Electrically small antenna
The far-field radiation pattern of an antenna is the sum of its near-field spherical modes, expressed using Legendre functions and spherical Bessel functions. In its simplest form, it is an omnidirectional radiation pattern with no variation in the azimuth plane. When the antenna becomes electrically small, the propagating modes are replaced by evanescent modes with high Q, where
The difficulties of designing an electrically small antenna includes:
- impedance matching,
- insertion loss from high current density flowing on a non-perfect conductor, resulting in joule heating, and
- a small radiation aperture with low radiation efficiency.
Fundamental limitations of antennas
Electrically small antennas belong to one of the four fundamental limitations of antennas addressed by R. C. Hansen. The four fundamental limitations of antennas are, electrically small antennas, superdirective antennas, superresolution antennas, and high-gain antennas.
Passive measurement of an electrically small antenna requires a quarter-wavelength RF choke or ferrite bead to be add to the end of the feeding coaxial cable to limit or prevent the current from flowing onto the surface of the cable. Current flowing on the exterior of the feeding cable increases the electrical size and radiation aperture of the antenna, resulting in erroneous measurement result. The quarter-wavelength choke are narrow-band and the ferrite beads are lossy at higher frequency greater than 1 GHz.
- H. A. Wheeler, "Fundamental Limitations of Small Antennas," Proceedings of the IRE, vol. 35, pp. 1479-1484, 1947.
- H. A. Wheeler, "The Radiansphere around a Small Antenna," Proceedings of the IRE, vol. 47, pp. 1325-1331, 1959.
- L. J. Chu, "Physical Limitations on Omni-Directional Antennas," J. Appl. Phys., Vol. 9, pp. 1163-1175, 1948.
- G. Goubau, "Multi-element Monopole Antennas," Proc. Workshop on Electrically Small Antennas, ECOM, Ft. Monmouth, NJ, pp. 63-67, May 1976.
- H. Foltz, J. McLean, G. Crook, "Disk-Loaded Monopoles with Parallel Strip Elements," IEEE Transactions on Antennas and Propagation, vol. 46, no.12, December 1998, pp. 1894-1896.
- J. A. Dobbins and R. L. Rogers, “Folded Conical Helix Antenna,” IEEE Trans. Antennas Propagation, vol. 49, No. 12, pp. 1777- 1781, December 2001.
- R. C. Hansen. Fundamental limitations in antennas. Proceedings of the IEEE, 69(2):170–182, February 1981.