Effective radiated power

Effective radiated power and effective isotropic radiated power both measure the power density a radio transmitter and antenna (or other source of electromagnetic waves) radiates in a specific direction: in the direction of maximum signal strength (the "main lobe") of its radiation pattern.[1][2][3][4] 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 desired direction. 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 a standard antenna. For example, a 1,000-watt transmitter feeding an antenna with a gain of 4 (6 dBi) will have the same signal strength in the direction of its main lobe, and thus the same ERP and EIRP, as a 4,000-watt transmitter feeding an antenna with a gain of 1 (0 dBi). So ERP and EIRP are measures of radiated power that can compare different combinations of transmitters and antennas on an equal basis.

In spite of the names, ERP and EIRP do not measure transmitter power, or total power radiated by the antenna, they are just a measure 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; an isotropic antenna and a half-wave dipole antenna:

• Isotropic gain is the ratio of the power density ${\displaystyle S_{\text{max}}}$ (signal strength in watts per square meter) received at a point far from the antenna (in the far field) in the direction of its maximum radiation (main lobe), to the power ${\displaystyle S_{\text{max,isotropic}}}$ received at the same point from a hypothetical lossless isotropic antenna, which radiates equal power in all directions

${\displaystyle \mathrm {G} _{\text{i}}={S_{\text{max}} \over S_{\text{max,isotropic}}}}$

Gain is often expressed in logarithmic units of decibels (dB). The decibel gain relative to an isotropic antenna (dBi) is given by

${\displaystyle \mathrm {G} {\text{(dBi)}}=10\log {S_{\text{max}} \over S_{\text{max,isotropic}}}}$

• Dipole gain is the ratio of the power density received from the antenna in the direction of its maximum radiation to the power density ${\displaystyle S_{\text{max,dipole}}}$ received from a lossless half-wave dipole antenna in the direction of its maximum radiation

${\displaystyle \mathrm {G} _{\text{d}}={S_{\text{max}} \over S_{\text{max,dipole}}}}$

The decibel gain relative to a dipole (dBd) is given by

${\displaystyle \mathrm {G} {\text{(dBd)}}=10\log {S_{\text{max}} \over S_{\text{max,dipole}}}}$

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, the gain of a half-wave dipole is greater than that of an isotropic antenna. The isotropic gain of a half-wave dipole is 1.64, or in decibels 10 log 1.64 = 2.15 dBi, so

${\displaystyle G_{\text{i}}=1.64G_{\text{d}}}$

In decibels

${\displaystyle G{\text{(dBi)}}=G{\text{(dBd)}}+2.15}$

The two measures EIRP and ERP are based on the two different standard antennas above:[1][3][2][4]

• EIRP is defined as the RMS power input in watts required to a lossless isotropic antenna to give the same maximum power density far from the antenna as the actual transmitter. It is equal to the power input to the transmitter's antenna multiplied by the isotropic antenna gain

${\displaystyle \mathrm {EIRP} =G_{\text{i}}P_{\text{in}}}$

The ERP and EIRP are also often expressed in decibels (dB). The input power in decibels is usually calculated with comparison to a reference level of one watt (W): ${\displaystyle P_{\text{in}}\mathrm {(dBW)} =10\log P_{\text{in}}}$. Since multiplication of two factors is equivalent to addition of their decibel values

${\displaystyle \mathrm {EIRP(dBW)} =G{\text{(dBi)}}+P_{\text{in}}\mathrm {(dBW)} }$

• ERP is defined as the RMS power input in watts required to a lossless half-wave dipole antenna to give the same maximum power density far from the antenna as the actual transmitter. It is equal to the power input to the transmitter's antenna multiplied by the antenna gain relative to a half-wave dipole

${\displaystyle \mathrm {ERP} =G_{\text{d}}P_{\text{in}}}$

In decibels

${\displaystyle \mathrm {ERP(dBW)} =G{\text{(dBd)}}+P_{\text{in}}\mathrm {(dBW)} }$

Since the two definitions of gain only differ by a constant factor, so do ERP and EIRP

${\displaystyle \mathrm {EIRP(W)} =1.64\cdot \mathrm {ERP(W)} }$

In decibels

${\displaystyle \mathrm {EIRP(dBW)} =\mathrm {ERP} {\text{(dBW)}}+2.15}$

The transmitter is usually connected to the antenna through a transmission line. Since the transmission line may have significant losses ${\displaystyle L}$, the power applied to the antenna is usually less than the output power of the transmitter ${\displaystyle P_{\text{TX}}}$. The relation of ERP and EIRP to transmitter output power is

${\displaystyle \mathrm {EIRP(dBW)} =P_{\text{TX}}\mathrm {(dBW)} -L\mathrm {(dB)} +G{\text{(dBi)}}}$

${\displaystyle \mathrm {ERP(dBW)} =P_{\text{TX}}\mathrm {(dBW)} -L\mathrm {(dB)} +G{\text{(dBi)}}-2.15}$

Losses in the antenna itself are included in the gain.

If the signal path is in free space (line-of-sight propagation with no multipath) the signal strength (power flux density in watts per square meter) ${\displaystyle S}$ of the radio signal on the main lobe axis at any particular distance ${\displaystyle r}$ from the antenna can be calculated from the EIRP or ERP. Since an isotropic antenna radiates equal power flux density over a sphere centered on the antenna, and the area of a sphere with radius ${\displaystyle r}$ is ${\displaystyle A=4\pi r^{2}}$ then

${\displaystyle S(r)={\mathrm {EIRP} \over 4\pi r^{2}}}$

Since ${\displaystyle \mathrm {EIRP} =\mathrm {ERP} /1.64}$,

${\displaystyle S(r)={\mathrm {ERP} \over 6.56\pi r^{2}}}$

However if the radio waves travel by ground wave as is typical for medium or longwave broadcasting, skywave, or indirect paths play a part in transmission, the waves will suffer additional attenuation which depends on the terrain between the antennas, so these formulas are not valid.

Because ERP is calculated as antenna gain (in a given direction) as compared with the maximum directivity of a half-wave dipole antenna, it creates a mathematically virtual effective dipole antenna oriented in the direction of the receiver. In other words, a notional receiver in a given direction from the transmitter would receive the same power if the source were replaced with an ideal dipole oriented with maximum directivity and matched polarization towards the receiver and with an antenna input power equal to the ERP. The receiver would not be able to determine a difference. Maximum directivity of an ideal half-wave dipole is a constant, i.e., 0 dBd = 2.15 dBi. Therefore, ERP is always 2.15 dB less than EIRP. The ideal dipole antenna could be further replaced by an isotropic radiator (a purely mathematical device which cannot exist in the real world), and the receiver cannot know the difference so long as the input power is increased by 2.15 dB.

Unfortunately, the distinction between dBd and dBi is often left unstated and the reader is sometimes forced to infer which was used. For example, a Yagi–Uda antenna is constructed from several dipoles arranged at precise intervals to create better energy focusing (directivity) than a simple dipole. Since it is constructed from dipoles, often its antenna gain is expressed in dBd, but listed only as dB. Obviously this ambiguity is undesirable with respect to engineering specifications. A Yagi–Uda antenna's maximum directivity is 8.77 dBd = 10.92 dBi. Its gain necessarily must be less than this by the factor η, which must be negative in units of dB. Neither ERP nor EIRP can be calculated without knowledge of the power accepted by the antenna, i.e., it is not correct to use units of dBd or dBi with ERP and EIRP. Let us assume a 100-watt (20 dBW) transmitter with losses of 6 dB prior to the antenna. ERP < 22.77dBW and EIRP < 24.92dBW, both less than ideal by η in dB. Assuming that the receiver is in the first side-lobe of the transmitting antenna, and each value is further reduced by 7.2 dB, which is the decrease in directivity from the main to side-lobe of a Yagi-Uda. Therefore, anywhere along the side-lobe direction from this transmitter, a blind receiver could not tell the difference if a Yagi-Uda was replaced with either an ideal dipole (oriented towards the receiver) or an isotropic radiator with antenna input power increased by 1.57 dB.[5]

Polarization has not been taken into account so far, but it must be properly clarified. When considering the dipole radiator previously we assumed that it was perfectly aligned with the receiver. Now assume, however, that the receiving antenna is circularly polarized, and there will be a minimum 3 dB polarization loss regardless of antenna orientation. If the receiver is also a dipole, it is possible to align it orthogonally to the transmitter such that theoretically zero energy is received. However, this polarization loss is not accounted for in the calculation of ERP or EIRP. Rather, the receiving system designer must account for this loss as appropriate. For example, a cellular telephone tower has a fixed linear polarization, but the mobile handset must function well at any arbitrary orientation. Therefore, a handset design might provide dual polarization receive on the handset so that captured energy is maximized regardless of orientation, or the designer might use a circularly polarized antenna and account for the extra 3 dB of loss with amplification.

Four bay crossed-dipole antenna of an FM broadcasting station.

For example, an FM radio station which advertises that it has 100,000 watts of power actually has 100,000 watts ERP, and not an actual 100,000-watt transmitter. The transmitter power output (TPO) of such a station typically may be 10,000 to 20,000 watts, with a gain factor of 5 to 10 (5× to 10×, or 7 to 10 dB). In most antenna designs, gain is realized primarily by concentrating power toward the horizontal plane and suppressing it at upward and downward angles, through the use of phased arrays of antenna elements. The distribution of power versus elevation angle is known as the vertical pattern. When an antenna is also directional horizontally, gain and ERP will vary with azimuth (compass direction). Rather than the average power over all directions, it is the apparent power in the direction of the antenna's main lobe that is quoted as a station's ERP (this statement is just another way of stating the definition of ERP). This is particularly applicable to the huge ERPs reported for shortwave broadcasting stations, which use very narrow beam widths to get their signals across continents and oceans.

For most microwave systems, a completely non-directional isotropic antenna (one which radiates equally and perfectly well in every direction – a physical impossibility) is used as a reference antenna, and then one speaks of EIRP (effective isotropic radiated power) rather than ERP. This includes satellite transponders, radar, and other systems which use microwave dishes and reflectors rather than dipole-style antennas.

In the case of medium wave (AM) stations in the United States, power limits are set to the actual transmitter power output, and ERP is not used in normal calculations. Omnidirectional antennas used by a number of stations radiate the signal equally in all directions. Directional arrays are used to protect co- or adjacent channel stations, usually at night, but some run directionally 24 hours. While antenna efficiency and ground conductivity are taken into account when designing such an array, the FCC database shows the station's transmitter power output, not ERP.

According to the Institution of Electrical Engineers (UK), ERP is often used as a general reference term for radiated power, but strictly speaking should only used when the antenna is a half-wave dipole,[7] and is used when referring to FM transmission.[8]

The height above average terrain for VHF and higher frequencies is extremely important when considering ERP, as the signal coverage (broadcast range) produced by a given ERP dramatically increases with antenna height. Because of this, it is possible for a station of only a few hundred watts ERP to cover more area than a station of a few thousand watts ERP, if its signal travels above obstructions on the ground.

• Nominal power (radio broadcasting)
• List of broadcast station classes