Monopole antenna
Encyclopedia
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...
surface, called a ground plane
Ground plane
In electrical engineering, a ground plane is an electrically conductive surface.-Radio antenna theory :In telecommunication, a ground plane structure or relationship exists between the antenna and another object, where the only structure of the object is a structure which permits the antenna to...
. The driving signal from the transmitter is applied, or for receiving antennas the output voltage 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
Dipole antenna
A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...
which consists of two identical rod conductors, with the signal from the transmitter applied between the two halves of the antenna. Common types of monopole antenna are the whip
Whip antenna
A whip antenna is an antenna consisting of a single straight flexible wire or rod, often mounted above some type of conducting surface called a ground plane. The bottom end of the whip is connected to the radio receiver or transmitter. They are designed to be flexible so that they won't break...
, rubber ducky
Rubber Ducky antenna
The Rubber Ducky antenna is an electrically short monopole antenna which functions somewhat like a base-loaded whip antenna and is sealed in a rubber or plastic jacket to protect the antenna...
, helical
Helical antenna
A helical antenna is an antenna consisting of a conducting wire wound in the form of a helix. In most cases, helical antennas are mounted over a ground plane. The feed line is connected between the bottom of the helix and the ground plane...
, random wire
Random wire antenna
A random wire antenna is a radio frequency antenna consisting of a wire whose length does not bear a relation to the wavelength of the radio waves used, but is typically chosen more for convenience. This type of antenna sometimes is called the zig-zag antenna, as it may be strung back and forth...
, mast radiator
Mast radiator
A mast radiator is a radio mast or tower in which the whole structure itself functions as an antenna. This design is commonly used for transmitting antennas operating at low frequencies, in the VLF, LF and MF ranges, in particular those used for AM broadcasting. The metal mast is electrically...
, and ground plane antennas.
Radiation pattern
Like a dipole antennaDipole antenna
A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...
, a monopole has an omnidirectional
Omnidirectional antenna
In radio communication, an omnidirectional antenna is an antenna which radiates radio wave power uniformly in all directions in one plane, with the radiated power decreasing with elevation angle above or below the plane, dropping to zero on the antenna's axis. This radiation pattern is often...
radiation pattern
Radiation pattern
In the field of antenna design the term radiation pattern most commonly refers to the directional dependence of the strength of the radio waves from the antenna or other source ....
. 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.
A monopole can be visualized as being formed by replacing one half of a dipole antenna
Dipole antenna
A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...
with a ground plane
Ground plane
In electrical engineering, a ground plane is an electrically conductive surface.-Radio antenna theory :In telecommunication, a ground plane structure or relationship exists between the antenna and another object, where the only structure of the object is a structure which permits the antenna to...
at right-angles to the remaining half. If the ground plane is large enough, the radio waves reflected from the ground plane will seem to come from an image antenna
Image antenna
In telecommunications and antenna design, an image antenna is an electrical mirror-image of an antenna element formed by the radio waves reflecting from a conductive surface called a ground plane, such as the surface of the earth...
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
Antenna gain
In electromagnetics, an antenna's power gain or simply gain is a key performance figure which combines the antenna's directivity and electrical efficiency. As a transmitting antenna, the figure describes how well the antenna converts input power into radio waves headed in a specified direction...
of twice (3 dBi
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
over) the gain of a similar dipole antenna, and a radiation resistance
Radiation resistance
Radiation resistance is that part of an antenna's feedpoint resistance that is caused by the radiation of electromagnetic waves from the antenna. The radiation resistance is determined by the geometry of the antenna, not by the materials of which it is made...
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, such as used in ground plane antennas, 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 groundGround (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
at the base of the antenna. This design is used for the mast radiator
Mast radiator
A mast radiator is a radio mast or tower in which the whole structure itself functions as an antenna. This design is commonly used for transmitting antennas operating at low frequencies, in the VLF, LF and MF ranges, in particular those used for AM broadcasting. The metal mast is electrically...
antennas employed in radio broadcasting at low frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
. At VHF
Very high frequency
Very high frequency is the radio frequency range from 30 MHz to 300 MHz. Frequencies immediately below VHF are denoted High frequency , and the next higher frequencies are known as Ultra high frequency...
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
Whip antenna
A whip antenna is an antenna consisting of a single straight flexible wire or rod, often mounted above some type of conducting surface called a ground plane. The bottom end of the whip is connected to the radio receiver or transmitter. They are designed to be flexible so that they won't break...
with a ground plane consisting of several wires or rods radiating horizontally 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 quarter-wave whip
Whip antenna
A whip antenna is an antenna consisting of a single straight flexible wire or rod, often mounted above some type of conducting surface called a ground plane. The bottom end of the whip is connected to the radio receiver or transmitter. They are designed to be flexible so that they won't break...
and "Rubber Ducky
Rubber Ducky antenna
The Rubber Ducky antenna is an electrically short monopole antenna which functions somewhat like a base-loaded whip antenna and is sealed in a rubber or plastic jacket to protect the antenna...
" antennas used with handheld radios such as walkie-talkie
Walkie-talkie
A walkie-talkie is a hand-held, portable, two-way radio transceiver. Its development during the Second World War has been variously credited to Donald L. Hings, radio engineer Alfred J. Gross, and engineering teams at Motorola...
s and cell phones are also monopole antennas. The hand and body of the person holding them 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 AMAM broadcasting
AM broadcasting is the process of radio broadcasting using amplitude modulation. AM was the first method of impressing sound on a radio signal and is still widely used today. Commercial and public AM broadcasting is carried out in the medium wave band world wide, and on long wave and short wave...
broadcasters comprises at least 120 buried copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
or phosphor bronze
Phosphor bronze
Phosphor bronze is an alloy of copper with 3.5 to 10% of tin and a significant phosphorus content of up to 1%. The phosphorus is added as deoxidizing agent during melting....
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
Welding
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...
, brazing
Brazing
Brazing is a metal-joining process whereby a filler metal is heated above and distributed between two or more close-fitting parts by capillary action. The filler metal is brought slightly above its melting temperature while protected by a suitable atmosphere, usually a flux...
or using coin silver solder to help reduce corrosion. Monopole antennas that use guy-wire
Guy-wire
A guy-wire or guy-rope, also known as simply a guy, is a tensioned cable designed to add stability to structures . One end of the cable is attached to the structure, and the other is anchored to the ground at a distance from the structure's base...
s 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
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
(RF) propagation, they are called directional antenna
Directional antenna
A directional antenna or beam antenna is an antenna which radiates greater power in one or more directions allowing for increased performance on transmit and receive and reduced interference from unwanted sources....
arrays.
The RF feed-point impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
across this antenna base insulator is well defined and is approximately as shown in the chart.
Federal Communications Commission
The Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...
(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,
, 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
Impedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
network matches the impedance of the antenna to the impedance of the transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
feeding it.
Examples of monopole antennas are:
- the whip antennaWhip antennaA whip antenna is an antenna consisting of a single straight flexible wire or rod, often mounted above some type of conducting surface called a ground plane. The bottom end of the whip is connected to the radio receiver or transmitter. They are designed to be flexible so that they won't break...
- the radio mast (when isolated from the ground and bottom-fed)
Monopole antennas have become one of the mainstay 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 antennaDual-band blade antenna- Introduction :A typical monopole antenna, pictured to the right, has an omni-directional pattern and is limited in its frequency range. Please keep in mind that an omni-directional radiation pattern applies to the azimuth patterns and does exhibit a null at zenith.A monopole antenna can be...
- Cellular repeaterCellular repeaterA cellular repeater, cell phone repeater, or wireless cellular signal booster, a type of bi-directional amplifier as commonly named in the wireless telecommunications industry, is a device used for boosting the cell phone reception to the local area by the usage of a reception antenna, a signal...
- Signal strengthSignal strengthIn telecommunications, particularly in radio, signal strength refers to the magnitude of the electric field at a reference point that is a significant distance from the transmitting antenna. It may also be referred to as received signal level or field strength. Typically, it is expressed in...
- Folded unipole antennaFolded unipole antennaThe folded-unipole antenna was first devised for broadcast use by the late John H. Mullaney, an American radio broadcast pioneer, and consulting engineer. A variation of a monopole antenna, it consists of a vertical metal rod or mast mounted over a conductive surface called a ground plane. The...
- Rubber Ducky antennaRubber Ducky antennaThe Rubber Ducky antenna is an electrically short monopole antenna which functions somewhat like a base-loaded whip antenna and is sealed in a rubber or plastic jacket to protect the antenna...
- Electrical lengtheningElectrical lengtheningElectrical lengthening is the modification of an aerial which is shorter than a whole-number multiple of a quarter of the radiated wavelength, by means of a suitable electronic device, without changing the physical length of the aerial, in such a way that it corresponds electrically to the next...
External links
- Errante's virtual ground monopole antenna www.Radiondistics.com
- Radiation of a quarter wavelength monopole