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<TD
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<P style="TEXT-ALIGN: center" align=center><FONT
face="Times New Roman"><B><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">NEAR-VERTICAL INCIDENCE
SKY-WAVE PROPAGATION CONCEPT</SPAN></B><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB"><o:p></o:p></SPAN></FONT></P>
<P><FONT face="Times New Roman"><B><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">M-1. Evaluation of
Communications Techniques</SPAN></B><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB"><o:p></o:p></SPAN></FONT></P>
<P><SPAN lang=EN-GB style="COLOR: black; mso-ansi-language: EN-GB"><FONT
face="Times New Roman">The standard communications techniques used in the
past will not support the widely deployed and the fast-moving formations
we intend to use to counter the modern threat. Coupling this with the
problems that can be expected in deploying multichannel LOS systems with
relays to keep up with present and future operation, high frequency (HF)
radio and the near-vertical incidence sky-wave (NVIS) mode take on new
importance. High frequency radio is quickly deployable, securable, and
capable of data transmission. It will be the first, and frequently the
only, means of communicating with fast-moving or widely separated units.
It may also provide the first long-range system to recover from a nuclear
attack. With this reliance on HF radio, communications planners,
commanders, and operators must be familiar with NVIS techniques and their
applications and shortcomings in order to provide more reliable
communications. <o:p></o:p></FONT></SPAN></P>
<P><FONT face="Times New Roman"><B><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">M-2. Problems Encountered
in Propagation of Radio Waves</SPAN></B><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB"><o:p></o:p></SPAN></FONT></P>
<P><FONT face="Times New Roman"><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">Under ideal conditions,
ground wave component of a radio wave becomes unusable at about 80
kilometers (50 mi) (</SPAN><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180014.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB">fig
2-12</SPAN></A></SPAN><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">). Under actual field
conditions, this range can be much less, sometimes as little as 3
kilometers (2 mi). Sky waves, generated by standard antennas (for example,
doublets) which efficiently launch the sky wave, will not return to earth
at a range of less than 161 kilometers (100 mi). This can leave a skip
zone of at least 80 to 113 kilometers (50 to 70 mi) where HF
communications will not function. This means that units such as long-range
patrols, armored cavalry deployed as advance or covering forces, air
defense early warning teams, and many division-corps, division-brigade,
division-DISCOM and division-DIVARTY stations are in the skip zone and
thus unreachable by HF radio even though HF is a primary means of
communication to these units. <o:p></o:p></SPAN></FONT></P>
<P><!-- [if gte vml 1] --><!-- [endif][if !vml] --><SPAN
style="COLOR: black"><FONT face="Times New Roman"><IMG height=261
src="cid:580473817@22062007-1D9A" width=610 align=bottom border=0
v:shapes="Picture15"><o:p></o:p></FONT></SPAN></P>
<P><FONT face="Times New Roman"><B><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">M-3. Concept of
Near-Vertical Incidence Sky-Wave Radiation</SPAN></B><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB"><o:p></o:p></SPAN></FONT></P>
<P><FONT face="Times New Roman"><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">Energy radiated in a
near-vertical incidence direction is not reflected down to a pinpoint on
the Earth's surface. If it is radiated on too high a frequency, the energy
penetrates the ionosphere and continues on out into space. Energy radiated
on a low enough frequency is reflected back to earth at all angles
(including the zenith), resulting in the energy striking the earth in an
omnidirectional pattern without dead spots (that is, without a skip zone).
Such a mode is called a near-vertical incidence sky wave (NVIS).
</SPAN><SPAN style="COLOR: black">The concept is illustrated in <A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180150.gif"><SPAN
style="COLOR: #a8975e">figure M-1</SPAN></A>.
<o:p></o:p></SPAN></FONT></P>
<P><!-- [endif] --><!-- [if gte vml 1] --><!-- [endif][if !vml] --><SPAN
style="COLOR: black"><FONT face="Times New Roman"><IMG height=512
src="cid:580473817@22062007-1DA1" width=610 align=bottom border=0
v:shapes="Picture17"><o:p></o:p></FONT></SPAN></P>
<P><FONT face="Times New Roman"><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">This effect is similar to
taking a hose with a fog nozzle and pointing it straight up. The water
falling back to earth covers a circular pattern continuously out to a
given distance. A typical receive signal pattern for antenna AS-2259/GR is
shown in </SPAN><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180151.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB">figure
M-2</SPAN></A></SPAN><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">, and the path length and
incident angle are shown in </SPAN><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180152.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB">figure
M-3</SPAN></A></SPAN><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">. A typical elevation plane
pattern is shown in </SPAN><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180153.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB">figure
M-4</SPAN></A></SPAN><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">. The main difference
between this short-range NVIS mode and the standard long-range sky-wave HF
mode is the lower frequency required to avoid penetrating the ionosphere
and the angle of incident signal upon the ionosphere. In order to attain a
NVIS effect, the energy must be radiated strong enough at angles greater
than about 75 or 80 degrees from the horizontal on a frequency that the
ionosphere will reflect at that location and time. The ionospheric layers
will reflect this energy in an umbrella-type pattern with no skip zone.
Any ground wave present with the NVIS signal will result in undesirable
wave interference effects (such as, fading) if the amplitudes are
comparable. However, proper antenna selection will reduce ground-wave
radiated energy to a minimum, and this will reduce the fading problems.
Ranges for the NVIS mode are shown in </SPAN><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180152.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB">figure
M-3</SPAN></A></SPAN><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB"> for typical ionosphere
height and take-off angles. Since NVIS paths are purely sky wave, the path
losses are nearly constant at about 110 dB +10 dB. Relative gain
performance of the AS-2259/GR NVIS antenna is shown in </SPAN><SPAN
style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180154.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB">figure
M-5</SPAN></A></SPAN><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">. This is significant for
the tactical communicator because all the energy arriving at the receiving
antenna is coming from above at about the same strength over all of the
communications ranges of interest. This means the effect of terrain and
vegetation (when operating from defiladed positions such as valleys) are
greatly reduced, and the receive signal strength will not vary greatly.
<o:p></o:p></SPAN></FONT></P>
<P><SPAN lang=EN-GB style="COLOR: black; mso-ansi-language: EN-GB"><FONT
face="Times New Roman"> <o:p></o:p></FONT></SPAN></P>
<P><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180150.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB"><FONT
face="Times New Roman">Figure M-1.</FONT></SPAN></A></SPAN><SPAN
lang=EN-GB style="COLOR: black; mso-ansi-language: EN-GB"><FONT
face="Times New Roman"> Near-vertical incidence sky-wave propagation
concept.<o:p></o:p></FONT></SPAN></P>
<P><!-- [endif] --><!-- [if gte vml 1] --><!-- [endif][if !vml] --><SPAN
style="COLOR: black"><FONT face="Times New Roman"><IMG height=562
src="cid:580473817@22062007-1DA8" width=565 align=bottom border=0
v:shapes="Picture20"><o:p></o:p></FONT></SPAN></P>
<P><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180151.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB"><FONT
face="Times New Roman">Figure M-2.</FONT></SPAN></A></SPAN><SPAN
lang=EN-GB style="COLOR: black; mso-ansi-language: EN-GB"><FONT
face="Times New Roman"> Near-vertical incidence sky-wave antenna typical
azimuth plane pattern.<o:p></o:p></FONT></SPAN></P>
<P><!-- [endif] --><!-- [if gte vml 1] --><!-- [endif][if !vml] --><SPAN
style="COLOR: black"><FONT face="Times New Roman"><IMG height=444
src="cid:580473817@22062007-1DAF" width=610 align=bottom border=0
v:shapes="Picture23"><o:p></o:p></FONT></SPAN></P>
<P><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180152.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB"><FONT
face="Times New Roman">Figure M-3.</FONT></SPAN></A></SPAN><SPAN
lang=EN-GB style="COLOR: black; mso-ansi-language: EN-GB"><FONT
face="Times New Roman"> Path length and incident angle (near-vertical
incidence sky-wave mode). <o:p></o:p></FONT></SPAN></P>
<P><!-- [endif] --><!-- [if gte vml 1] --><!-- [endif][if !vml] --><SPAN
style="COLOR: black"><FONT face="Times New Roman"><IMG height=338
src="cid:580473817@22062007-1DB6" width=610 align=bottom border=0
v:shapes="Picture26"><o:p></o:p></FONT></SPAN></P>
<P><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180153.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB"><FONT
face="Times New Roman">Figure M-4.</FONT></SPAN></A></SPAN><SPAN
lang=EN-GB style="COLOR: black; mso-ansi-language: EN-GB"><FONT
face="Times New Roman"> Typical elevation plane
pattern.<o:p></o:p></FONT></SPAN></P>
<P><!-- [endif] --><!-- [if gte vml 1] --><!-- [endif][if !vml] --><SPAN
style="COLOR: black"><FONT face="Times New Roman"><IMG height=562
src="cid:580473817@22062007-1DBD" width=541 align=bottom border=0
v:shapes="Picture29"><o:p></o:p></FONT></SPAN></P>
<P><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180154.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB"><FONT
face="Times New Roman">Figure M-5.</FONT></SPAN></A></SPAN><SPAN
lang=EN-GB style="COLOR: black; mso-ansi-language: EN-GB"><FONT
face="Times New Roman"> Relative gain performance of AS-2259
antenna.<o:p></o:p></FONT></SPAN></P>
<P><FONT face="Times New Roman"><B><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">M-4. Assessment of
Characteristics of Common Antennas</SPAN></B><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB"><o:p></o:p></SPAN></FONT></P>
<P><FONT face="Times New Roman"><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">It is obvious that the Army
needs short-range HF communications in the 2-30 MHz frequency band in the
1985-1990 time frame and beyond. The problem, however, is to obtain the
required radiation characteristics. This is not difficult, because
half-wave dipole antennas located from one-quarter to one-tenth wavelength
above the ground will cause the radiated energy to be directed vertically
(</SPAN><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180156.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB">fig
M-6</SPAN></A></SPAN><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">).</SPAN><SPAN
style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180155.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB">Table
M-1</SPAN></A></SPAN><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB"> shows the relative gain
toward the zenith of the most common types of HF antennas. This table
shows that the half-wave Shirley folded dipole (</SPAN><SPAN
style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180157.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB">fig
M-7</SPAN></A></SPAN><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">) has the most gain towards
the zenith (with the other dipoles being almost as good). The Shirley
dipole is a good NVIS base station antenna, but it is limited to a band of
frequencies within about 10 percent of the design frequency. The fan
dipole (</SPAN><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180158.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB">fig
M-8</SPAN></A></SPAN><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB"> and </SPAN><SPAN
style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180155.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB">table
M-1</SPAN></A></SPAN><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">) performs almost as well,
and it provides more frequency flexibility (for example, day, night, and
transition period frequencies). For tactical communications, these dipoles
can be easily deployed in a field expedient manner because they can be
located close to the ground. For mobile or shoot-and-scoot type
operations, vehicular-mounted antennas are required. This is the standard
5-meter (161/2-foot) whip bent down to a horizontal position (</SPAN><SPAN
style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180159.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB">fig
M-9</SPAN></A></SPAN><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">). In this configuration,
the whip is essentially an asymmetrical dipole (with the vehicle body
forming one side) located close to the Earth. A significant amount of
energy is directed upward (</SPAN><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180156.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB">fig
M-6</SPAN></A></SPAN><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB"> for typical pattern) to be
reflected back by the ionosphere in an umbrella pattern. For use, while
operating on the move, the whip antenna must be tied across or parallel to
the vehicle or shelter. This configuration is like an asymmetrical
open-wire line, and it also directs some energy upwards although with less
efficiency. There are still no skip zones, but received signal levels are
weaker than with the whip tied back as shown in </SPAN><SPAN
style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180159.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB">figure
M-9</SPAN></A></SPAN><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">.
<o:p></o:p></SPAN></FONT></P>
<P><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180155.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB"><FONT
face="Times New Roman">Table M-1.</FONT></SPAN></A></SPAN><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB"><FONT
face="Times New Roman"> Summary of relative gain toward the zenith for
field-expedient high frequency antennas (in dB)
<o:p></o:p></FONT></SPAN></P>
<P><!-- [endif] --><!-- [if gte vml 1] --><!-- [endif][if !vml] --><SPAN
style="COLOR: black"><FONT face="Times New Roman"><IMG height=550
src="cid:580473817@22062007-1DC4" width=750 align=bottom border=0
v:shapes="Picture32"><o:p></o:p></FONT></SPAN></P>
<P><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180156.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB"><FONT
face="Times New Roman">Figure M-6.</FONT></SPAN></A></SPAN><FONT
face="Times New Roman"><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB"> Typical elevation plane
patterns for half-wavelength antennas one-eighth </SPAN><!-- [endif] --><!-- [if gte vml 1] --><!-- [endif][if !vml] --><SPAN
style="COLOR: black"><IMG height=414 src="cid:580473817@22062007-1DCB"
width=610 align=bottom border=0 v:shapes="Picture35"></SPAN><SPAN
lang=EN-GB style="COLOR: black; mso-ansi-language: EN-GB">wavelength or
less above ground. <o:p></o:p></SPAN></FONT></P>
<P><SPAN lang=EN-GB style="COLOR: black; mso-ansi-language: EN-GB"><FONT
face="Times New Roman"> <o:p></o:p></FONT></SPAN></P>
<P><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180157.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB"><FONT
face="Times New Roman">Figure M-7.</FONT></SPAN></A></SPAN><SPAN
lang=EN-GB style="COLOR: black; mso-ansi-language: EN-GB"><FONT
face="Times New Roman"> Half-wave Shirley folded dipole.
<o:p></o:p></FONT></SPAN></P>
<P><!-- [endif] --><!-- [if gte vml 1] --><!-- [endif][if !vml] --><SPAN
style="COLOR: black"><FONT face="Times New Roman"><IMG height=588
src="cid:580473817@22062007-1DD2" width=610 align=bottom border=0
v:shapes="Picture38"><o:p></o:p></FONT></SPAN></P>
<P><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180158.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB"><FONT
face="Times New Roman">Figure M-8.</FONT></SPAN></A></SPAN><SPAN
lang=EN-GB style="COLOR: black; mso-ansi-language: EN-GB"><FONT
face="Times New Roman"> Fan dipole NVIS base station
antenna.<o:p></o:p></FONT></SPAN></P>
<P><!-- [endif] --><!-- [if gte vml 1] --><!-- [endif][if !vml] --><SPAN
style="COLOR: black"><FONT face="Times New Roman"><IMG height=376
src="cid:580473817@22062007-1DD9" width=610 align=bottom border=0
v:shapes="Picture40"><o:p></o:p></FONT></SPAN></P>
<P><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180159.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB"><FONT
face="Times New Roman">Figure M-9.</FONT></SPAN></A></SPAN><SPAN
lang=EN-GB style="COLOR: black; mso-ansi-language: EN-GB"><FONT
face="Times New Roman"> Tying the whip antenna
down.<o:p></o:p></FONT></SPAN></P>
<P><!-- [endif] --><!-- [if gte vml 1] --><!-- [endif][if !vml] --><SPAN
style="COLOR: black"><FONT face="Times New Roman"><IMG height=502
src="cid:580473817@22062007-1DE0" width=610 align=bottom border=0
v:shapes="Picture43"><o:p></o:p></FONT></SPAN></P>
<P><FONT face="Times New Roman"><B><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">M-5. Orientation of
Antenna</SPAN></B><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB"><o:p></o:p></SPAN></FONT></P>
<P><FONT face="Times New Roman"><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">Wire dipole antennas have
always been sited so that the broadside of the antenna was pointed toward
the receiving station(s). This is still the correct approach for long-haul
paths. This antenna orientation is not necessary when using the NVIS mode.
For NVIS operation, antenna orientation does not matter since all the
energy is directed upward and returns to earth in an omnidirectional
pattern. This means that the dipole should be erected at any orientation
that is convenient at the particular radio site without regard to the
location of other stations. This holds true except when operating in the
region of the magnetic dip equator (</SPAN><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180160.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB">fig
M-10</SPAN></A></SPAN><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">). When operating near the
dip equator (such as, within 500 kilometers (311 mi)), the dipole antennas
should be oriented in a magnetically north-south direction for greater
receive signal levels for all NVIS path bearings. Antenna orientation
broadside to the path direction must be retained near the dip equator and
elsewhere for longer sky-wave paths. <o:p></o:p></SPAN></FONT></P>
<P><FONT face="Times New Roman"><B><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">M-6. Problems in Using the
NVIS Concept</SPAN></B><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB"><o:p></o:p></SPAN></FONT></P>
<P><SPAN lang=EN-GB style="COLOR: black; mso-ansi-language: EN-GB"><FONT
face="Times New Roman">While use of the NVIS technique does provide beyond
line-of-sight, skip-zone-free communications, there are some drawbacks in
its use that must be understood in order to minimize them.
<o:p></o:p></FONT></SPAN></P>
<P><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180160.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB"><FONT
face="Times New Roman">Figure M-10.</FONT></SPAN></A></SPAN><SPAN
lang=EN-GB style="COLOR: black; mso-ansi-language: EN-GB"><FONT
face="Times New Roman"> Magnetic dip equator.<o:p></o:p></FONT></SPAN></P>
<P><!-- [endif] --><!-- [if gte vml 1] --><!-- [endif][if !vml] --><SPAN
style="COLOR: black"><FONT face="Times New Roman"><IMG height=410
src="cid:580473817@22062007-1DE7" width=610 align=bottom border=0
v:shapes="Picture45"><o:p></o:p></FONT></SPAN></P>
<P><FONT face="Times New Roman"><B><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">Interference Between Ground
Wave and Sky Wave.</SPAN></B><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB"><o:p></o:p></SPAN></FONT></P>
<P><SPAN lang=EN-GB style="COLOR: black; mso-ansi-language: EN-GB"><FONT
face="Times New Roman">Where both a NVIS and ground-wave signal are
present, the ground wave can cause destructive interference. Proper
antenna selection will suppress ground-wave radiation and minimize this
effect while maximizing the amount of energy going into the NVIS mode.
<o:p></o:p></FONT></SPAN></P>
<P><FONT face="Times New Roman"><B><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">High Take-Off
Angles.</SPAN></B><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB"><o:p></o:p></SPAN></FONT></P>
<P><FONT face="Times New Roman"><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">In order to produce
radiation which is nearly vertical, antennas must be selected and located
carefully in order to minimize the ground-wave radiation and maximize the
energy radiated towards the zenith. This can be accomplished by using
specially designed antennas such as AS-2259/GR or by locating standard
dipole (doublet) antennas one-quarter to one-tenth wavelength from the
ground in order to direct the energy toward the zenith (</SPAN><SPAN
style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180161.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB">fig
M-11</SPAN></A></SPAN><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">). A typical measured
dipole pattern (power gain) is shown in </SPAN><SPAN
style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180162.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB">figure
M-12</SPAN></A></SPAN><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">.
<o:p></o:p></SPAN></FONT></P>
<P><FONT face="Times New Roman"><B><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">Critical Frequency
Selection.</SPAN></B><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB"><o:p></o:p></SPAN></FONT></P>
<P><SPAN lang=EN-GB style="COLOR: black; mso-ansi-language: EN-GB"><FONT
face="Times New Roman">As in all sky-wave propagation, there is a critical
frequency (fo) above which radiated energy will not be reflected by the
ionosphere but will pass through it (TM 11-666). This frequency is related
approximately to the angle of incidence. <o:p></o:p></FONT></SPAN></P>
<P><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180161.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB"><FONT
face="Times New Roman">Figure M-11.</FONT></SPAN></A></SPAN><SPAN
lang=EN-GB style="COLOR: black; mso-ansi-language: EN-GB"><FONT
face="Times New Roman"> Recommended dipole height for NVIS
applications.<o:p></o:p></FONT></SPAN></P>
<P><!-- [endif] --><!-- [if gte vml 1] --><!-- [endif][if !vml] --><SPAN
style="COLOR: black"><FONT face="Times New Roman"><IMG height=562
src="cid:580473817@22062007-1DEE" width=505 align=bottom border=0
v:shapes="Picture47"><o:p></o:p></FONT></SPAN></P>
<P><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180162.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB"><FONT
face="Times New Roman">Figure M-12.</FONT></SPAN></A></SPAN><SPAN
lang=EN-GB style="COLOR: black; mso-ansi-language: EN-GB"><FONT
face="Times New Roman"> Measured radiation pattern of the 8-MHz 23-foot
high unbalanced dipole.<o:p></o:p></FONT></SPAN></P>
<P><!-- [endif] --><!-- [if gte vml 1] --><!-- [endif][if !vml] --><SPAN
style="COLOR: black"><FONT face="Times New Roman"><IMG height=562
src="cid:580473817@22062007-1DF5" width=554 align=bottom border=0
v:shapes="Picture49"><o:p></o:p></FONT></SPAN></P>
<P><FONT face="Times New Roman"><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">This means that the useful
frequency range varies in accordance with the path length. The shorter the
path, the lower the MUF and the smaller the frequency range. In practice,
this limits the NVIS mode of operation to the 2-to 4-MHz range at night
and to the 4- to 8-MHz range during the day (</SPAN><SPAN
style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180163.gif"><SPAN
lang=EN-GB style="COLOR: #a8975e; mso-ansi-language: EN-GB">fig
M-13</SPAN></A></SPAN><SPAN lang=EN-GB
style="COLOR: black; mso-ansi-language: EN-GB">). These nominal limits
will vary with the 11-year sunspot cycle and they will be smaller during
sunspot minimums (for example, 1985-86). This restriction of the frequency
range is due to the physics of the situation and cannot be overcome. Some
problems can be expected when operating on the NVIS mode in this portion
of the HF spectrum. <o:p></o:p></SPAN></FONT></P>
<P><!-- [endif] --><!-- [if gte vml 1] --><!-- [endif][if !vml] --><SPAN
style="COLOR: black"><FONT face="Times New Roman"><IMG height=489
src="cid:580473817@22062007-1DFC" width=610 align=bottom border=0
v:shapes="Picture51"><o:p></o:p></FONT></SPAN></P>
<P><SPAN lang=EN-GB style="COLOR: black; mso-ansi-language: EN-GB"><FONT
face="Times New Roman">The range of frequencies between the MUF and the
LUF is limited, and frequency assignment may be a problem.
<o:p></o:p></FONT></SPAN></P>
<P><SPAN lang=EN-GB style="COLOR: black; mso-ansi-language: EN-GB"><FONT
face="Times New Roman">The lower portion of the band which supports NVIS
is somewhat congested with aviation, marine, broadcast, and amateur radio
which limits frequencies available. <o:p></o:p></FONT></SPAN></P>
<P><SPAN style="COLOR: black"><FONT face="Times New Roman">Atmospheric
noise is higher in this portion of the HF spectrum in the afternoon and
night. <o:p></o:p></FONT></SPAN></P>
<P><SPAN style="COLOR: black"><FONT face="Times New Roman">Man-made noise
tends to be higher in this portion of the HF spectrum.
<o:p></o:p></FONT></SPAN></P>
<P><FONT face="Times New Roman"><B><SPAN style="COLOR: black">M-7.
Advantages in Using the NVIS Concept</SPAN></B><SPAN
style="COLOR: black"><o:p></o:p></SPAN></FONT></P>
<P><SPAN style="COLOR: black"><FONT face="Times New Roman">After the
foregoing problems are overcome, there are many advantages in using the
NVIS concept. <o:p></o:p></FONT></SPAN></P>
<P><FONT face="Times New Roman"><B><SPAN style="COLOR: black">The tactical
environment.</SPAN></B><SPAN
style="COLOR: black"><o:p></o:p></SPAN></FONT></P>
<UL type=disc>
<LI class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; COLOR: black; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto; mso-list: l0 level1 lfo1; tab-stops: list 36.0pt"><FONT
face="Times New Roman">There are skip-zone-free omnidirectional
communications. <o:p></o:p></FONT></LI></UL>
<UL type=disc>
<LI class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; COLOR: black; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto; mso-list: l5 level1 lfo2; tab-stops: list 36.0pt"><FONT
face="Times New Roman">Terrain does not effect loss of signal. This
gives a more constant received signal level over the operational range
instead of one which varies widely with distance.
<o:p></o:p></FONT></LI></UL>
<UL type=disc>
<LI class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; COLOR: black; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto; mso-list: l6 level1 lfo3; tab-stops: list 36.0pt"><FONT
face="Times New Roman">Operators are able to operate from protected,
dug-in positions. Thus tactical commanders do not have to control the
high ground for HF communications purposes. <o:p></o:p></FONT></LI></UL>
<UL type=disc>
<LI class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; COLOR: black; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto; mso-list: l7 level1 lfo4; tab-stops: list 36.0pt"><FONT
face="Times New Roman">Orientation of doublets and inverted antennas
become noncritical. <o:p></o:p></FONT></LI></UL>
<P><FONT face="Times New Roman"><B><SPAN style="COLOR: black">The EW
environment</SPAN></B><SPAN
style="COLOR: black"><o:p></o:p></SPAN></FONT></P>
<P><FONT face="Times New Roman"><B><SPAN style="COLOR: black">ù There is a
lower probability of geolocation.</SPAN></B><SPAN style="COLOR: black">
NVIS energy is received from above at very steep angles, which makes
direction finding (DF) from nearby (but beyond ground-wave range)
locations more difficult. <o:p></o:p></SPAN></FONT></P>
<P><FONT face="Times New Roman"><B><SPAN style="COLOR: black">ù
Communications are harder to jam.</SPAN></B><SPAN style="COLOR: black">
Ground-wave jammers are subject to path loss. Terrain features can be used
to attenuate a ground wave jammer without degrading the desired
communication path. The jamming signal will be attenuated by terrain,
while the sky-wave NVIS path loss will be constant. This will force the
jammer to move very close to the target or put out more power. Either
tactic makes jamming more difficult. <o:p></o:p></SPAN></FONT></P>
<P><FONT face="Times New Roman"><B><SPAN style="COLOR: black">ù Operators
can use low-power successfully.</SPAN></B><SPAN style="COLOR: black"> The
NVIS mode can be used successfully with very low-power HF sets. This will
result in much lower probabilities of intercept/detection (LPI/LPD). <A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180164.gif"><SPAN
style="COLOR: #a8975e">Figures M-14</SPAN></A> and <A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180165.gif"><SPAN
style="COLOR: #a8975e">M-15</SPAN></A> show results obtained in Thailand
jungles and mountains with the 15-watt AN/PRC-74 operating on one SSB
voice frequency (3.6 MHz) over a 24-hour period.
<o:p></o:p></SPAN></FONT></P>
<P><SPAN style="COLOR: black"><FONT face="Times New Roman">15-watt
AN/PRC-74 operating on one SSB voice frequency (3.6 MHz) over a 24-hour
period.<o:p></o:p></FONT></SPAN></P>
<P><FONT face="Times New Roman"><B><SPAN style="COLOR: black">M-8.
Conditions Under Which to Use the NVIS Concept</SPAN></B><SPAN
style="COLOR: black"><o:p></o:p></SPAN></FONT></P>
<P><SPAN style="COLOR: black"><FONT face="Times New Roman">Near-vertical
incidence sky-wave techniques must be considered under the following
conditions: <o:p></o:p></FONT></SPAN></P>
<UL type=disc>
<LI class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; COLOR: black; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto; mso-list: l3 level1 lfo5; tab-stops: list 36.0pt"><FONT
face="Times New Roman">The area of operations is not conducive to
ground-wave HF communications (for example, mountains).
<o:p></o:p></FONT></LI></UL>
<UL type=disc>
<LI class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; COLOR: black; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto; mso-list: l8 level1 lfo6; tab-stops: list 36.0pt"><FONT
face="Times New Roman">Tactical deployment places stations in
anticipated skip zones when using traditional frequency selection
methods and operating procedures. <o:p></o:p></FONT></LI></UL>
<UL type=disc>
<LI class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; COLOR: black; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto; mso-list: l2 level1 lfo7; tab-stops: list 36.0pt"><FONT
face="Times New Roman">When operating in heavy wet jungle (or other
areas of high signal attenuation). <o:p></o:p></FONT></LI></UL>
<UL type=disc>
<LI class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; COLOR: black; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto; mso-list: l1 level1 lfo8; tab-stops: list 36.0pt"><FONT
face="Times New Roman">When prominent terrain features are not under
friendly control. <o:p></o:p></FONT></LI></UL>
<UL type=disc>
<LI class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; COLOR: black; mso-margin-top-alt: auto; mso-margin-bottom-alt: auto; mso-list: l4 level1 lfo9; tab-stops: list 36.0pt"><FONT
face="Times New Roman">When operating against enemy ground-wave jammers
and direction finders. <o:p></o:p></FONT></LI></UL>
<P><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180163.gif"><SPAN
style="COLOR: #a8975e"><FONT face="Times New Roman">Figure
M-13.</FONT></SPAN></A><FONT face="Times New Roman"> Maximum usable
frequencies in Vietnam.<o:p></o:p></FONT></SPAN></P>
<P><!-- [endif] --><!-- [if gte vml 1] --><!-- [endif][if !vml] --><SPAN
style="COLOR: black"><FONT face="Times New Roman"><IMG height=489
src="cid:580473817@22062007-1E03" width=610 align=bottom border=0
v:shapes="Picture53"><o:p></o:p></FONT></SPAN></P>
<P><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180164.gif"><SPAN
style="COLOR: #a8975e"><FONT face="Times New Roman">Figure
M-14.</FONT></SPAN></A><FONT face="Times New Roman"> Communications
success with the AN/PRC-74 as a function of time of day and antenna type
over a 12-mile path in low mountains, spring and summer
1963.<o:p></o:p></FONT></SPAN></P>
<P><!-- [endif] --><!-- [if gte vml 1] --><!-- [endif][if !vml] --><SPAN
style="COLOR: black"><FONT face="Times New Roman"><IMG height=562
src="cid:580473817@22062007-1E0A" width=569 align=bottom border=0
v:shapes="Picture55"><o:p></o:p></FONT></SPAN></P>
<P><SPAN style="COLOR: black"><A
href="http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/24-18/24180165.gif"><SPAN
style="COLOR: #a8975e"><FONT face="Times New Roman">Figure
M-15.</FONT></SPAN></A><FONT face="Times New Roman"> Communications
success as a function of range for the AN/PRC-74 in mountainous and varied
terrain--including jungle in Thailand.<o:p></o:p></FONT></SPAN></P>
<P><!-- [endif] --><!-- [if gte vml 1] --><!-- [endif][if !vml] --><SPAN
style="COLOR: black"><FONT face="Times New Roman"><IMG height=562
src="cid:580473817@22062007-1E11" width=566 align=bottom border=0
v:shapes="Picture57"><o:p></o:p></FONT></SPAN></P></TD></TR>
<TR style="mso-yfti-irow: 2; mso-yfti-lastrow: yes">
<TD
style="BORDER-RIGHT: #ece9d8; PADDING-RIGHT: 0cm; BORDER-TOP: #ece9d8; PADDING-LEFT: 0cm; PADDING-BOTTOM: 0cm; BORDER-LEFT: #ece9d8; PADDING-TOP: 0cm; BORDER-BOTTOM: #ece9d8; BACKGROUND-COLOR: transparent">
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt"><SPAN
style="COLOR: black"><o:p><FONT
face="Times New Roman"> </FONT></o:p></SPAN></P></TD></TR></TBODY></TABLE>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt"><o:p><FONT
face="Times New Roman"> <FONT face=Arial size=2> (Excerpt from US Army
Field Manual 24-18)</FONT></FONT></o:p></P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt"><o:p><FONT face=Arial
size=2></FONT></o:p> </P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt"><o:p>Ionospheric
Map </o:p><o:p><IMG alt="North Atlantic Ionospheric Map"
src="http://www.ips.gov.au/Images/HF%20Systems/North%20Atlantic/Ionospheric%20Map/NorthAtlanticIMap.gif"></o:p></P><o:p>
<P style="TEXT-ALIGN: justify">A feature of the ionosphere is its ability to
reflect radio waves. However, only radio waves within a certain frequency range
will be reflected and this range varies with a number of factors. <BR><BR>The
most widely used instrument for ionospheric measurement is the ionosonde. The
ionosonde is essentially a high frequency radar which sends short pulses of
radio energy into the ionosphere. If the radio frequency is not too high, the
pulses are reflected back to earth. <BR><BR>The ionosonde records the time delay
between transmission and reception of the pulses. By varying the frequency of
the pulses (typically 1-22MHz), a record is obtained of the time delay at
different frequencies. This record is referred to as an ionogram. <BR><BR>The
highest (see note) frequency which the ionosphere will reflect vertically is
called foF2. These foF2 measurements from various sites can be used to create a
map of foF2. The data used to produce the map of the North Atlantic region are
from:
<UL>
<LI><A class=ext1 href="" target=blank>The Space Environment Centre, Boulder
Colorado</A>
<LI><A class=ext1 href="" target=blank>The Instituto Nazionale di Geofisica in
Rome</A>
<LI><A class=ext1 href="" target=blank>The Rutherford Appleton Laboratory
(Ionosondes Group) in England</A>
<LI><A class=ext1 href="" target=blank>The Swedish Institute of Space
Physics</A> </LI></UL>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt">The above map can be used as a
guide to NVIS ionospheric frequency support. <BR><BR>Map is updated at
approximately 40 minutes past the hour. <BR><BR><B>Note: </B>The geomagnetic
field splits a radio wave in the ionosphere into two separate components, termed
the ordinary (o) and extraordinary (x) waves. It is the o-wave which is
routinely scaled from ionograms. <BR><BR>The data presented in this page are
experimental, and are derived from the automated interpretation of regional
ionograms.</P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt"> </P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt"> ( vide em: <A
href="http://www.ips.gov.au/HF_Systems/5/2">http://www.ips.gov.au/HF_Systems/5/2</A> )</P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt"><o:p><FONT face=Arial
size=2></FONT></o:p> </P></o:p><!-- Converted from text/plain format --></DIV></BODY></HTML>