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IEEE Transactions on Antennas and Propagation
Volume 48 Number 2, February 2000

Table of Contents for this issue

Complete paper in PDF format

The Coaxial Beam-Rotating Antenna (COBRA): Theory of Operation and Measured Performance

Clifton C. Courtney, Senior Member, IEEE and Carl E. Baum Fellow, IEEE

Page 299.

Abstract:

Many microwave generators, especially high-power sources, utilize an azimuthally symmetric output mode such as the TM01 circular waveguide or the coaxial TEM mode. If such a mode is projected into an antenna aperture and radiated directly, then a doughnut-shaped radiation pattern with a boresight null will result. Antenna designs to directly accommodate an azimuthally symmetric output mode and the high electric fields of high-power sources have been considered, but they tend to be low gain, do not radiate a boresight peak along the axis of the source, and the pattern peak direction changes with frequency. Mode conversion techniques to alter the aperture field distribution (i.e., TM01 to TE11 in circular waveguide) have also been explored, but losses and weight, size and cost additions impact negatively on total system design. This paper describes a novel antenna we call the coaxial beam-rotating antenna (COBRA) that mitigates many of the problems normally associated with the azimuthally symmetric output modes of high-power microwave sources. The COBRA accepts directly (without the need for mode conversion) an azimuthally symmetric guided mode of a microwave source and radiates a high-gain pattern with a boresight peak. In addition, the COBRA operates with a wide bandwidth, is compatible with the intense electric fields associated with high-power microwave sources, and the geometry of the antenna can be easily configured to produce an arbitrarily (elliptically) polarized boresight field. This paper presents the fundamental theory of operation, derives pertinent design and performance equations, and gives the measured operating characteristics of a COBRA prototype.

References

  1. R. Lemke, S. Calico and M. Clark, "Investigation of a load-limited, magnetically insulated transmission line oscillator (MILO)", IEEE Trans. Plasma Sci., vol. 25, pp.  364-374, Apr.  1997.
  2. K. J. Hendricks, et al. "Increasing the RF energy per pulse of an RKO", IEEE Trans. Plasma Sci., vol. 26, pp.  320-325, June  1998 .
  3. R. A. Koslover, C. D. Cremer, W. P. Geren, D. E. Voss and L. M. Miner, "Compact, broadband, high power circular TM01 to TE11 mode converter", U.S. Patent 4 999 591,
  4. S. N. Vlasov and I. M. Orlova, "Quasi-optical transformer which transforms the waves in a waveguide having circular cross-section into a highly directional wave beam", Radiophys. Quantum Electron., vol. 17, pp.  115 -119, 1975.
  5. E. Baca, et al. "Computational and experimental investigations of shaped end radiators", in AMEREM'96, Albuquerque, NM, 1996.
  6. J. H. Beggs, R. J. Luebber and B. G. Ruth, "Analysis of electromagnetic radiation from shaped-end radiators using the finite-difference time domain method", IEEE Trans. Antennas Propagat., vol. 41, p.  1324, Sept.  1993.
  7. 10 017 "IEEE Standard Definitions of Terms for Radio Wave Propagation", IEEE Std 211-1977,
  8. A. D. Olver, P. J. B. Clarricoats, A. A. Kishk and L. Shafai, Microwave Horns and Feeds, New York: IEEE Press, 1994 .
  9. C. E. Baum and H. N. Kritikos, "Symmetry in electromagnetics,"in Electromagnetic Symmetry, C. E. Baum, and H. Kritikos, Eds. Washington, DC: Taylor and Francis, 1995, ch. 1.
  10. W. Stutzman and G. Thiele, Antenna Theory and Design, New York: Wiley, 1981.
  11. M. Abramowitz and I. Stegun, Handbook of Mathematical Functions, New York: Dover, 1965, p.  10. 
  12. 10 017 "IEEE Standard Test Procedures for Antennas", 1979.
  13. C. C. Courtney, et al. "Design and measurement of a Cassegrain-type coaxial beam-rotating antenna", Sensor and Simulation Note 427, Nov.  1998.
  14. C. C. Courtney and C. E. Baum, "Design and optimization of a conical transmission-line feed for a coaxial beam-rotating antenna", Sensor and Simulation Note 429, Dec.  1998.
  15. C. C. Courtney and C. E. Baum, "Coaxial beam-rotating antenna (COBRA) concepts", Sensor and Simulation Note 395, Apr.  1996.
  16. C. C. Courtney, D. Slemp, D. Baum, C. E. Baum, R. Torres and W. Prather, "Coaxial beam-rotating antenna (COBRA) prototype measurements", Sensor and Simulation Note 408, July  1997.