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Small Antenna Handbook by Robert E. Collin, Robert C. Hansen

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APPENDIX B

Definitions of Terms Useful to ESA

Bandwidth The frequency range over which the impedance mismatch factor does not exceed a specified value. Common values are half-power (img) and 88% power (img). Bandwidth is often expressed as a percent; the frequency range is divided by the center frequency.
Broadband antenna An antenna whose bandwidth is roughly an octave or more.
Directivity The ratio of the peak of the radiation intensity to the radiation intensity averaged over all directions.
Efficiency The ratio of peak radiation intensity to the one that would be obtained if conduction and dielectric losses were absent.
ESA An electrically small antenna; usually taken as fitting in a sphere of diameter img.
Gain Directivity reduced by conduction and dielectric losses, and by impedance mismatch losses.   Note: The IEEE definition does not include mismatch losses; industry uses the definition given above.
Radian sphere A hypothetical sphere of radius img; the sphere encloses the antenna.   Note: ESA usually use a radian sphere whose radius is half that above.
Superdirective antenna ...

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