A
monopole antenna is a type of radio antenna formed by replacing one half of a
dipole antennaA dipole antenna, created by Heinrich Rudolph Hertz around 1886, is an antenna that can be made by a simple wire, with a center-fed driven element for transmitting or receiving radio frequency energy...
with a
ground planeIn 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 monopole behaves exactly like a dipole, as if its reflection in the ground plane formed the missing half of the dipole (see
image antennaIn telecommunication, an image antenna is a theoretical mirror-image of an antenna, an antenna element considered to extend below the ground plane as the actual antenna is above the ground plane....
). However, a monopole will have a directive gain of 5.19dB (gain is twice (3dB over) that for a half-wave
dipole antennaA dipole antenna, created by Heinrich Rudolph Hertz around 1886, is an antenna that can be made by a simple wire, with a center-fed driven element for transmitting or receiving radio frequency energy...
), and a lower input resistance, resulting in overall lower efficiency.
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 is the process of radio broadcasting using amplitude modulation.-History:AM was the dominant method of broadcasting during the first eighty years of the 20th century and remains widely used into the 21st....
broadcasters comprises 120 buried
copperCopper 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 rather soft and malleable and a freshly-exposed surface has a pinkish or peachy color...
or
phosphor bronzePhosphor 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. These alloys are notable for their toughness, strength, low coefficient of friction, and fine grain...
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
brazingBrazing is a metal-joining process whereby a filler metal or alloy is heated to melting temperature 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 or flux...
or using coin silver solder to help reduce corrosion. Monopole antennas that use
guy-wiresA guy-wire or guy-rope 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...
for support are called masts in some countries. In the United States, the term “mast” is not generally used to relate to antennas, 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 FrequencyRadio frequency is a frequency, or rate of oscillation, of electromagnetic radiation within the range of about 3 Hz to 300 GHz. This range corresponds to the frequency of alternating current electrical signals used to produce and detect radio waves...
(RF) propagation, they are called
directional antennaA 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
impedanceElectrical impedance, or simply impedance, describes a measure of opposition to a sinusoidal alternating current . Electrical impedance extends the concept of resistance to AC circuits, describing not only the relative amplitudes of the voltage and current, but also the relative phases...
across this antenna base insulator is well defined and is approximately as shown in the chart.
In the United States, the
Federal Communications CommissionThe Federal Communications Commission is an independent agency of the United States government, created, directed, and empowered by Congressional statute , and with the majority of its commissioners appointed by the current President...
(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 matchingImpedance matching is the electronics design practice of setting the input impedance of an electrical load equal to the fixed output impedance of the signal source to which it is ultimately connected, usually in order to maximize the power transfer and minimize reflections from the load...
network matches the impedance of the antenna to the impedance of the
transmission lineA transmission line is the material medium or structure that forms all or part of a path from one place to another for directing the transmission of energy, such as electromagnetic waves or acoustic waves, as well as electric power transmission....
feeding it.
Examples of monopole antennas are:
- the whip antenna
A whip antenna is the most common example of a monopole antenna, an antenna with a single driven element and a ground plane.The whip antenna is a stiff but flexible wire mounted, usually vertically, with one end adjacent to a ground plane...
- the radio mast (when isolated from the ground and bottom-fed)
See also
- Cellular repeater
A 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 to boost the cell phone reception to the local area by the usage of a reception antenna, a signal...
- Signal strength
In 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 antenna
The folded-unipole antenna was first devised for broadcast use by the late John H. Mullaney, an American radio broadcast pioneer, and consulting engineer...
- Rubber Ducky antenna
The Rubber Ducky antenna is an electrically short antenna which functions somewhat like a base-loaded whip or monopole antenna and is sealed in a rubber or plastic jacket to protect the antenna...
External links