BANDWIDTH AND Q OF ANTENNAS RADIATING TE AND TM MODES

The time-domain Poynting theorem is used to develop a general expressi on for the complex Poynting vector applicable to any single-frequency electromagnetic radiation field, It is found that the traditional comp lex Poynting vector applies to TE or TM fields, which we call simple f ields, but that it does not apply to TE and TM fields, which we call c ompound fields, Either TE or TM fields are generated by most antennas, We show that previously imposed theoretical minimum size-to-wavelengt h ratios for useful antenna operation apply to simple fields but not a lways to compound ones, We conclude that electrically small, efficient compound antennas may be possible, As an example, the general form of the Poynting vector is used to analyze a compound source consisting o f four antenna elements; idealized, superimposed, properly phased and oriented, coherent, electric and magnetic, dipole and quadrupole radia tors, When properly driven, the antenna supports zero reactance on a c ircumscribing virtual surface of radius a, even in the limit as the ra dius-to-wavelength ratio of that surface goes to zero, The directivity pattern has a fixed 9 dB gain; the radiative Q of the surface is less and the bandwidth more by a factor of (ka)(2). where k is the wave nu mber, than for similarly sized radiators of simple fields.