Figure 5 shows that the offsets of the three types of slots are almost the same when the slot width is not greater than 22 mm.

After optimization, the areas of the rectangular, end-rounded and elliptical slots are 1554, 1018 and 2206 mm2, respectively. The optimized elliptical slot has the largest aperture area (1.4x that of the rectangular slot). Because the radiated power of each slot is equal, a larger area results in a lower power density and therefore a higher power handling capability.

Figure 5

Figure 5 Optimized slot offset from the waveguide central axis as a function of slot width.

Figure 6

Figure 6 Four-element antenna array with rounded rectangular slots.

MEASUREMENT

A four-element array is connected to a high-power source (see Figure 6). There are three arrays, designed with either rectangular, end-rounded rectangular or elliptical slots. The widths of the rectangular, end-rounded rectangular and elliptical slots are 15, 20 and 27.5 mm, respectively. The waveguide is vacuum sealed to maximize its microwave power handling capability.

With a microwave source power of about 100 MW, the maximum electric field produced within the rectangular, end-rounded rectangular and elliptical slots is 46, 43 and 38 MV/m, respectively. According to Kilpatrick,17 the following relationship exists in a vacuum:

where f is the frequency in MHz and E is the breakdown electric field in MV/m.

Figure 7

Figure 7 Array radiated waveform.

According to Equation (2), E is ∼35 MV/m at 1.575 GHz. When the electric field in the waveguide exceeds the breakdown electric field, there is evidence of tail erosion (pulse shortening).18 Figure 7 compares the envelopes of a pulsed radiated waveform at the output of each of the slotted arrays. Tail erosion is the most serious in the rectangular slot array and least pronounced in the elliptical slot array. This shows that the elliptically shaped slot array has the largest handling power capability.

CONCLUSION

An L-Band waveguide array is designed with longitudinal shunt slots to investigate the power handling capability of longitudinal shunt slot waveguide arrays with various slot geometries and slot widths. Three different types of array slots are considered: rectangle, end-rounded rectangle and elliptical. Simulation shows that the power handling capability of the rounded-end rectangle slot is almost 1.1x that of the rectangle slot and the power handling capability of the elliptical slot array is almost 1.5x that of the rectangular slot. This is supported by experimental measurements of tail erosion in slotted array radiation measurements.

References

  1. R. C. Johnson, Antenna Engineering Handbook, Third Edition, McGraw Hill, 1993.
  2. J. Benford, J. A. Swegle and E. Schamiloglu, High Power Microwaves, Second Edition, Taylor & Francis, 2007.
  3. X. -Q. Li, Q. -X. Liu, X. -J. Wu, L. Zhao, J. -Q. Zhang and Z. -Q. Zhang, “A GW Level High-Power Radial Line Helical Array Antenna,” IEEE Transactions on Antennas and Propagation, Vol. 56, No. 9, September 2008, pp. 2943-2948.
  4. J. W. Li, W. H. Huang, Z. Q. Zhang, H. J. Huang, K. Y. Wang, T. Z. Li, C. H. Chen and J. Y. Fang, “Testing of an X-band HPM Antenna Based on Leaky Waveguide,” High Power Laser Particle Beams, Vol. 23, No. 12, December 2011, pp. 3363–3366.
  5. L. Guo, W. Hang, C. Chao, J. Li, Y. Liu and R. Meng, “Studies of a Leaky-Wave Phased Array Antenna for High-Power Microwave Applications,” IEEE Transactions on Plasma Science, Vol. 44, No.10, August 2016, pp. 2366–2375.
  6. R. S. Elliott, Antenna Theory and Design, IEEE Press, 2003.
  7. J. L. Volakis, G. A. Roland, “Waveguide Slot Antenna Arrays,” Antenna Engineering Handbook, McGraw Hill, Fourth Edition, Chapter 9, 2007, pp. 32–34.
  8. S. Bemal, F. Vega, F. Roman and A. Valero, “A High-Gain, Broad-Wall Slotted Waveguide Antenna Array to be Used as Part of a Narrowband High Power Microwaves System,” International Conference on Electromagnetics in Advanced Applications, September 2015.
  9. C. E. Baum, “Sidewall Waveguide Slot Antenna for High Power,” Sensor and Simulation Notes, Note 503, January 2005.
  10. M. Al-Husseini, A. El-Hajj and K. Y. Kabalan, “High-Gain S-Band Slotted Waveguide Antenna Arrays with Elliptical Slots and Low Sidelobe Levels,” Progress In Electromagnetics Research Symposium Proceedings, August 2013.
  11. M. Gilden and L. Gould, Handbook on High Power Capabilities of Waveguide Systems, Microwave Associate Inc., 1963.
  12. H. Y. Yee, “Impedance of a Narrow Longitudinal Shunt Slot in a Slotted Waveguide Array,” IEEE Transactions on Antennas and Propagation, Vol. 22, No. 4, July 1974, pp. 589–592.
  13. S. Silver, Microwave Antenna Theory and Design, McGraw Hill, 1949, pp. 297-300.
  14. R. S. Elliott, “An Improved Design Procedure for Small Arrays of Shunt Slots,” IEEE Transactions on Antennas and Propagation, Vol. 31, No. 1, January 1983, 48–53.
  15. G. J. Stern and R. S. Elliott, “Resonant Length of Longitudinal Slots and Validity of Circuit Representation: Theory and Experiment,” IEEE Transactions on Antennas and Propagation, Vol. 33, No. 11, 33, November 1985, pp. 1264–1271.
  16. R. S. Elliott and W. O’Loughlin, “The Design of Slot Arrays Including Internal Mutual Coupling,” IEEE Transactions on Antennas and Propagation, Vol. 34, No. 9, September 1986, pp. 1149–1154.
  17. W. D. Kilpatrick, “Criterion for Vacuum Sparking Designed to Include Both RF and DC,” Review of Scientific Instruments, Vol. 28, No. 10, October 1957, pp. 824–826.
  18. S. D. Korovin, G. A. Mesyats, I. V. Pegel, S. D. Polevin and V. P. Tarakanov, “Pulsewidth Limitation in the Relativistic Backward Wave Oscillator,” IEEE Transactions on Plasma Science, Vol. 28, No. 3, June 2000, pp. 485–495.