EIRP

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For non-isotropic, highly directional dipole antennas (e.g. point-to-point antennas used in bridging systems, satellite dishes and antennas used in cellular phone cell sites where the cell is divided up into sectors), the power radiated inside the footprint or area of coverage of these antennas is known as the equivalent (or effective) isotropic (or isotropically) Radiated Power (EIRP). EIRP should be distinguished from the simple, raw, Effective Radiated Power (ERP). The EIRP is defined in the direction of strongest antenna radiation—in line with the beam axis or "biggest lobe" of antenna coverage. EIRP in a particular direction is the power that would have to be radiated by an imaginary isotropic antenna to achieve the same field strength as the real, more directional, dipole antenna provides, after taking into account all cable losses and antenna gain. Thus the EIRP will be a higher figure than the ERP.

Given the ERP, one can derive the EIRP using the following formula:

• EIRP = ERP + 2.14 dB

• or EIRP (in watts) = ERP (in watts) * 1.64

The radius by which a receiving site may be separated from the transmitting station is proportional to the square root of the EIRP of the transmitter-antenna unit. Thus, if the EIRP is suddenly reduced by 50 percent, the maximum receiving distance reduces to 70 percent of its original radius. Therefore, it is important that the cable that connects the transmitter to the antenna dissipate as little of the signal power as possible, since such power dissipation (attenuation), reduces the power available to the antenna. A high quality waveguide tube (instead of a "lossy" coaxial cable) may only lose up to 0.49 dB per 100 feet, yet a 600 ft. run of such a waveguide would have an attenuation of approximately 3 dB and reduce the transmitter power to only 50% of the value leaving the transmitter. Nevertheless, coaxial cables almost always connect a transmitter and antenna, as long as the run between the two is kept very short.

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