Effective radiated power
ERP measures directional radio power using a half-wave dipole reference.
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Effective radiated power (ERP), also called equivalent radiated power, is an IEEE-standardized measure of directional radio frequency power from a transmitter. It represents the total power that a half-wave dipole antenna would need to radiate in order to produce the same radiation intensity—meaning signal strength or power flux density per unit area—at a distant receiver in the direction of the actual antenna’s strongest beam, known as the main lobe. ERP combines the transmitter’s output power with the antenna’s ability to focus that power in a specific direction, calculated as the input power to the antenna multiplied by its gain. This metric is commonly used in electronics and telecommunications, especially in broadcasting, to describe the apparent power of a station as experienced by listeners in its coverage area.
A related measure is effective isotropic radiated power (EIRP), which instead compares the actual antenna to a theoretical isotropic antenna that radiates equally in all directions. The difference is that ERP uses a half-wave dipole as the reference, while EIRP uses an isotropic antenna. Since a half-wave dipole has a gain of about 1.64 (or roughly 2.15 dB) over an isotropic radiator, the relationship between ERP and EIRP depends on whether they are expressed as power or in decibels.
Both ERP and EIRP measure the power density radiated in the direction of maximum signal strength, not the total power output or total radiated power. They depend on two factors: the transmitter’s total power and the antenna’s radiation pattern, quantified by antenna gain relative to a specified standard.
For instance, a 1,000-watt transmitter feeding an antenna with a gain of 4 times that of an isotropic antenna (about 6 dBi) produces the same EIRP as a 4,000-watt transmitter feeding an isotropic antenna. Thus, ERP and EIRP allow comparison of different transmitter-antenna combinations on an equal basis. They are always greater than the actual total power radiated by the antenna.
The two measures differ because antenna gain is traditionally expressed relative to two different theoretical standards: an isotropic antenna and a perfect, lossless half-wave dipole. Isotropic gain is the ratio of power density received in the main lobe to that from a hypothetical isotropic antenna, often given in decibels (dBi).
Lore & Background
Effective radiated power and effective isotropic radiated power both measure the power density a radio transmitter and antenna radiate in a specific direction: in the direction of maximum signal strength (the main lobe) of its radiation pattern. This apparent power is dependent on two factors: the total power output and the radiation pattern of the antenna – how much of that power is radiated in the direction of maximal intensity. The latter factor is quantified by the antenna gain, which is the ratio of the signal strength radiated by an antenna in its direction of maximum radiation to that radiated by some necessarily named standard antenna. For ERP, the standard is a half-wave dipole antenna.
In spite of the names, ERP and EIRP do not measure transmitter power or total power radiated by the antenna; they are just measures of signal strength along the main lobe. They give no information about power radiated in other directions or total power. ERP and EIRP are always greater than the actual total power radiated by the antenna. The difference between ERP and EIRP is that antenna gain has traditionally been measured in two different units, comparing the antenna to two different standard antennas, both theoretical; an isotropic antenna and a (perfect, lossless) half-wave dipole.
Dipole gain is the ratio of the power density received at a point far from the antenna in the direction of its maximum radiation to the power density received at the same point from a hypothetical lossless half-wave dipole antenna. In contrast to an isotropic antenna, the dipole has a 'donut-shaped' radiation pattern; its radiated power is maximum in directions perpendicular to the antenna, declining to zero on the antenna axis. Since the radiation of the dipole is concentrated in horizontal directions (assuming the antenna axis is vertical), the gain of a half-wave dipole is greater than that of an isotropic antenna. The isotropic gain of a half-wave dipole is about 1.64, or, in decibels, 2.15 dB.
Reader's Guide
ERP is significant because it provides a standardized, practical measure for comparing the effective directional power of different transmitter and antenna combinations, especially in broadcasting. By referencing a half-wave dipole—a realizable antenna—ERP gives engineers and regulators a tangible benchmark for signal strength in the main lobe. Its legacy lies in its widespread use in telecommunications and broadcasting to quantify the apparent power experienced by listeners in the reception area. The relation to transmitter output power accounts for losses in components such as filters, transmission lines, and impedance matching networks, with antenna losses included in the gain.
ERP is always 2.15 dB less than EIRP, a constant difference arising from the gain of a half-wave dipole over an isotropic radiator. This distinction is often left unstated, and the reader is sometimes forced to infer which reference is being used. ERP does not measure total radiated power or power in other directions, only the signal strength along the main lobe. In free space, signal strength at a distance can be calculated from ERP, but ground wave, skywave, or indirect paths introduce additional attenuation not covered by simple formulas.
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Effective radiated power (CC BY-SA 4.0).
- Word definitions: the Codexery glossary, each quoted from its Wikipedia article.
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