SIMULATION AND MEASUREMENT

Simulation was performed using ANSOFT HFSS 13.0, and the predicted performance is shown in Figure 3. Measurements made with a Keysight Technologies 8531B network analyzer (see Figure 4) show a center frequency at 580 MHz with |S21| and |S31| = -3.0 dB. For |S21| and |S31| within -3 ± 0.5 dB, the measured fractional bandwidths are 24.1 and 24.2 percent, respectively. Figure 5 shows the phase difference between S21 and S31. With a criterion of ±1 degree around the nominal 90-degree phase difference, the frequency range is 570 to 600 MHz, corresponding to a bandwidth of 5.2 percent.

Figure 5

Figure 5 Phase difference between S21 and S31.

Figure 6

Figure 6 Prototype coupler.

Table 1

Figure 4a shows that eleventh harmonic signals are effectively suppressed with |S21| and |S31| below -10 dB. This means the new coupler will protect any following circuitry from interference from 1.1 to 6.5 GHz, such as from the IEEE 802.11 a/b/g standard.

The circuit area of a conventional branch-line coupler at the same frequency is approximately 3900 mm2. By comparison, this prototype represents a surface area of 14.8 percent (see Figure 6). Table 1 compares the performance of the prototype coupler with previous work.

CONCLUSION

A new microstrip branch-line coupler uses modified radial stub loaded resonators to achieve compact size and wideband harmonic suppression. With eight modified radial stubs placed inside its free area, the occupied area is reduced to 14.8 percent of a conventional design at 580 MHz. The in-band performance is comparable to that of a conventional design and suppresses up to the eleventh harmonic. The measured performance agrees closely with the design simulation.

Acknowledgments

This work was supported by the Natural Science Foundation of China under Grant Nos. 61377080 and 61302842.

References

  1. A. Mohra, A. F. Sheta and S. F. Mahmoud, “New Compact 3 dB 0/180 Microstrip Coupler Configurations,” Applied Computational Electromagnetics Society (ACES) Journal, Vol. 19, No. 2, July 2004, pp. 108–112.
  2. B. Xiao, J. Hong and B. Wang, “A Novel UWB Out-of-Phase Four-Way Power Divider,” Applied Computational Electromagnetics Society (ACES) Journal, Vol. 26, No. 10, October 2011, pp. 863–867.
  3. K. A. Shamaileh, A. Qaroot, N. Dib and A. Sheta, “Design of Compact Unequal Split Wilkinson Power Divider with Harmonics Suppression Using Non-Uniform Transmission Lines,” Applied Computational Electromagnetics Society Journal, Vol. 26, No. 6, June 2011, pp. 530–538.
  4. K. W. Eccleston and S. H. M. Ong, “Compact Planar Microstripline Branch-Line and Rat-Race Couplers,” IEEE Transactions on Microwave Theory and Techniques, Vol. 51, No. 10, 2003, pp. 2119–2125.
  5. P. Mondal and A. Chakrabarty, “Design of Compact Branch-Line and Rat-Race Hybrid Couplers with Harmonics Suppression,” IET Microwaves Antennas and Propagation, Vol. 3, No. 1, 2009, pp. 109–116.
  6. J. Gu and X. Sun, “Miniaturization and Harmonic Suppression of Branch-Line and Rat-Race Hybrid Coupler Using Compensating Spiral Compact Microstrip Resonant Cell,” IEEE MTT-S International Microwave Symposium Digest, June 2005, pp. 1211–1214.
  7. J. Wang, B. Z. Wang, Y. X. Guo, L. C. Ong and S. Xiao, “A Compact Slow-Wave Microstrip Branch-Line Coupler with High Performance,” IEEE Microwave and Wireless Components Letters, Vol. 17, No. 7, July 2007, pp. 501–503.
  8. V. K. Velidi, B. Patel and S. Sanval, “Harmonic Suppressed Compact Wideband Branch-Line Coupler Using Unequal Length Open-Stub Units,” International Journal of RF and Microwave Computer-Aided Engineering, Vol. 21, No. 1, November 2010, pp. 115–119.
  9. K. Y. Tsai, H. S. Yang, J. H. Chen and Y. J. Chen, “A Compact 3 dB Branch-Line Hybrid Coupler with Harmonics Suppression,” IEEE Microwave and Wireless Components Letters, Vol. 21, No. 10, October 2011, pp. 537–539.
  10. V. K. Velidi, A. Pal and S. Sanyal, “Harmonics and Size Reduced Microstrip Branch-Line Baluns Using Shunt Open-Stubs,” International Journal of RF and Microwave Computer-Aided Engineering, Vol. 21, No. 2, March 2011, pp. 199–205.
  11. X. Yang, Z. Liao and X. C. Zhang, “Design of Compact Rat-Race Couplers with Arbitrary Power Division Ratios,” Progress In Electromagnetics Research Letters, Vol. 74, 2018, pp. 83–89.
  12. K. O. Sun, S. J. Ho, C. C. Yen and D. Van Der Weide, “A Compact Branch-Line Coupler Using Discontinuous Microstrip Lines,” IEEE Microwave and Wireless Components Letters, Vol. 15, No. 8, August 2005, pp. 519–520.
  13. M. Nosrati, “An Extremely Compact Microstrip Branch‐Line Coupler,” Microwave and Optical Technology Letters, Vol. 51, No. 6, June 2009, pp. 1403–1406.
  14. K. M. Cheng and F. L. Wong, “A Novel Approach to the Design and Implementation of Dual-Band Compact Planar 90°Branch-Line Coupler,” IEEE Transactions on Microwave Theory and Techniques, Vol. 52, No. 11, November 2004, pp. 2458–2463.
  15. H. L. Zhang and K. J. Chen, “A Stub Tapped Branch-Line Coupler for Dual-Band Operations,” IEEE Microwave and Wireless Components Letters, Vol. 17, No. 2, February 2007, pp. 106–108.
  16. M. Nosrati, M. Daneshmand and B. S. Virdee, “Novel Compact Dual‐Narrow-Wideband Branch‐Line Couplers Using T‐Shaped Stepped‐Impedance‐Stub Lines,” International Journal of RF and Microwave Computer‐Aided Engineering, Vol. 21, No. 6, September 2011, pp. 642–649.
  17. H. Zhang and Z. Zhang, "Miniaturized Microstrip Branch-Line Coupler with Good Harmonic Suppression Based on Radial Stub Loaded Resonators," Progress In Electromagnetics Research Letters, Vol. 87, 15-20, 2019.