диафрагмированные волноводные фильтры / b7a1cad5-b088-4d84-ae67-5d6c78c15ca8

Abstract-A novel microstrip antenna with filtering characteristics is designed for wireless communication system. This antenna with filtering characteristics consists of two microstrip open-loop resonators, a coupled line and a microstrip antenna. The coupled line plays a role of admittance inverter, meanwhile it provides energy for antenna. By this way, the antenna performs not only a radiator, but also the last resonator of the bandpass filter. Through this integrated design, it not only reduces the overall size of the filter and the antenna, but also reduces the loss. The simulation results show that the filterantenna achieves good filtering and impedance bandwidth characteristic.

I.INTRODUCTION

In wireless communication system, antenna and filter, whose performance decides performance of wireless communication system, are crucial components in RF frontend. Thus, it’s significant to design a compact module with good radiation and filtering characteristics.

Several researches on integration design of antenna and filter could be seen in [1]-[3]. In [1], the stacked four patches couple each other to generate resonance, which makes microstrip antenna with filtering characteristics. Though this structure has radiation and filtering characteristics, synthesis approach is not used. It is not easy to generalize. In [2]-[3], an extra impedance transformation structure was used in between filter and antenna. By this method, good impedance bandwidth and filtering characteristics have been got. However, this transformation structure increases the complexity of the overall system, together with its weight, size, and losses. In recent years, synthesis approach has been used to integration design of antenna and filter [4]-[7]. This integration approach reduces the filter-antenna size and the transition loss between the filter and antenna. However, in [4]-[5], the couple structure between filter and antenna is very complex, which is hard to design. In [6]-[7], filter-antenna doesn’t have good filtering characteristics, especially the band-edge selectivity.

In this paper, a novel microstrip antenna with filtering characteristics is designed for wireless communication system. By synthesis approach, two microstrip square open-loop resonators, a coupled line, and a microstrip antenna are integrated to a filter-antenna. In this method, the last resonator of bandpass filter and the load is replaced by microstrip antenna. It reduces transition loss between filter and microstrip. The total loss of the filter-antenna is almost

identical to the insertion loss of filter alone. The filter-antenna design and simulation results are given blow. The results indicate that filter-antenna has great radiation and filtering performance.

II. MICROSTRIP FILTER-ANTENNA DESIGN

Before the integrated filter-antenna is designed, a prototype three-pole microstrip square open-loop resonator filter is synthesized as shown in Fig.1. A three-pole Chebyshev bandpass filter with a 0.1-dB equal-ripple and a fractional bandwidth (FBW) of 10% at 2.45GHz center frequency is designed using standard filter synthesis techniques [8]. The coupling coefficients between resonators are k12=k23=0.092, and the external quality factors of the resonators at the input

and output port are Qext,1=Qext,2=10.3.This filter is printed on a substrate with dielectric constant 2.65 and 0.6 mm thickness.

Resonator2

Figure 1 Reference three-pole passband filter

Fig.2 (a) shows the proposed filter-antenna which consists of two microstrip square open-loop resonators, a coupled line and a microstrip antenna. There is not needed an impedance transformation structure between the filter and the antenna in the proposed integration approach. Therefore, there is nearzero transition loss between the filter and the antenna. Since the antenna is to be designed as the last resonator of the filter, the first step is to synthesize the quality factor of the antenna QA , which can be extracted using [9]

Where Zin is the input impedance of the structure at the antenna feed point shown in Fig. 2(b). The ground plane of the antenna, which is also the ground plane of the circuitry, has a

fixed size of L W=78 80mm2. The resonant frequency of the antenna is decided by the length of the antenna, which is 39mm and is approximately a quarter-wavelength for 2.45GHz. The QA of the antenna is dependent on the antenna width. Finally, W1 is selected to be 45mm. In order to connect the antenna to the second square open-loop resonator, a coupled line as admittance inverter is used. The structure ensures that the microstrip antenna acts as a resonator of the filter.

The length (y1+y2) of the coupled line is nearly equal to a quarter-wavelength at 2.45 GHz. The more information about coupled line could be seen in [10]. The width and the gap of the coupled line are 0.6mm and 0.54mm. The HFSS 17 (High Frequency Structure Simulator) is used to simulate and optimize. The final optimized size is shown in table 1.

W1

L1

s2

w1

y

(b)

Figure 2 (a) Integrated three-pole filter-antenna . (b) Reference microstrip antenna..

TABLE I THE OPTIMIZED PARAMETERS

III. SIMULATION RESULT

The HFSS17 is used to simulate reflection coefficient and gain curve of the filter-antenna. The reflection coefficient is below -12dB in the entire band, the bandwidth is about 6.4% (2.37-2.53GHz), as shown in Fig.3. It also provides the curve of gain varies with the frequency. The curve shows the filterantenna has flat gain in the pass-band with a variation of amplitude about 0.5dB. The out-of-band filer-antenna gain response is less than -10dBi, which indicates the filter-antenna with good selectivity in the out-of-band.

Frequency (GHz)

Figure 3 Simulated reflection coefficient and gain of the proposed filterantenna

The filter-antenna simulated results of the E-plane and H- plane radiation patterns at 2.45 GHz are presented in Fig. 4. The filer-antenna exhibits good radiation pattern with cross polarization levels below -30dB within the main beam. The E- plane and H-plane pattern are symmetrical. The 3dB beamwidths of E-plane and H-plane pattern are 78 and 70 , respectively.

(b)

Figure 4 Simulated radiation patterns of the proposed filter-antenna

(a) E-plane (b) H-plane.

IV. CONCLUSION

A new compact microstrip antenna with filtering characteristics is designed by synthesis method. A quarterwavelength coupled line is used to integrate the antenna and the microstrip filter resonators, given that the antenna containing the radiating element is one of the filter resonators. The proposed filter-antenna provides great skirt selectivity as the conventional band-pass filter, flat antenna gain in the passband, and high suppression in the out-of-band. The electrical response of the microstrip filter-antenna presents great filtering and radiation functions. Owing to these results, the

microstrip filter-antenna is very suitable for use in the RF front-end of modern wireless communication system.

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