Monopole antenna

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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 current from the transmitter is applied, or for receiving antennas the output signal voltage to the receiver is taken, between the monopole and the ground plane. One side of the feedline to the transmitter or receiver is connected to the lower end of the monopole element, and the other side is connected to the ground plane, which may be the Earth. This contrasts with a dipole antenna which consists of two identical rod conductors, with the current from the transmitter applied between the two halves of the antenna. The monopole antenna is related mathematically to the dipole.

The vertical monopole is an omnidirectional antenna with a low gain of 2 - 5 dBi, and radiates most of its power in horizontal directions or low elevation angles. Common types of monopole antenna are the whip, rubber ducky, umbrella, inverted-L and T-antenna, inverted-F, folded unipole antenna, mast radiator, and ground plane antennas.

The monopole is usually used as 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 form is the quarter-wave monopole, in which the antenna is approximately one quarter of the wavelength of the radio waves.

It is said to be the most widely used antenna in the world. Monopoles shorter than one-quarter wavelength, called electrically short monopoles, are also widely used since they are more compact. Monopoles five-eights (5/8 = 0.625) of a wavelength long are also common, because at this length a monopole radiates a maximum amount of its power in horizontal directions. A capacitively loaded or top-loaded monopole is a monopole antenna with horizontal conductors such as wires or screens insulated from ground attached to the top of the monopole element, to increase radiated power.

Large top-loaded monopoles, the T and inverted L antennas and umbrella antenna are used as transmitting antennas at longer wavelengths, in the L.F. and V.L.F. bands.

The monopole antenna was invented in 1895 by radio pioneer Guglielmo Marconi; for this reason it is also called the Marconi antenna although Alexander Popov independently invented it at about the same time.

Types and uses

Due to their omnidirectional radiation pattern, vertical monopole antennas are commonly used in terrestrial radio communication systems in which the direction to the transmitter or receiver is unknown or constantly changing, such as radio broadcasting, mobile two-way radios, beacons, and wireless devices like cellphones and Wi-Fi networks, because they radiate equal radio power in all horizontal directions but little power up into the sky where it would be wasted. The quarter-wave monopole is the smallest antenna that is resonant, making it an efficient radiator; it is said to be the most widely used antenna in the world.

- Lower frequency monopole antennas

Large monopoles are the main transmitting antennas used in the lower frequencies below 3 megahertz, the M.F., L.F., and V.L.F. bands, because the radio propagation mode used in these bands, ground waves, requires a vertically polarized antenna with good horizontal radiation characteristics. At these frequencies, the Earth itself is used as the antenna's ground plane. The most common antenna is the mast radiator, a vertical mast mounted on the ground but insulated from it electrically. ranging from about one-sixth to five-eighths wavelengths tall. One side of the feedline from the transmitter is connected to the conductive metal mast which serves as the radiating element, and the other to an Earth ground connection") consisting of a radial network of buried wires stretching outward from a terminal at the base of the antenna. This design is used for A.M. radio broadcasting antennas in the M.F. and L.F. bands. Another variant is the folded unipole antenna. At lower frequencies in the L.F. and V.L.F. band, the tallest antenna masts that can be practically constructed are electrically short, significantly shorter than one-quarter wavelength. Simple monopoles this short are inefficient due to their very low radiation resistance, so to increase efficiency and radiated power, capacitively top-loaded monopoles such as the inverted-L, T antenna and umbrella antenna are used.

In the shortwave bands variations such as the folded monopole, J-pole antenna, and normal mode helical are used.

- Higher frequency monopole antennas

- !Rubber ducky antenna on walkie-talkie, 144 megahertz

- !Cell phone U.H.F. whip antenna on car

- !3 fiberglass half-wave whip antennas

- U.S. Navy broadband conical monopole antenna

- !V.H.F. ground plane antenna

- !Dual band 2.4 and 5 gigahertz monopole antenna on a home Wi-Fi router

At higher frequencies in the V.H.F. and U.H.F. bands, the size of the ground plane needed is smaller, so artificial metal ground planes of screen or rods are used to allow the antenna to be mounted above the ground. A common type for mounting on masts or stationary structures is the ground plane antenna, consisting of a quarter-wave whip antenna with a ground plane of 3 or 4 wires or rods a quarter-wavelength long radiating horizontally or diagonally from its base, connected to the ground side of the feedline. Another variation is the discone antenna, which is notable for having a very broad bandwidth"). At frequencies above 30 megahertz an automobile or aircraft body makes an adequate ground plane, so whip antennas for two-way radios and cell phones are mounted on car bumpers or roofs, 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 quarter-wave whip and rubber ducky antennas used with handheld radios such as walkie-talkies and portable F.M. radios in the V.H.F. and U.H.F. bands are also monopole antennas. In these portable devices the antenna does not have an effective ground plane, the ground side of the transmitter or receiver is just connected to the chassis ground connection on its circuit board. Since these "ground" conductors are no larger than the element itself the antenna usually functions more like an asymmetrical dipole than a monopole antenna.

A monopole type widely used in wireless devices and cell phones operating at microwave frequencies is the inverted F antenna I.F.A. The monopole element is bent over in an L shape parallel to the ground area on the circuit board, to make it compact enough to be enclosed in the device case; the antenna may be fabricated of copper foil on the printed circuit board itself. To improve the impedance match with the feed circuit the antenna is shunt fed, the feedline is connected to an intermediate point along the element, and the base of the element is grounded. Many variants of this antenna are used in handheld devices, such as multiband versions and meander antennas.