диафрагмированные волноводные фильтры / bc0d3e0b-c30e-4c89-bd65-d6233c814e62

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© 2008 Wiley Periodicals, Inc.

AN ULTRA-WIDE BANDPASS FILTER WITH GOOD OUT-OF-BAND PERFORMANCE

Zhong Li, Guang-Ming Wang, Chen-Xin Zhang, and Ge-Nong Long

Radar Engineering Department, Missile Institute of Air Force Engineering University, Sanyuan, Shanxi 713800, China; Corresponding author: lihejun_5@sina.com

Received 19 November 2007

ABSTRACT: An ultra-wideband microstrip filter with good out-of-band performance is designed and demonstrated. The filter design is based on a circuit model for an optimum short-circuited stub transmission-line filter whose unit elements or connecting lines are nonredundant. Substituting the connecting lines by the lowpass filter using stepped impedance hairpin resonator, the undesired spurious bands can be efficiently suppressed, and the designed filter length is reduced. According to the different requirements, the broadband bandpass and lowpass filter can be independently designed and modified, which makes the filter more convenient and easily implemented. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 1735–1737, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop. 23517

Key words: ultra-wideband; microstrip filter; bandpass filter; stepped impedance hairpin resonator

1. INTRODUCTION

Owing to the fast development of broadband wireless communication systems, the design of wide and ultra-wide bandpass filters draws much attention. Many researchers have been investigating various kinds of ultra-wideband (UWB) bandpass filters using the microstrip structure, which is suitable for the printed circuit boards. The multiple-mode resonators (MMR) is used firstly to design UWB band pass filters by Zhu et al., the designed filter can

make up a 110% passband covering the whole UWB band [1]. Coupled lines using backside aperture on ground plane for enhancing coupling [2] and interdigital coupled lines [3] with multimode resonator are also used for realizing ultra-wide pass band. The hybrid microstrip and coplanar waveguide structure with tightened coupling degree are proposed to make ultra-wide band filters in [4]. For flatter group delay in a wideband bandpass filter, the shunt short-circuited stubs filter are employed in [5]; the optimum UWB filter bandwidth is 10.65 GHz at a midband frequency of 8.9 GHz for a factional bandwidth of about 120%. The shunt two-section open-circuited stubs are employed to produce transmission zeros, the filter has sharp attenuations and high selectivity [6].

Although these filters can achieve the UWB bandpass characteristic, they usually introduce some spurious bands in the communication system. These undesired bands become an important drawback for ultra-wide bandpass filters performance due to their proximity to the pass band of interest. To gain good stopband rejection, the traditional design approach is cascading the broadband bandpass and bandstop filters. Obviously, the method makes the filter to have greater size and more loss. In this article, a novel method to suppress the spurious response of microstrip bandpass filter is proposed. By embedding the lowpass filter using stepped impedance hairpin resonator (SIHR) into the shunt-stub UWB bandpass filter, the stopband rejection bandwidth of the bandpass filter can be significantly improved, and the total filter structure is more compact.

2. THEORY ANALYSIS AND DESIGN

2.1. Shunt-Stub UWB Bandpass Filter

The configuration of the optimum short-circuited stub bandpass filters consists of a cascade of shunt stubs of equal length alternating with uniform transmission lines, each of them twice the stub electrical length. Although the filter consists of only n stubs, it has an insertion function of degree 2n 1 in frequency so that its passband response has 2n 1 ripple. Therefore, this kind filter will have a fast rate of cutoff compared with n ripples for an n-stub bandpass filter.

The designed UWB bandpass filter has a pass band of 1.4 –5.8 GHz, the central frequency is about 3.6 GHz. To determine the impedance values of the short circuit stubs and line elements, the tabulated element values supplied in [7] for optimum distributed highpass filters are used. The characteristic impedances of the connecting lines are all close to the terminal impedance of 50 . The filter implemented on a substrate with dielectric constant r 4.1, thickness h 1.0 mm. Considering the impedance consistency with the embedded lowpass filter, through the simulation and optimization by the commercial software Ensemble 8.0 based on method of moment (MOM), all the connecting lines can be modified to have a 50 characteristic impedance, and the three stubs impedance is decreased to best allowing the fabrication tolerances. The resulting filter size values are shown in Figure 1(a), it has a vertical symmetry structure and the three stubs have the same length 11.5 mm. The simulation results depicted in Figure 1(b), five reflection zeros can be observed in the pass bands for the three-degree UWB filter.

2.2. SIHR Lowpass Filter

The compact elliptic-function lowpass filter use a single microstrip SIHR with direct-connected feed lines, as shown in Figure 2(a). The SIHR consists of the single transmission line l2 (the characteristic impedance is Z2) and coupled lines with a length of l1 (the evenand odd-mode impedance are Zoe and Zoo, respectively). By selecting Z2 [mt] ZooZoe , the size of the SIHR is smaller than that

Frequency (GHz)

(b)

Figure 1 (a) The short-circuited stubs ultra-wideband filter and (b) simulated S-parameters

network analysis (VNA) Agilent 8720ET. The measured S- parameters in Figure 3(b) show that the UWB bandpass filter 3-dB bandwidth is 4.4 GHz (1.4 –5.8 GHz), the huge stopband is obtained up to 20 GHz, the in-band insert loss is lower than 1.5 dB, and return losses are better than 10 dB. The insert loss tended to increase toward higher frequencies, which would account for the frequency-dependent losses of the two SMA connectors and the dielectric material. The measured group delay in Figure 3(c) varies between 0.4 and 0.6 ns in the pass band. This means that the developed UWB filter has a good linearity as well. A comparison of the topologies of the filter in Figures 1(a) and 3(a), the proposed filter obtains 23% length reduction.

w1 Zoe, Zoo

w2

Z2

(a)

of the conventional hairpin resonator. This lowpass filter can provide the advantages of compact size, sharp cutoff, and wide stop band frequency response [8].

According to the equivalent circuit in [9], the elliptic-function low pass filter is synthesized that the 3-dB cutoff frequency is [mt] 5.8 GHz and the attenuation pole is about 11.4 GHz, which is the center frequency of the first spurious pass band in Figure 1. Through the electromagnetic simulation and optimization, the dimensions of the SIHR are as follows: w1 2.5 mm, l1 2.3 mm, w2 0.8 mm, l2 4.3 mm, and gap size s 0.2 mm. To gain wideband attenuation, two resonators are linked directly to construct the cascaded SIHR, and the layout is shown in Figure 2(b). The parallel-coupled lines length l1 can be turned to adjust transmission pole locations so as to improve the passband characteristics and change the location of the attenuation pole. These different layout and size are simulated, and the result is shown Figure 2(c). From the Figure 2(c), we find that the cascaded SIHR structure provides a much sharper cutoff frequency response and deeper rejection band when compared with the results of using the single SIHR, and the attenuation pole will tend to drop with the l1 increasing.

3. EXPERIMENTAL RESULTS

The proposed filter structure is shown in Figure 3(a), substituting the connecting lines by the cascaded SIHR lowpass filter. To gain broad stop band, two different couple line length (l1 2.3 mm, 2.7 mm, respectively) cascaded SIHR are employed. Something is important that the electrical length between the adjacent shunt stubs must be constant after substituting.

The UWB filter is fabricated on a substrate with dielectric constant of 4.1, thickness of 1.0 mm, measured by the vector

(c)

Figure 2 (a) Single SIHR layout, (b) cascaded SIHR layout, and (c) the simulated insert loss under different SIHR layout and size

34.2mm

(a)

Frequency (GHz)

Frequency (GHz)

(c)

Figure 3 (a) The proposed ultra-wideband filter, (b) measured S-param- eters, and (c) measured group delay

4. CONCLUSION

A novel UWB bandpass filter topology with broad stopband rejection is proposed. The experiment filter has 122% 3-dB bandwidth, less than 1.5 dB in-band insert losses, the variation of group delay less than 0.2 ns, and a wide stopband bandwidth with 17 dB attenuation up to 20 GHz. The design method is that the broadband stopband filter is internally embedded within the section between two short-circuited stubs of the broadband bandpass filter, which is not increase the size. With the lowpass and bandpass filter designed separately, the method is more convenient and easily implemented. This compact and high performance lowpass filter should be useful for many broad system applications.

REFERENCES

1.L. Zhu, S. Sun, and W. Menzel, Ultra-wideband (UWB) bandpass filters using multiple-mode resonator, IEEE Microwave Wireless Compon Lett 15 (2005), 796-798.

2.L. Zhu, H. Bu, and K. Wu, Aperture compensation technique for innovative design of ultra-broadband and microstrip bandpass filter, IEEE MTT-S Int Microwave Symp Dig 1 (2000), 315-318.

3.S. Sun and L. Zhu, Capacitive-ended interdigital coupled lines for UWB bandpass filters with improved out-of-band performances, IEEE Microwave Wireless Compon Lett 16 (2006), 440-442.

4.N. Thomson and J.S. Hong, Compact ultra-wideband microstrip/coplanar waveguide bandpass filter, IEEE Microwave Wireless Compon Lett 17 (2007), 184-186.

5.J.S. Hong and H. Shaman, An optimum ultra-wideband microstrip filter, Microwave Opt Technol Lett 47 (2005), 230-233.

6.P. Cai, Z.W. Ma, X.H. Guan, T. Anada, and G. Hagiwara, Synthesis and realization of ultra-wideband bandpass filters using the z-transform technique, Microwave Opt Technol Lett 48 (2006), 1398-1401.

7.J.S. Hong and M.J. Lancaster, Microstrip filters for RF/microwave applications, Wiley, New York, 2001.

8.L.H. Hsieh and K. Chang, Compact lowpass filter using stepped impedance hairpin resonator, Electron Lett 37 (2001), 899-900.

9.L.H. Hsieh and K. Chang, Compact elliptic-function low-pass filters using microstrip stepped-impedance hairpin resonators, IEEE Trans Microwave Theory Tech 51 (2003), 193-199.

© 2008 Wiley Periodicals, Inc.

RESONANT MICROSTRIP MEANDER LINE ANTENNA ELEMENT FOR WIDE SCAN ANGLE ACTIVE PHASED ARRAY ANTENNAS

K. S. Beenamole,1 Prem N. S. Kutiyal,1 U. K. Revankar,1 and V. M. Pandharipande2

1 Electronics and Radar Development Establishment, Bangalore, India; Corresponding author: ksbeena@yahoo.com

2 Department of ECE, Osmania University, Hyderabad, India

Received 19 November 2007

ABSTRACT: A compact, wide bandwidth, wide beamwidth resonant microstrip meander line antenna element is reported for active phased array applications. The antenna element offers a return loss better than10 dB over a bandwidth of 12% in S-band with a beamwidth of 130° in E-plane. The element has been tested in an array for its wide-angle scan performance. Simulated and measured results are presented. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 1737–1740, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23531

Key words: meander line antenna; wide band wide beam patch antenna; phased array element

1. INTRODUCTION

Active phased arrays have now become a practical proposition for modern day radar systems, by overcoming the major problems of low reliability and low efficiency inherent in the passive phased array configurations. Active phased arrays require wide band, wide beam antenna elements with low cross polarization levels for obtaining a wide array scan zone over a broad bandwidth. Many different types of radiating elements have been used in phased array radars operating in different frequency bands. In this work, a new type of microstrip antenna element has been studied theoretically and experimentally, to be employed in an active phased