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Monopole antenna

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A typical mast radiator monopole antenna of an AM radio station in Chapel Hill, North Carolina. The mast itself is connected to the transmitter and radiates the radio waves. It is mounted on a ceramic insulator to isolate it from the ground. The other terminal of the transmitter is connected to a ground system consisting of cables buried under the field.

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.[1][2][3] 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 often used as a resonant antenna. The rod functions as an open resonator for radio waves and oscillates with standing waves of voltage and current along its length. The length of the antenna, therefore, is determined based on the wavelength of the desired radio waves. The most common form is the quarter-wave monopole, in which the antenna length is approximately one quarter of the wavelength of the radio waves. In broadcasting monopole antennas, however, lengths equal to ⁠5/8⁠ wavelength are also popular because in a monopole this length maximizes the power radiated perpendicular to the axis of the radiator, which with a vertical radiator optimizes efficiency for terrestrial broadcast. The monopole antenna was invented in 1895 by radio pioneer Guglielmo Marconi; for this reason it is sometimes called the Marconi antenna.[4][5][6]

The load impedance of the quarter-wave monopole is half that of the dipole antenna or 37.5 ohms.

Common types of monopole antenna are

History

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Drawing from Marconi's 1896 patent[7] showing his first monopole antennas, consisting of suspended metal plates (u, w) attached to the transmitter (left) and receiver (right), with the other side grounded (E). Later he found that the plates were unnecessary and a suspended wire was adequate.
Marconi's first monopole transmitter
One of Marconi's early monopole antennas at his Poldhu, Cornwall transmitting station, 1900, consisting of a small metal plate suspended from a wooden arm with a long wire running down to the transmitter in the building.

The monopole antenna was invented in 1895 and patented in 1896[7] by radio pioneer Guglielmo Marconi during his historic 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. He found by experiment that if instead of the dipole, one side of the transmitter and receiver 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,[4][5][6] although Alexander Popov independently invented it at about the same time.[8][9][10][11]

Radiation pattern

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Showing the monopole antenna has the same radiation pattern over perfect ground as a dipole in free space with twice the voltage
Vertical radiation patterns of ideal monopole antennas over a perfect infinite ground. The distance of the line from the origin at a given elevation angle is proportional to the power density radiated at that angle.

Like a vertically suspended dipole antenna, a monopole has an omnidirectional radiation pattern: It radiates with equal power in all azimuthal directions perpendicular to the antenna. 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. Since vertical halfwave dipoles must have their center raised at least a quarter wave above the ground, whereas monopoles must be mounted directly on the ground, the monopoles' radiation patterns are more greatly affected by resistance in the earth, and the radiation pattern with elevation inherently differs.

A monopole can be visualized (right) as being formed by replacing the bottom half of a vertical dipole antenna (c) with a conducting plane (ground 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 (a) reflected from the ground plane will seem to come from an image antenna (b) 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.

Up to a length of a half-wavelength () the antenna has a single lobe with maximum gain in horizontal directions, perpendicular to the antenna axis. Below the quarter wavelength () resonance the radiation pattern is nearly constant with length. Above () the lobe flattens, radiating more power in horizontal directions.

Above a half-wavelength the pattern splits into a horizontal main lobe and a small second conical lobe at an angle of 60° elevation into the sky. However, the horizontal gain keeps increasing and reaches a maximum at a length of five-eighths wavelength: (this is an approximation valid for a typical thickness antenna, for an infinitely thin monopole the maximum occurs at ). The maximum occurs at this length because the opposite phase radiation from the two lobes interferes destructively and cancels at high angles, "compressing" more of the power into the horizontal lobe.

Slightly above the horizontal lobe rapidly gets smaller and the high angle lobe gets larger, reducing power radiated in horizontal directions, and hence reducing gain. Because of this, not many antennas use lengths above or 0.625 wave. As the antenna is made longer, the pattern divides into more lobes, with nulls (directions of zero radiated power) between them.

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 and reduce the gain.[12] The gain of actual quarter wave antennas with typical ground systems is around 2–3 dBi.

Gain and input impedance

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Multi-lobed radiation pattern of a ⁠3/ 2 ⁠ wavelength monopole. Monopole antennas up to ⁠1/ 2 ⁠ wavelength long have a single "lobe", with field strength declining monotonically from a maximum in the horizontal direction, but longer monopoles have more complicated patterns with several conical "lobes" (radiation maxima) directed at angles into the sky.

Because it radiates only into the space above the ground plane, or half the space of a dipole antenna, a monopole antenna over a perfectly conducting infinite ground plane will have a gain of twice (3 dB greater than) the gain of a similar dipole antenna, and a radiation resistance half that of a dipole. Since a half-wave dipole has a gain of 2.19 dBi and a radiation resistance of 73 Ohms, a quarter-wave ( ⁠1/ 4 ⁠ λ ) monopole will have a gain of 2.19 + 3.0 = 5.2 dBi and a radiation resistance of about 36.5 Ohms.[13] The antenna is resonant at this length, so its input impedance is purely resistive. The input impedance has capacitive reactance below ⁠1/ 4 ⁠ λ and inductive reactance from ⁠1/ 4 ⁠ to ⁠1/ 2 ⁠ λ .

The gains given in this section are only achieved if the antenna is mounted over a perfectly conducting infinite ground plane. With typical artificial ground planes smaller than several wavelengths, the gain will be 1 to 3 dBi lower, because some of the horizontal radiated power will diffract around the plane edge into the lower half space, where it dissipates in the soil. Similarly over a resistive earth ground, the gain will be lower due to power absorbed in the earth.

As the length is increased to approach a half-wavelength (⁠1/ 2 ⁠ λ) – the next resonant length – the gain increases some, to 6.0 dBi. Since at this length the antenna has a current node at its feedpoint, the input impedance is very high. A hypothetical infinitesimally thin antenna would have infinite impedance, but for finite thickness of typical monopoles it is around 800–2,000 Ohms; high, but manageable by feeding through a substantial step-up transformer.

The horizontal gain continues to increase up to a maximum of about 6.6 dBi at a length of five-eighths wavelength ⁠5/ 8 ⁠ λ so this is a popular length for ground wave antennas and terrestrial communication antennas, for frequencies where a larger antenna size is feasible. The input impedance drops to about 40 Ohms at that length. The antenna's reactance is capacitive from ⁠1/ 2 ⁠ to ⁠3/ 4 ⁠ λ . However, above ⁠5/ 8 ⁠ λ the horizontal gain drops rapidly because progressively more power is radiated at high elevation angles in the second lobe.

Types

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Main articles: Whip antenna, Umbrella antenna, and Folded unipole antenna
VHF ground plane antenna, a type of monopole antenna used at high frequencies. The three conductors projecting downward are the ground plane.

For monopole antennas operating at lower frequencies, below 20 MHz, the ground plane is usually the Earth; in this case the antenna is a vertical mast mounted on the ground on an insulator to isolate it electrically from the ground. One side of the feedline is connected to the mast and the other to an Earth ground at the base of the antenna. In transmitting antennas to reduce ground resistance this is often a radial network of buried wires stretching outward from a terminal near the base of the antenna. This design is used for the mast radiator transmitting antennas employed for radio broadcasting in the MF and LF bands. At lower frequencies the antenna mast is electrically short giving it a very small radiation resistance, so to increase efficiency and radiated power capacitively toploaded monopoles such as the T-antenna and umbrella antenna are used.

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.[14] A common type of monopole antenna at these frequencies for mounting on masts or structures consists of a quarter-wave whip antenna with a ground plane consisting of 3 or 4 wires or rods a quarter-wave long radiating horizontally or diagonally from its base connected to the ground side of the feedline; 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,[14] and aircraft communication antennas frequently consist of a short conductor in an aerodynamic fairing projecting from the fuselage; this is called a blade antenna.[13]

The quarter-wave whip and rubber ducky antennas used with handheld radios such as walkie-talkies and portable FM radios are also monopole antennas. In these portable devices the antenna does not have an effective ground plane, the ground side of the transmitter is just connected to the ground connection on its circuit board. Since the circuit board ground is often smaller than the antenna, the antenna and ground combination may function more as an asymmetrical dipole antenna than a monopole. The hand and body of the person holding them may function as a rudimentary ground plane.

Wireless devices and cell phones use a monopole variant called the inverted-F antenna.[15] The monopole element is bent over parallel to the ground area on the circuit board, so it can be enclosed in the device case; usually the antenna is fabricated of copper foil on the printed circuit board itself.[15][16] This geometry would give the antenna a very low impedance if it was driven at the base. To improve the impedance match with the feed circuit (typically 50 Ohms impedance) the antenna is shunt fed, the feedline is instead connected to an intermediate point along the element, and the element end is grounded.

See also

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References

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  1. ^ Poisel, Richard (2012). Antenna Systems and Electronic Warfare Applications. Artech House. p. 223. ISBN 9781608074846 – via Google Books.
  2. ^ Bevelacqua, Peter J. (2016). "The Monopole Antenna". Antenna Types. Antenna-Theory.com website. Retrieved 20 August 2020.
  3. ^ Straw, R. Dean; et al., eds. (2000). The ARRL Antenna Book (19th ed.). American Radio Relay League. p. 2.17. ISBN 9780872598041 – via Google Books.
  4. ^ a b Das, Sisir K. (2016). Antenna and Wave Propagation. Tata McGraw-Hill Education. p. 116. ISBN 978-1259006326 – via Google Books.
  5. ^ a b Wong, K. Daniel (2011). Fundamentals of Wireless Communication Engineering Technologies. John Wiley and Sons. p. 94. ISBN 978-1118121092 – via Google Books.
  6. ^ a b Kishore, Kamal (2009). Antenna and Wave Propagation. IK International Ltd. p. 93. ISBN 978-9380026060 – via Google Books.
  7. ^ a b US patent 586193, Guglielmo Marconi Transmitting electrical signals, filed December 7, 1896, granted July 13, 1897
  8. ^ Visser, Hubregt J. (2006). Array and Phased Array Antenna Basics. John Wiley and Sons. p. 31. ISBN 0470871180 – via Google Books.
  9. ^ Howeth, L.S. (1963). The History of Communications - Electronics in the U.S. Navy. U.S. Navy. p. 19.
  10. ^ Meinel, Christoph; Sack, Harald (2014). Digital Communication: Communication, Multimedia, Security. Springer Science and Business Media. p. 55. ISBN 978-3642543319.
  11. ^ Stutzman, Warren L.; Thiele, Gary A. (2012). Antenna Theory and Design. John Wiley and Sons. p. 8. ISBN 978-0470576649 – via Google Books.
  12. ^ Weiner, Melvin M. (2003). Monopole Antennas. Boca Raton, FL: CRC Press. p. vi. ISBN 0-8247-4844-1 – via Google Books.
  13. ^ a b Macnamara, Thereza (2010). Introduction to Antenna Placement and Installation. John Wiley and Sons. p. 145. ISBN 978-0-470-01981-8 – via Google Books.
  14. ^ a b Kissick, W. A. (April 2001). Antenna System Guide. U.S. National Institute of Standards and Technology (NIST) for National Institute of Justice, US Dept. of Justice. pp. 17–19. NIJ Guide 202-00 – via Google Books.
  15. ^ a b Chen, Zhi Ning; Chia, Michael Yan Wah (2006). Broadband Planar Antennas: Design and applications. John Wiley and Sons. pp. 135–138. ISBN 9780470871751 – via Google Books.
  16. ^ Bevelacqua, Peter J. (2016). "Inverted F Antenna". antenna-theory.com website. Antenna Tutorial. Retrieved 8 June 2021.
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