measurement both in lower and upper band edge. The radiation efficiency of the filtering DRA as a function of frequency is depicted in figure 10(a). The radiation efficiency plot also confirms the filtering response. It can be observed that the efficiency remains stable approximately at 90% within passband. It drops sharply beyond passband. Three resonance dip at 3.1 GHz, 3.3 GHz and 3.45 GHz are chosen to examine the broadside radiation characteristics of the DRA.

The radiation beams pattern in E-plane (u = 0L) and H-plane (u = 90L) are shown in figures 11 (a)-(c) for those three frequencies. TE111 mode of an RDRA radiate like a short magnetic dipole in the broadside direction [16]. Measurement results verify that the main beam is pointed in broadside with a beamwidth of 90L in E-plane and 110L in H-plane. Over the operating band, co-to- cross-pol isolation is greater than 30 dB both in E-plane and H-plane. The comparison of filtering technique along with some notable performance of different filtering DRA are summarized in Table 1. It can be comprehended from the table that our design provides high selectivity with maximum number of radiation nulls. The footprint (xk0 9 yk0) of our design is optimum compared to the most of designs tabulated in Table 1. The proposed design has offered a wide controlling range of radiation nulls individually at lower and upper band-edge compared to the other designs.

4. Conclusion

A high-selective filtering DRA with multiple radiation nulls have been introduced and the performance characteristics have been verified experimentally. First, a highselective feed network (HSFN) is modeled using practically realizable simple quarter wavelength band-rejection resonators pairs and its performance is verified both in circuit simulation and EM-simulation. Next, by integrating this HSFN with an aperture coupled DRA, four controllable radiation nulls are generated both side of passband edge producing a sharp filtering performance. Footprint of the proposed structure is same as the conventional DRA. The proposed structure yields 11.8% bandwidth with almost steady broadside gain of 5.6 dBi and reasonably good skirt-be- havior beyond passband with four radiation nulls. This design could be a best fit candidate for high demanding WiMAX and TD-LTE communication as it dodges the nearby Wi-Fi bands.

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