient as possible.

Considering the monopole as having no "gain" as such, it is still possible to
make a single element directional. Where there is a ground element, the field
is as strong as the system can make. Where there is no ground element, the
field is as weak as the system can make, assuming that the earth is not as
efficient a ground as the "artificial" one (truth is, the artificial ground
BECOMES the real ground as far as the antenna is concerned). Use this
phenomenon by building a virtually solid ground in the direction desired.
Since the antenna is by nature omnidirectional, several directions can be
"maximized" at once, both transmit and receive.

Where one of the "radialless" antennas is being used, a ground grid under the
antenna will stabilize the effect of the earth ground under the antenna. A
good ground grid brings the system closer to the theoretical "perfect ground."

A "good" ground system is not hard to do, but is often hard to define. The
following information has been obtained from a variety of sources, some the
literature researched for this NVIS report.


The combination of mobile and portable NVIS antennas gives good, practical
tactical antennas. Appropriate use of a vertical antenna gives long range
performance. Selecting the components of the two BEFORE the need means that
you are READY for any need. Being NVIS mobile means that you can go out beyond
the range of most local repeaters, or into territory where V/Uhf communication
isn't possible, and maintain contact from the net control site to a radius of
300 miles or more.

System design should include all modes, all frequencies in the HF spectrum and
all kinds of operation. It is not that difficult, but with an understanding of
the NVIS concept as well as the usual long-range "DX" concept, tactical
communicators are well armed for any situation.

Some thoughts:
This information was sent out on packet, instead of any other medium, because
this is where the action is. My guess is that most serious public service
hams are either on packet or have access to it via a friend who is. Therefore,
this information can get to a lot of people. One wag suggested I submit it
to a magazine. That's a good idea, but only the subscribers would have access.

ACCESS. That is the key. Packet offers more access to information than any of
our magazines; more immediately, and interpersonal, too.

One group is already anticipating me and setting up a statewide packet net
using NVIS HF concepts. I think there is opportunity here to make better
networks and maybe relieve the pressure on the U/VHF net. In any case, there
is food for thought and enough theory here to work from - if I learn any
more, or if I am advised of errors, I will let you know and pass the new
info along. I would appreciate your doing the same to me. I have already
had a lot of positive feedback. And one guy who feels that this is too much
verbage from one ham, so he is killing ALL of these buls! So much for public
service from his BBS! I do hope he, and any others of like mind, change their
opinions and store the buls for a while so local hams have a chance to read
and investigate.

Best of Holidays and thanks for your patience. I have been loading these 3
at a time, with a 2/3 or more day span in between, to avoid clutter. I can
only hope it worked out that way!




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