3 20 mm, which corresponds to 0.20 k 1 3 0.12 k 1 where, k 1 is the guiding wavelength of the microstrip constructed on the same substrate as the diplexer and operated at 1.8 GHz. The measured and simulated S 11 and S 23 of the prototype superim- posed on Figure 9(a) shows that the input return losses of the input port at the two passbands are about 15 dB in the desired passbands and the isolation is better than 30 dB. Figures 9(b) and 9(c) depict the simulated and measured S 22 , S 33 , S 21 , and S 31 parameters from which maximum insertion losses of 2.6 dB are obtained in the desired passbands associated with the designed quad-channel diplexer. 4. CONCLUSION The design of the quad-channel diplexer, which is composed of ring and lumped resonators associated with the embedded impedance transformers, was presented. By inserting embedded transformers and connecting a shunt grounded stub at the com- mon input of the channel filters, both channel filters could be effectively eliminate interference between them. The design method was validated by the fabrication and measurement of a prototype on a commercial PCB substrate. Table 2 shows the comparison of the prototype of the quad-channel diplexer and other quad-channel diplexers. When compared with the works presented in [8,11], the size of our work was only about 55.6 and 58% of those reported in [8,11], respectively. Besides, the operation frequency of the highest channel of the proposed diplexer was higher than that of the diplexers of these previous works; however, similar insertion losses were achieved in the passbands for the proposed structure. This implies that the pro- posed structure of the quad-channel diplexer not only has the most compact circuit size but also exhibits the highest Q factor for minimizing the insertion losses in the passbands. However, the proposed quad-channel diplexer can also achieve more trans- mission zeros in the stopbands (near the corners of passbands) for obtaining fast roll-off beside the passbands’ corners. REFERENCES 1. B.J. Chen, T.M. Shen, and R.B. Wu, Design of tri-band filters with improved band allocation, IEEE Trans Microwave Theory Tech 7 (2009), 1790–1797. 2. Q.X. Chu, and F.C. Chen, A Compact Dual-Band Bandpass Filter Using Meandering Stepped Impedance Resonators, IEEE Microwave Wireless Compon Lett 5 (2008), 320–322. 3. K.K.M. Cheng, and C. Law, A New Approach to the Realization of a Dual-Band Microstrip Filter with Very Wide Upper Stopband, IEEE Trans Microwave Theory Tech 6 (2008), 1461–1467. 4. E.E. Djoumessi, and K. 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COMPACT, DUAL POLARIZED, MUTLIBAND FREQUENCY SELECTIVE SURFACE WITH WIDEBAND SPURIOUS REJECTION Hossein Nasrollahi, 1 Ali Nooraei Yeganeh, 2 Seyed Hassan Sedighy, 3 and Sajad Mohammad-Ali-Nezhad 4 1 Electrical Engineering Department, Iran University of Science and Technology, Tehran, Iran 2 Electrical Engineering Department, Khaje Nasir Toosi University, Tehran, Iran 3 School of New Technologies, Iran University of Science and Technology, Tehran, Iran; Corresponding author: sedighy@iust.ac.ir 4 Engineering Department, University of Qom, Iran Received 15 September 2016 ABSTRACT: A miniaturized multiband bandstop frequency selective surface (FSS) based on square loop structures with wideband spurious rejection is proposed. This structure can eliminate the spurious frequen- cy up to 30 GHz when the loop structure act as a spatial band stop filter at 7.2 GHz. Significant capacitor achieved by parasitic patch loading of the structure create a new resonance at a lower frequency when the lumped inductor added and made a new compact multiband FSS struc- ture. The proposed FSSs has similar attribute for TE and TM incident polarizations, also. Moreover, the measurement results show good agreement with the simulation ones. The simplicity, compact size, flexi- bility in multiband tuning, wideband spurious rejection and insensitive polarization operation verify the ability and capability of the proposed TABLE 2 Comparison of the Prototype with Other Quad-Channel Diplexers in the Published Article Ref. No Channels’ frequencies (GHz) Channels’ fractional bandwidth Order of filters No. of Zeros Channels’ Insertion Loss (dB) Isolation (dB) Circuit Size (k 1 3 k 1 ) [8] 0.6/0.9/1.2/1.6 3.6%,5.5%,5.7%,6% 3 0 3.2, 2.6, 2.7, 2.2 >55 0.27 3 0.16 [9] 1.5/2.0/2.4/3.5 8%,4%,6%,2% 2 0 0.8, 1, 0.7, 1.5 >30 0.42 3 0.13 [10] 2.5/3.5/3.0/4.0 4.4%,5.1%,3.3%,4.8% 2 4 2.4, 2.5 2.5, 2.9 >30 0.48 3 0.19 [11] 0.9/2.4/1.5/3.5 4.3%,4.6%, 3.3%,4.0% 2 2 2.0, 2.1 1.5, 2.5 >29 0.23 3 0.18 This work 1.8/3.6/2.4/5.0 10%,6%,4%,3% 2 8 1.9, 2.4, 2.5, 2.6 >30 0.20 3 0.12 888 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 59, No. 4, April 2017 DOI 10.1002/mop