Minimum Q Electrically Small Antennas

O. S. Kim

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Theoretically, the minimum radiation quality factor Q of an isolated resonance can be achieved in a spherical electrically small antenna by combining TM1m and TE1m spherical modes, provided that the stored energy in the antenna spherical volume is totally suppressed. Using closed-form expressions for the stored energies obtained through the vector spherical wave theory, it is shown that a magnetic-coated metal core reduces the internal stored energy of both TM1m and TE1m modes simultaneously, so that a self-resonant antenna with the Q approaching the fundamental minimum is created. Numerical results for a multiarm spherical helix antenna confirm the theoretical predictions. For example, a 4-arm spherical helix antenna with a magnetic-coated perfectly electrically conducting core (ka=0.254) exhibits the Q of 0.66 times the Chu lower bound, or 1.25 times the minimum Q.
Original languageEnglish
JournalI E E E Transactions on Antennas and Propagation
Issue number8
Pages (from-to)3551-3558
Publication statusPublished - 2012


  • Chu lower bound
  • Circular polarization
  • Electric dipole
  • Electrically small antennas
  • Fundamental limitations
  • Magnetic dipole
  • Quality factor

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