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In RF engineering, radial has three distinct meanings, both referring to lines which radiate from (or intersect at) a radio antenna, but neither meaning is related to the other. ;Ground system radial wires When used in the context of antenna construction, radial wires are physical objects: Wires running away from the base of the antenna, used to augment or replace the conductivity of the ground near the base of the antenna. The radial wires either may run above the surface of the earth (''elevated radials''), on the surface (''on ground radials''), or buried a centimeter or so under the earth (''buried radials''). The ends of the wires nearest the antenna base are connected to the antenna system electrical ground, and the far ends are either unconnected, or connected to metal stakes driven into the earth. ;Top loading radial wires Symmetricly arranged radial wires may also be attached to the top of an antenna, running horizontally away from its apex. For practical length radials, their effect is to improve feedpoint impedance of a short antenna almost the same as extending the height of the antenna by an amount equal to the combined length of all the radials, up to a point of diminishing returns around about a dozen radials. The radials do not themselves radiate, but may indirectly cause a small improvement in antenna radiation of short antennas by raising their point of maximum current upward along the main part of the mast. ;Map radial lines When used in the context of planning for a transmission system, radial lines are a concept used when describing a
radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30  hertz (Hz) and 300  gigahertz (GHz). They are generated by an electronic device called a tr ...
station's
broadcast range A broadcast range (also listening range or listening area for radio, or viewing range or viewing area for television) is the service area that a broadcast station or other transmission covers via radio waves (or possibly infrared light, which is ...
: The radials in this case are several lines drawn on a
map A map is a symbolic depiction emphasizing relationships between elements of some space, such as objects, regions, or themes. Many maps are static, fixed to paper or some other durable medium, while others are dynamic or interactive. Although ...
, radiating from the transmitter, with evenly spaced horizontal bearings. The radial extends as far as the transmitted signal can reach either by calculation or by measurement.


Ground system radial wires

Stations transmitting at low frequencies like the
mediumwave Medium wave (MW) is the part of the medium frequency (MF) radio band used mainly for AM radio broadcasting. The spectrum provides about 120 channels with more limited sound quality than FM stations on the FM broadcast band. During the dayt ...
and
longwave In radio, longwave, long wave or long-wave, and commonly abbreviated LW, refers to parts of the radio spectrum with wavelengths longer than what was originally called the medium-wave broadcasting band. The term is historic, dating from the e ...
AM broadcast bands, and some lower
shortwave Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 me ...
frequencies, have frequencies so low that any feasible antenna is necessarily short compared to the
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
, the most common being a quarter wave vertical antenna. These wires are called radials, ground radials, grounding radials, ground system radials, or earthing radials. The radials at the antenna base provide a proper
ground plane In electrical engineering, a ground plane is an electrically conductive surface, usually connected to electrical ground. The term has two different meanings in separate areas of electrical engineering. *In antenna theory, a ground plane is a ...
for the types of radio antennas used for long
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
s. These "half dipole" antennas require ''
ground Ground may refer to: Geology * Land, the surface of the Earth not covered by water * Soil, a mixture of clay, sand and organic matter present on the surface of the Earth Electricity * Ground (electricity), the reference point in an electrical c ...
ing'' or ''earthing wires'' in order to function well, since the virtual image of the antenna electrically reflected by the mirror-like ground system is an essential part of the operation of the actual antenna standing above the ground system. The radials are typically buried in the soil or laid on the soil in a flat, radial pattern.


Practical issues for ground system radials

The ground system radials do not have to be absolutely straight nor absolutely horizontal. Although they provide an electrical "ground", they do not require any actual contact with the surrounding earth, even though advisable. When the radials are mechanically incorporated into the structure of a small antenna it is called a ''
ground plane antenna A whip antenna is an antenna consisting of a straight flexible wire or rod. The bottom end of the whip is connected to the radio receiver or transmitter. A whip antenna is a form of monopole antenna. The antenna is designed to be flexible so th ...
''. For these antennas the radials slope off at an angle and are also called a ''skirt''. Radials lying upon the ground or within it are not resonant, and there is a great deal of practical latitude in the length and number of radials, although licensing requirements may demand excessive numbers and lengths. Elevated radials are far more efficient at intercepting electrical fields before they reach the earth, so that only three or four may suffice, but since their electrical properties are not dampended by adjacent soil they must be cut to a resonant length.


Electrical hazards

When well designed, the far ends of the wires in the ground system carry extremely high voltages. If elevated above the soil, the ends are often connected to ground rods as a safety measure, rather than to improve the function of the antenna. Because of the same hazard, elevated radials are placed at least eight feet above the ground surface, to be out of reach of passers by, or over a fenced area. Any metal object within the near field of the radiator must also be directly tied to the ground system, or the metal will become charged with radio-frequency
voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...
, and become an
electric shock Electrical injury is a physiological reaction caused by electric current passing through the body. The injury depends on the density of the current, tissue resistance and duration of contact. Very small currents may be imperceptible or produce a ...
hazard. If large enough to act as a parasitic radiator it may also affect or distort the antenna radiation pattern.


Top loading radial wires

Similar radial wires can be placed on the top of antennas (instead of at the base) that also promote more efficient distribution of current in the antenna, but the structure of radial wires added to the top end of the antenna is called a ''capacitance hat'' or ''top loading''. Like ground radials, top radials are symmetricly arranged wires placed to radiate away from the apex of the antenna, ideally running horizontally away from the top. Electrically, this is equivalent to attaching a capacitor at the top of the antenna, whose other contact is wired to the ground system, which constitutes the opposite plate of the capacitor. Top loading antennas is a way to effectively increase the height of an antenna (for some purposes, but not all) either to reduce the reactance at the feedpoint, or to indirectly increase
radiation resistance Radiation resistance, \ R_\mathsf\ or \ R_\mathsf\ , is proportional to the part of an antenna's feedpoint electrical resistance that is caused by power loss from the emission of radio waves from the antenna. Radiation resistance is an ''effecti ...
by increasing the amount of current in the vertical part of the mast. Although coils can be used to similarly load an antenna, using coils introduces resistance losses from the substantial amount of wire needed; using radials for capacitive loading effectively adds no loss.


Practical considerations for loading radials

For radials 9 electrical degrees or less ( 
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
, each, or shorter) the effect on feedpoint impedance and the current in the mast is the same as extending the height of the antenna by a length of wire equal to 99% of the total length of all the radials, up to about a dozen radials. The radials may cause a small to moderate improvement in antenna radiation by raising the point of maximum current upward along the main part of the antenna, although symmetrical radials themselves do not radiate. Horizontal radial wires are ideal, but sometimes difficult to support. As a substitute same-lengths of conductive top segments of down-sloping guy wires can be electrically connected to the antenna apex. This is less than ideal, but often more feasible for long top radials on towers. When guy wires are used for top loading instead of horizontal radial wires, they must not extend very far down the mast, since the guy wires will block some radiation from the mast, which causes a conflict between loss caused by the radiating mast being "shadowed" by the guy wires, and the indirect gains realized by the increased current in the radiating part of the mast.


Map radial lines

The use of radial lines on a
map A map is a symbolic depiction emphasizing relationships between elements of some space, such as objects, regions, or themes. Many maps are static, fixed to paper or some other durable medium, while others are dynamic or interactive. Although ...
for measurement, planning, and regulation of radio transmissions is called the ''radial method''. It has no relation to grounding radials described above.


Radials for transmission planning

In the field of transmission planning, radials are evenly spaced points (
vector Vector most often refers to: *Euclidean vector, a quantity with a magnitude and a direction *Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematic ...
s) along evenly spaced
line Line most often refers to: * Line (geometry), object with zero thickness and curvature that stretches to infinity * Telephone line, a single-user circuit on a telephone communication system Line, lines, The Line, or LINE may also refer to: Art ...
s ( bearings) from a common point on a
map A map is a symbolic depiction emphasizing relationships between elements of some space, such as objects, regions, or themes. Many maps are static, fixed to paper or some other durable medium, while others are dynamic or interactive. Although ...
, which are used to determine the
average In ordinary language, an average is a single number taken as representative of a list of numbers, usually the sum of the numbers divided by how many numbers are in the list (the arithmetic mean). For example, the average of the numbers 2, 3, 4, 7 ...
elevation The elevation of a geographic location is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface (see Geodetic datum § ...
above mean sea level Height above mean sea level is a measure of the vertical distance ( height, elevation or altitude) of a location in reference to a historic mean sea level taken as a vertical datum. In geodesy, it is formalized as '' orthometric heights''. Th ...
(AMSL) within a
radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30  hertz (Hz) and 300  gigahertz (GHz). They are generated by an electronic device called a tr ...
station's
broadcast range A broadcast range (also listening range or listening area for radio, or viewing range or viewing area for television) is the service area that a broadcast station or other transmission covers via radio waves (or possibly infrared light, which is ...
(including broadcast stations and
cellphone A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive calls over a radio frequency link whi ...
base station Base station (or base radio station) is – according to the International Telecommunication Union's (ITU) Radio Regulations (RR) – a "land station in the land mobile service." The term is used in the context of mobile telephony, wireless c ...
s, among others). This in turn determines the station's
height above average terrain Height above average terrain (HAAT), or (less popularly) effective height above average terrain (EHAAT), is the vertical position of an antenna site is above the surrounding landscape. HAAT is used extensively in FM radio and television, as it i ...
(HAAT), which greatly affects its coverage area (more so than
effective radiated power Effective radiated power (ERP), synonymous with equivalent radiated power, is an IEEE standardized definition of directional radio frequency (RF) power, such as that emitted by a radio transmitter. It is the total power in watts that would h ...
), and therefore the potential for
RF interference Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrost ...
with other adjacent stations or cells. This information must be submitted with an application for a
construction permit Planning permission or developmental approval refers to the approval needed for construction or expansion (including significant renovation), and sometimes for demolition, in some jurisdictions. It is usually given in the form of a building perm ...
. The points used for calculation may differ if a
directional antenna A directional antenna or beam antenna is an antenna which radiates or receives greater power in specific directions allowing increased performance and reduced interference from unwanted sources. Directional antennas provide increased performanc ...
is used.


Background of the radial method

The use of the radial method is more common in North America, where the FCC and CRTC use it in mediumwave transmission planning and regulation. In Europe and Asia, the use of radials has fallen out of favor since the 1970s, and in many nations the radial antenna proof is only acceptable as an ancillary antenna proof. Canada and Mexico, due to lower population densities, never implemented the fully complete radial models that the US FCC did. The radial method has been falling out of favor for methods based on Cartesian coordinates. Cartesian methods require more CPU time (and memory) to compute, but are understood to more realistically represent antenna systems. The main importance of the radial methods is that a quick ''antenna system proof'' can be ''completed in less than 15 minutes'' (often in only 5 minutes) of a typical home computer's CPU time, regardless of antenna system complexity.


Regulatory use of radials

The ITU over the past 50 years — in consideration of the various population densities of its members — officially mandates a minimum of 5 radials for an entire antenna system. Although many broadcasting regulators around the world had to find some way of regulating longwave and mediumwave antenna patterns and power, only the FCC chose to implement the radial method in its fullest form. The FCC decision to fully implement radials evolved from 1925 to 1975. Technology had changed, and by the 1980s, computer terrain simulation of station interference and station patterns could be done on mainframes, typically using Cartesian or other non-radial methods. The FCC rules on radials were relaxed in stages from 1996 to 2013. It is expected that the 2013 ruleset for radials will probably endure without change for a decade.


Footnotes


See also

*
broadcast range A broadcast range (also listening range or listening area for radio, or viewing range or viewing area for television) is the service area that a broadcast station or other transmission covers via radio waves (or possibly infrared light, which is ...
*
ground plane antenna A whip antenna is an antenna consisting of a straight flexible wire or rod. The bottom end of the whip is connected to the radio receiver or transmitter. A whip antenna is a form of monopole antenna. The antenna is designed to be flexible so th ...
*
ground plane In electrical engineering, a ground plane is an electrically conductive surface, usually connected to electrical ground. The term has two different meanings in separate areas of electrical engineering. *In antenna theory, a ground plane is a ...
* radio antenna


References

{{DEFAULTSORT:Radial (Radio) Broadcast engineering