World Library  
Flag as Inappropriate
Email this Article

Monopole antenna

Article Id: WHEBN0002874126
Reproduction Date:

Title: Monopole antenna  
Author: World Heritage Encyclopedia
Language: English
Subject: T-antenna, Dipole antenna, Electrical length, Whip antenna, Helical antenna
Collection: Antennas (Radio), Radio Frequency Antenna Types
Publisher: World Heritage Encyclopedia
Publication
Date:
 

Monopole antenna

Mast radiator monopole antenna used for broadcasting. AM radio station WARE, Warren, Massachusetts, USA.

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.

Contents

  • History 1
  • Radiation pattern 2
  • Types 3
  • Monopole broadcasting antennas 4
  • See also 5
  • External links 6
  • References 7

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.

Showing the monopole antenna has the same radiation pattern over perfect ground as a dipole in free space with twice the voltage

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, 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 (3 dB over) 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.[1]

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.[2]

Types

Radiation pattern of 3/2 wavelength monopole. Monopole antennas up to 1/4 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.

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.[1] 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.

VHF ground plane antenna, a type of monopole antenna used at high frequencies. The three conductors projecting downward are the 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 (RF) propagation, they are called directional antenna arrays.

In the United States, the Federal Communications Commission (FCC) 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, i, flowing into the antenna from the transmission line multiplied by the real part of the antenna's feed-point impedance, r.

P = i ^2 r

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:

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

External links

  • Radiation of a quarter wavelength monopole

References

  1. ^ a b Macnamara, Thereza (2010). Introduction to Antenna Placement and Installation. USA: John Wiley and Sons. p. 145.  
  2. ^ Weiner, Melvin M. Weiner (2003). Monopole antennas. USA: CRC Press. pp. vi.  
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 


Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.