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Chapter 3

Antennas

One of the most important considerations when operating a radio is the type
of antenna to be used. For good communications with a radio operating in the HF
range (2.000 kHz to 29.999 MHz), you must consider the--

Type of antenna.

Operating frequency.

Terrain around the radio site.

Time of day.

Location of and distance between radios.

Atmospheric conditions.

The operator can sometimes control the first four or five factors. The
antenna and frequency are the most important considerations under his control.
Both should be selected to suit the distance between the radios and the
propagation characteristics. The operator will most likely have two or three
different frequencies assigned for the operation or exercise. These will be
found in the SOI under the net in which he is operating.

Section I. Antenna Selection

a. The field environment, tactical situation, and distance between radio
sites determine the type of antenna used. If the radio set is used while on the
move, the whip antenna supplied with the equipment is normally used. The whip
antenna, using the ground wave, is satisfactory for most short-range missions.

b. If the tactical situation permits, a simple half-wave dipole antenna
(doublet) or the NVIS antenna is used to extend the range of the radio by using
the skip phenomenon. Skip means the radio waves are bounced off the ionosphere
and back to earth giving coverage of 300 miles or more. Figure
3-1 shows the ground wave and sky wave using the skip phenomenon. The NVIS
can be used at frequencies above 12 MHz but automatic tuning of the radio
(AN/PRC-104A and/or AN/GRC-213) may not be possible at all frequencies.

c. When using an antenna with directional characteristics, orient the antenna
so that it is most sensitive in the direction of the other station(s). Figure
3-2 shows the radiation pattern of the antennas.

d. Standard and
optional antennas that can be used with IHFRs are listed below.

WHIP ANTENNA

Omnidirectional (360-degree radiation pattern).

Easily and quickly assembled and erected.

Lightweight and easy to carry.

Limited range (10 miles or less) over land.

AS-2259/GR (NVIS)

Omnidirectional.

Requires large clear area (80 feet square) for proper erection.

Long range (0 to 300 miles).

HALF-WAVE DIPOLE DOUBLET

Bidirectional (broadside to wire).

Good portability.

Quickly assembled and erected. Requires two or more vertical supports
(trees, poles).

Extended range (to 300 miles and beyond).

QUARTER-WAVE SLANT WIRE

Bidirectional (broadside to wire).

Good portability.

Quickly assembled and erected. Requires only one vertical support (tree
or pole).

Range up to 1,000 miles.

NOTE: These are a few of the antennas that can be used.

Section II. Whip Antenna

a. When using a quarter-wave or whip antenna, ground the antenna to increase
its effectiveness. Using this characteristic of the ground, an antenna a
quarter-wavelength long can be made into the equivalent of a half-wave antenna.
If such an antenna is erected vertically and its lower end is grounded, the
ground takes the place of the missing quarter-wavelength, and the reflections
supply that part of the radiated energy normally supplied by the lower half of
an ungrounded half-wave antenna.

b. The antenna is grounded by grounding the vehicle itself. Use a ground
stake at least 4 feet long, a hammer, and a ground strap. You may substitute a
steel reinforcing rod, a steel fence poet, or a metal water or gas pipe cut to
the right length for the ground stake. Ensure paint and rust are removed from
such items. Substitutes for the ground strap include battery cables or any heavy
gauge wire. Do not use Field Wire WD-1 because it does not provide a suitable
ground. Drive the ground stake into the ground until the top of the stake is 2
to 4 inches above the ground. Attach one end of the ground strap to the stake
and the other end to the vehicle body. Ensure all paint and rust are removed
from connecting point of the vehicle body to allow a good metal-to-metal
contact.

c. When a whip antenna is mounted on a vehicle, the metal of the vehicle will
affect the operation of the antenna. As a result, the direction in which the
vehicle is facing may also affect transmission and reception, particularly of
distant or weak signals. A vehicle with a whip antenna mounted on its left rear
side transmits its strongest signal in a line running diagonally from the
antenna through the right front side of the vehicle. Similarly, an antenna
mounted on the right rear side of the vehicle radiates its strongest signal in a
direction diagonally toward the left front side. In some cases, the best
direction can be determined by driving the vehicle in a small circle until the
best reception is obtained.

Section III. Antenna RC-292

Antenna RC-292 is used to extend the distance range of the old and new
generation of FM field radio sets. The antenna consists of one vertical
radiating element and three ground plane elements. The lengths of these elements
are determined by the operating frequency of the radio set. Refer to the antenna
element selection chart at Table
3-1. The antenna is elevated on a 30-foot sectional mast which in turn is
held erect by guy ropes and stakes. When the operating frequency is changed,
check the antenna element selection chart. If the new frequency requires a
change in element length, lower the antenna and add or subtract the required
number of elements.

Section IV. Antenna Group OE-254/GRC

Description

Antenna Group OE-254/GRC is used to extend the range of the old and new
generation of FM field radio sets. The antenna consists of three upward and
three downward extended radials. These radials remain the same length for all
frequencies from 30 to 88 MHz. The antenna is elevated on a 30-foot sectional
mast held erect with guy ropes and stakes.

NOTE: No change needs to be made in the number of radials when a change of
frequency is necessary.

Antenna Group OE-254/GRC

Installation

Section V. Doublet Antenna

Determining Doublet Antenna Length

Section V. Doublet Antenna

Determining Doublet Antenna Height

The height of the doublet antenna above the ground determines the radiation
pattern, or take-off angle. The radiation pattern for a distance of 0 to 250
miles should be straight up. To determine the height of your doublet antenna,
use OTF. For daytime uses of 4.8 to 9.7 MHz for a distance of 100 miles, use the
formula for a quarter-wave. For nighttime uses of the same frequency of 4.8 to
9.7 MHz, use the formula for a tenth of a wave.

Doublet Antenna

Orientation

Doublet Antenna

Installation Using AB-155A/U

Figure
3-15 shows correct installation of a doublet antenna using mast assembly
AB-155A/U. Pay particular attention to mast height. For maximum effectiveness,
erect a doublet antenna at least a quarter-wavelength off the ground. Forty-foot
masts provide adequate height in most instances.

Section VI. NVIS Antenna AS-2259/GR

Description

a. The NVIS Antenna AS-2259/GR is a lightweight, sloping dipole,
omnidirectional antenna. It is designed to be used with an AM radio operating in
the HF range of 2 to 30 MHz. It provides high angle radiation (near vertical
incidence) to permit short-range sky wave propagation over communications
circuits varying from 0 to 300 miles. It can be used with tactical HF radio
communications equipment that tunes a 15-foot whip antenna. Examples of such
equipment are the AN/GRC-106 and the IHFRs (AN/PRC-104A and AN/GRC-213/193A). Figure
3-16 shows an operational AS2259/GR NVIS.

b. An adapter MX-10618/GRC-193A is used to interface the vehicle's whip
antenna base with the AS-2259/GR. Do not use the adapter MX-9313/GR provided
with some of the earlier antenna kits with the AN/PRC-104A or AN/GRC-213/193A.
It is used to interface radios with greater output power (AN/MRC83/87). The
antenna weighs about 14.7 pounds and can be erected by two men in about 5
minutes. The antenna is polarized horizontally and vertically at the same time,
radiating RF energy in all directions at the same time. It consists of eight
lightweight mast sections that function as the antenna coax feed line and four
radiating elements that also serve as guys supporting the mast.

NVIS Antenna AS-2259/GR

Installation

Section VII. Field Expedients for Antennas

a. Antennas are sometimes broken or damaged resulting in failed or poor
communications. If a spare is available, replace the damaged antenna. When there
is no spare, fabricate an emergency antenna. For more information on how to
repair and fabricate antennas refer to FM
24-18. The following suggest ways to construct an emergency antenna.

(1) The best wire for antennas is copper or aluminum. In an emergency,
however, use any wire available. WD-1/TT is suitable for this purpose.

(2) The exact length of many antennas is critical. Therefore, the length of
the emergency antenna should be the same as the length of the antenna it
replaces.

(3) Antennas supported by trees can usually survive heavy windstorms if the
trunk of a tree or a stout limb is used for support. To keep the antenna taut
and to prevent it from breaking or stretching, attach a spring or a strip of old
inner tube to one end of the antenna, or pass a rope through a pulley or eye
hook, attach the rope to the end of the antenna, and load the rope with a heavy
weight to keep the antenna taut.

(4) Guys used to hold antenna supports are made of rope or wire. To ensure
that wire guys will not affect the operation of the antenna, cut the wire into
several short lengths and connect the pieces with insulators. Small pieces of
dry wood, bottles, or even suitably shaped stones may be used.

b. An improvised antenna may change the performance of a radio set. Use
either of the two following expedient methods to determine whether the
improvised antenna is operating properly.

(1) The distant receiver may be used to test the antenna. If the signal
received from a station is strong, the antenna is operating satisfactorily. If
the signal is weak, adjust the height and length of the antenna and transmission
line to receive the strongest signal at a given setting of the volume control of
the receiver. If your set is equipped with a power or SWR meter, use this device
to adjust your antenna.

(2) In some radio sets, the transmitter is used to adjust the antenna. First,
set the controls of the transmitter in the proper position for normal operation;
then, tune the system by adjusting the antenna height, length, and the
transmission line length to obtain the best transmission output.

c. When a whip antenna is broken into two sections, the portion of the
antenna that is broken off can be connected to the portion attached to the base
fitting by joining the sections together. When both parts of the broken whip are
available and usable, connect the two broken ends together and wrap with wire,
ensuring that wrapping is clean and tight. Lash pole or branch to antenna until
antenna will stand alone. When the portion of whip that is broken is missing or
unusable, add a piece of wire that is nearly the same length as the broken
section. Then, lash a pole the length of the antenna securely to the base
section of the antenna and tie wire to the top of pole. If possible, solder the
connections.

d. Emergency repair of wire antennas can be grouped into two categories:
repair or replacement of the wire used as an antenna or transmission line; and
repair or replacement of the assembly used to support the antenna wires.

(1) When one or more wires on an antenna are broken, the antenna can be
repaired by reconnecting the broken wires. To do this, lower the antenna to the
ground, clean the surface of the wire, and twist the wires together. Whenever
possible, solder the connections.

(2) If the antenna is damaged beyond repair, substitute another antenna. If
antenna sections are not available, WD-1 (field wire) can be used as a
substitute. Ensure the length of the wire is the same length as the original
antenna and you have a good wire to antenna contact.

e. Building a good field-expedient vertical half-rhombic antenna calls for a
good resistor, not a dead BA-30 battery or a C-rat can full of sand and oil.
Plan ahead. If you can, lay in a supply of the 600-ohm 2-watt resistors. Since
600-ohms is not a standard resistor value, you have a choice. Get a 620-ohm,
2-watter with NSN 5905-00-407-6167, or wire 2 1200-ohm, l-watt resistors in
parallel. Get them with NSN 5905-00-369-6916. Then, follow the directions on how
to fabricate antennas. Put the resistor on the end nearest your receiving
station. Remember, resistor wattage must beat least half of the radio's output
wattage. (See Figure
3-21.)

Section VIII. Grounding Systems