A monopole antenna is a class of radio antenna consisting of a straight
rod-shaped conductor, often mounted perpendicularly over some type of
conductive surface, called a ground plane. The driving signal from the
transmitter is applied, or for receiving antennas the output signal to the
receiver is taken, between the lower end of the monopole and the ground plane.
One side of the antenna feedline is attached to the lower end of the
monopole, and the other side is attached to the ground plane, which is often the
Earth. This contrasts with a dipole antenna which consists of two identical
rod conductors, with the signal from the transmitter applied between the two
halves of the antenna. The monopole is a resonant antenna; the
rod functions as an open resonator for radio waves, oscillating with standing
waves of voltage and current along its length. Therefore the length of the
antenna is determined by the wavelength of the radio waves it is used with. The
most common or fundamental form is the quarter-wave monopole, in which the
antenna is approximately 1/4 of a wavelength of the radio waves. The
monopole antenna was invented in 1895 by radio pioneer Guglielmo Marconi, for
this reason it is sometimes called a Marconi antenna. Common types of
monopole antenna are the whip, rubber ducky, helical, random wire, umbrella,
inverted-L and T-antenna, inverted-F, mast radiator, and ground plane
antennas. History
The monopole antenna was invented in 1895 by radio pioneer Guglielmo Marconi
during his historic first experiments in radio communication. He began by using
Hertzian dipole antennas consisting of two identical horizontal wires ending in
metal plates. He found by experiment that if instead of the dipole, one side
of the transmitter was connected to a wire suspended overhead, and the other
side was connected to the Earth, he could transmit for longer distances. For
this reason the monopole is also called a Marconi antenna, although Alexander
Popov independently invented it at about the same time. Common types of monopole
antenna are the whip, rubber ducky, helical, random wire, umbrella,
inverted-L and T-antenna, mast radiator, and ground plane antennas.
A definition of a Marconi antenna is an antenna which uses a ground plane as
part of its resonant circuit. A Hertzian antenna is one that does not require a
ground to oscillate, such as a half-wave.
Radiation pattern Like a dipole antenna, a monopole has an
omnidirectional radiation pattern. That is it radiates equal power in all
azimuthal directions perpendicular to the antenna, but the radiated power
varies with elevation angle, with the radiation dropping off to zero at the
zenith, on the antenna axis. It radiates vertically polarized radio waves.
A monopole can be visualized as being formed by replacing the bottom half of a
vertical dipole antenna with a conducting plane at right-angles to the
remaining half. If the ground plane is large enough, the radio waves from the
remaining upper half of the dipole reflected from the ground plane will
seem to come from an image antenna forming the missing half of the dipole,
which adds to the direct radiation to form a dipole radiation pattern. So the
pattern of a monopole with a perfectly conducting, infinite ground plane is
identical to the top half of a dipole pattern, with its maximum radiation in
the horizontal direction, perpendicular to the antenna. Because it radiates only
into the space above the ground plane, or half the space of a dipole antenna, a
monopole antenna will have a gain of twice the gain of a similar dipole
antenna, and a radiation resistance half that of a dipole. Thus a quarter-wave
monopole, the most common type, will have a gain of 5.19 dBi and a radiation
resistance of about 36.8 ohms if it is mounted above a good ground plane.
The general effect of electrically small ground planes, as well as imperfectly
conducting earth grounds, is to tilt the direction of maximum radiation up to
higher elevation angles. Types
The ground plane used with a monopole may be the actual earth; in this case
the antenna is mounted on the ground and one side of the feedline is connected to
an earth ground at the base of the antenna. This design is used for the
mast radiator antennas employed in radio broadcasting at low frequencies, as well
as other low frequency antennas such as the T-antenna and umbrella antenna. At
VHF and UHF frequencies the size of the ground plane needed is smaller, so
artificial ground planes are used to allow the antenna to be mounted above
the ground. A common type of monopole antenna at these frequencies consists of
a quarter-wave whip antenna with a ground plane consisting of several wires
or rods radiating horizontally or diagonally from its base; this is called
a ground-plane antenna. At gigahertz frequencies the metal surface of a car
roof or airplane body makes a good ground plane, so car cell phone antennas
consist of short whips mounted on the roof, and aircraft communication
antennas frequently consist of a short conductor in an aerodynamic fairing
projecting from the fuselage; this is called a blade antenna. The most common
antenna used in mobile phones is the inverted-F antenna, which is a variant
of the inverted-L monopole. Bending over the antenna saves space and keeps the it
within the bounds of the mobile's case but the antenna then has a very low
impedance. To improve the match the antenna is not fed from the end, rather
some intermediate point, and the end is grounded instead. The quarter-wave whip
and "Rubber Ducky" antennas used with handheld radios such as walkie-talkies
and cell phones are also monopole antennas. These don't use a ground
plane, and the ground side of the transmitter is just connected to the
ground connection on its circuit board. The hand and body of the person holding
them may function as a rudimentary ground plane.
Monopole broadcasting antennas When used for radio broadcasting, the
radio frequency power from the broadcasting transmitter is fed across
the base insulator between the tower and a ground system. The ideal ground system
for AM broadcasters comprises at least 120 buried copper or phosphor bronze
radial wires at least one-quarter wavelength long and a ground-screen in
the immediate vicinity of the tower. All the ground system components are bonded
together, usually by welding, brazing or using coin silver solder to help reduce
corrosion. Monopole antennas that use guy-wires for support are called masts
in some countries. In the United States, the term “mast” is generally used to
describe a pipe supporting a smaller antenna, so both self-supporting and
guy-wire supported radio antennas are simply called monopoles if they stand
alone. If multiple monopole antennas are used in order to control the direction
of radio frequency propagation, they are called directional antenna arrays.
In the United States, the Federal Communications Commission requires that
the transmitter power input to the antenna be measured and maintained. The
power input is calculated as the square of the measured current, , flowing into
the antenna from the transmission line multiplied by the real part of the
antenna's feed-point impedance, . This impedance is periodically measured
to verify the stability of the antenna and ground system. Normally, an
impedance matching network matches the impedance of the antenna to the
impedance of the transmission line feeding it.
Examples of monopole antennas are: the whip antenna
the mast radiator Monopole antennas have become one of the
components of mobile and internet networks across the globe. Their
relative low cost and fast installation makes them an obvious choice for
developing countries. See also
Dual-band blade antenna Cellular repeater
Signal strength Folded unipole antenna
Electrical lengthening External links
Radiation of a quarter wavelength monopole
References