Radiated Power and Field Strength from UHF ISM Transmitters

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  • Author: Larry Burgess
Short-range radios that operate in the Industrial, Scien-tific, and Medical (ISM) frequency allocations from 260 MHz to 470 MHz are widely used for remote keyless entry (RKE), home security, and remote control. A critical performance measurement for the radio transmitter is the power that it radiates from the antenna. This power must be high enough to make the link between the transmitter and receiver reliable, yet it must not be so high that it exceeds the radiation limits established in Part 15.231 of the FCC regulations. This application note discusses the relationship between FCC field-strength requirements in the 260 MHz to 470 MHz frequency range and the radiated power and typical quantities measured on a test receiver. By understanding this relationship and knowing some conversion factors, the user can determine whether measurements made at a test receiver indicate that the transmitter is close to its radiated power goal. Tables will illustrate the values that a designer can expect to obtain in field tests.