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 LF and VLF 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.
Large monopoles are the main transmitting antennas used in the lower frequencies below 3 MHz, the MF, LF, and VLF 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 AM radio broadcasting antennas in the MF and LF bands. Another variant is the folded unipole antenna. At lower frequencies in the LF and VLF 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.
At higher frequencies in the VHF and UHF 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 MHz 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 FM radios in the VHF and UHF 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 (IFA). 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.
History
The monopole antenna was invented in 1895 and patented in 1896British patent 189612039 Marconi, Guglielmo Improvements in transmitting electrical impulses and signals, and in apparatus therefore, Applied: 2 June 1896, full specification: 2 March 1897, granted: 2 July 1897. British patents allowed the full specification to be submitted after the application. Marconi's monopole antenna did not appear in his initial June 1896 application but in his March 1897 specification. Corresponding US patent 586193, Marconi, Guglielmo, Transmitting electrical signals, filed 7 December 1896, granted: 13 July 1897 by radio entrepreneur Guglielmo Marconi during his first experiments in radio communication. He began by using dipole antennas invented by Heinrich Hertz consisting of two identical horizontal wires ending in metal plates, and by his mentor Augusto Righi consisting of four metal spark balls, but was unable to transmit further than about a half mile. He found by experiment that if instead of the dipole, one side of the transmitter and receiver terminals was connected to a wire attached to a metal plate suspended overhead, and the other side was connected to a conductor buried in the Earth, he could transmit for longer distances. discusses the reason Marconi, a self-educated tinkerer, made this discovery, and not the many educated physicists and professors who were also experimenting with wireless. This is generally attributed to the influence on physics of James Clerk Maxwell's 1873 electromagnetic theory, that proved that both light and radio waves were the same phenomenon; electromagnetic waves. Maxwell's theory conditioned physicists to think of radio waves as "invisible light". Thus they assumed that radio waves traveled in straight lines like light and were limited by the horizon. Therefore they believed radio systems could transmit no further than existing optical blinker light or semaphore communication systems, and so were useless for long distance communication. Marconi, an experimentalist free from this bias, thought of radio more as a telegraph without wires, and therefore he tried the telegraphy technique of using the ground as one side of the circuit. He found the plate was unnecessary and a suspended wire was adequate. The monopole is also called a Marconi antenna, although Alexander Popov independently invented it at about the same time for his lightning detection receiver.
In the next few years, using higher monopole antennas and better transmitters and receivers Marconi steadily increased the range of his radiotelegraphy communication system to hundreds of kilometers (miles), convincing the world that radio was a practical communication method. In 1901 he achieved transatlantic radio transmission using a monopole transmitting antenna consisting of 50 vertical wires suspended in a fan shape from a support cable between 60 meter (200 foot) poles.
Before Marconi, several inventors experimented with wireless communication between vertical aerials, although without creating a practical system. In October 1866 Mahlon Loomis demonstrated communication between two grounded wire aerials supported by kites on mountaintops apart. When one aerial wire was touched to a grounded contact, currents of atmospheric electricity in it apparently generated radio waves which induced currents in the other wire, detected by a sensitive galvanometer. Starting in 1882, Amos Dolbear also used grounded vertical wire antennas during his development of a ground conduction telephone, but his system seems to have worked by electrostatic induction instead of radio waves, and by 1895 he had only achieved distances of 1/4 mile. A suit claiming Marconi infringed Dolbear's 1882 and 1886 wireless patents was dismissed in 1901. In 1885 Thomas Edison patented a system of harbor communication between vertical towers on shore and vertical wires suspended from a ship's mast, but this also worked by electrostatic induction and was never tried.
In the primitive spark transmitters used in Marconi's time, in addition to radiating the radio waves the antenna also served as the resonator which generated the oscillating currents which determine the frequency and thus the wavelength of the waves. Marconi's new antenna functioned as a quarter-wave monopole which radiated with a wavelength of approximately four times its height. This longer antenna greatly increased the wavelength, reducing the frequency of Marconi’s transmitter from the VHF and UHF bands generated by Hertz's antennas which could not transmit beyond the horizon, to the MF band. Also, it emitted vertically polarized radio waves, instead of the horizontally polarized waves produced by the Hertz antenna. Longer radio waves have less attenuation with distance. These longer vertically polarized waves could propagate as ground waves which can follow the curvature of the Earth, and could also reflect off the ionosphere (called the 'skip' or skywave mechanism), and thus travel beyond the visual horizon. This explains the increased range.