6.3.1 Describing Transmitter Power
Transmitter power can be specified in several ways, each describing a different characteristic of the transmitting system. Common terms include rated power, maximum power, carrier power, average power, peak power, peak-envelope power (PEP), effective radiated power (ERP), and effective isotropic radiated power (EIRP). These quantities are related, but they are not interchangeable. Any quoted power should therefore be interpreted together with its reference point, modulation, duty cycle, averaging interval, and antenna assumptions.
Rated transmitter power is the output that equipment is specified to deliver under defined operating, loading, modulation, and thermal conditions. It is usually measured at the transmitter output connector, before feeder losses and antenna gain are taken into account. Maximum transmitter power is the highest RF output that the equipment can produce under permitted operating conditions. Many communications systems use automatic power control and therefore operate below their maximum power for much of the time.
Carrier power is the average power supplied to the antenna transmission line during one RF cycle under an unmodulated-carrier condition. Average power is power averaged over a stated time interval and is particularly important for intermittent or pulsed transmissions. Peak power is the highest power reached during a pulse or another short interval. For an ideal rectangular pulse train, average power is approximately equal to peak power multiplied by the duty factor, where the duty factor is the pulse duration multiplied by the pulse repetition frequency.
For amplitude-varying modulation, particularly single-sideband transmission, PEP is commonly used. PEP is the RF-cycle-average power supplied to the antenna transmission line at the highest crest of the modulation envelope under normal operating conditions. Here, average refers only to averaging the instantaneous RF power over one carrier cycle. PEP is not averaged over the modulation envelope or over multiple envelope peaks. It describes the power reached at modulation peaks rather than the average power transmitted over a longer interval. For a constant-envelope signal, such as an unmodulated carrier or ideal frequency modulation, PEP equals carrier power.
Conducted transmitter-output power does not account for feeder losses or the directional concentration of energy produced by an antenna. ERP is referenced to an ideal half-wave dipole, whereas EIRP is referenced to an ideal isotropic radiator. An ideal half-wave dipole has a maximum gain 2.15 dB greater than that of an isotropic radiator. Consequently, for the same transmitting system:
or equivalently:
ERP and EIRP incorporate feeder losses and antenna gain and describe the equivalent power radiated in a specified direction. They should not be interpreted as the total power emitted in every direction.
Transmitters by frequency range. Human-made RF transmitters operate across frequency ranges extending from kilohertz-range systems to microwave and millimeter-wave applications. Communications, broadcasting, radar, navigation, sensing, industrial, medical, and short-range consumer systems differ substantially in the way their output is generated and directed. Operating frequency influences antenna dimensions, propagation, field distribution, and interaction with the body. Appendix D groups representative transmitters by frequency range and provides typical power ratings together with the qualifications needed to interpret them.
Mobile phones. Mobile phones merit particular attention because they are adaptive transmitters that are commonly used close to the body. Their output power changes with the radio technology and frequency band, the quality of the connection, the type and volume of traffic, and network power control. Maximum rated power therefore describes a capability rather than the phone’s typical output, while time-averaged power varies substantially between standby, voice, and data operation. The manner in which the phone is used and its distance from the body are often more important to personal exposure than small differences in rated power. Appendix D summarizes the relevant source characteristics; Section 6.4 considers the resulting exposure.
Industrial applications. Industrial RF equipment may operate at high generator powers to heat, weld, dry, or otherwise process materials. In induction heating, energy is coupled principally through magnetic fields into conductive loads; in dielectric heating, alternating electric fields deposit energy in insulating or weakly conducting materials. The equipment’s nameplate power should not be interpreted as free-space radiated power. Operator exposure depends on factors including frequency, applicator geometry, shielding, field leakage, access, maintenance condition, and working position. Representative applications and power ratings are described in Appendix D; occupational exposure is considered in Section 6.4.
Medical applications. Medical systems use electromagnetic energy intentionally and must therefore be considered in terms of how that energy is applied. Magnetic resonance imaging combines a static magnetic field, switched gradient fields, and pulsed RF fields, while diathermy, electrosurgery, and ablation employ different frequencies, coupling arrangements, powers, and treatment durations. Generator power alone does not describe patient or staff exposure; applicator geometry, duty cycle, treatment time, tissue coupling, and equipment-specific controls must also be considered. Appendix D describes the principal source types, while Sections 6.4 and 6.5 address exposure and RF dosimetry.
Domestic RF sources. Domestic RF sources include wireless communications devices, induction cooktops, microwave ovens, wireless chargers, and security or identification systems. Two distinctions are particularly important. The electrical input or heating power of an appliance is not the same as the RF field that leaks into the surrounding environment, and the maximum output of a wireless transmitter is not the same as its time-averaged output. Exposure depends on such factors as proximity, duty cycle, enclosure and shielding, correct operation, and equipment condition. Appendix D provides representative examples and ratings. Together, these sources illustrate why source power must be followed by an assessment of the external fields actually experienced by a person, as discussed in the next section.