Efficient radiation from an electrically small antenna: control of higher order modes

The time domain Poynting theorem is used to develop a complex Poynting vector that is applicable to all electromagnetic radiation fields. We find that the traditional complex Poynting vector applies to fields generated by either solenoidal, TE, or irrotational, TM, sources but not both; most antennas generate one or the other. A direct result is that the commonly accepted proof that Q of electrically small antennas is large and increases precipitously with decreasing size breaks down with mixed TE and TM modes. A compound antenna generating mixed TE and TM modes is analyzed and it is determined that the input impedance is purely resistive and independent of the electrical size; Q increases with decreasing size but not as rapidly as previously believed to be required. There is reasonable hope of obtaining an electrically small, efficient radiator of electromagnetic radiation. Difficulties in the implementation of compound antennas include control of unwanted higher order modes and the large differences between the input impedances of TE and TM sources. The significance and control of higher order modes for TE and TM radiators are discussed.