диафрагмированные волноводные фильтры / a13d4cdf-4d4d-4b03-9c66-2692d6a356d8

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

Figure 1 Simulated return loss plots of rectangular patch, square patch, and meander line antenna elements

array for application in ground based as well as airborne radar. Patch antenna has the advantage of being low profile, lightweight, and is easy, as well as economical to manufacture and could be made available at a lower cost [1].

The major drawback of the patch antenna is its inherently narrow bandwidth. Several techniques have been proposed in the literature to improve the bandwidth which include the addition of parasitic patches [2], employing a low dielectric thick substrate, by using aperture coupled stacked microstrip antenna elements and by using meander line probe-fed antennas [3]. This design employs a meander line patch antenna element etched on a thick glass epoxy substrate. The design is experimentally validated meeting the simulated results.

2. CHOICE OF MEANDER LINE ANTENNA ELEMENT

Recently, meander line antennas are being increasingly used in personal wireless communications [4] due to their compact structure to get size reduction for a particular operating frequency. As on today, this element has not been reported for active phased array applications. In this work, a resonant microstrip meander line antenna element has been chosen to meet with the wide beam and wide bandwidth requirements, which cannot be achieved by its square and rectangular patch antenna counterparts within the available array grid size. A comparative study has been conducted among the square, rectangular, and meander line patch antennas. Simulated results conducted on electromagnetic CAD shows that the bandwidth of a square patch of size 20 mm 20 mm is 205 MHz (see Fig. 1) and that of rectangular patch of size 20 mm 32.5 mm is 223 MHz (see Fig. 1). The meander line antenna gives a bandwidth 400 MHz (Fig. 1), wherein the bandwidth enhancement is almost double when compared with square patch and rectangular patch, and the results are summarized in Table 1, where f is the bandwidth and f0 is the center frequency.

TABLE 1 Comparison Between Square, Rectangle, and Meander Line Patch Antennas

The meander line antenna element gives good radiation characteristics in E- and H-planes and a wide impedance bandwidth over the frequency of operation. Also the cross polarization levels are low ( 20 dB) to realize low sidelobe radiation patterns.

3. ELEMENT STRUCTURE

The proposed new microstrip antenna element is a resonant meander line antenna (also called rampart line antenna) [1], the

Figure 2 Meander line antenna element: (a) front view, (b) side view, and (c) photograph

Figure 3 16-Element E-plane linear array using meander line antenna element

geometry and the photograph of the element is shown in Figure 2. The meander line element consists of a meandering microstrip line formed by a series of sets of right-angled bends. The fundamental element in this case is formed by four right-angled bends and the radiation mainly occurs from the discontinuities (bends) of the structure. The right-angled bends are chamfered to reduce the right-angled discontinuity sucesptance for impedance matching. This case employs two meander sections in a total size of 2.5 wavelengths. The current directions are changing in every half wavelength and there are more than four half wavelength changes in this design. The radiations from the bends add up to produce the desired polarization depends on the dimensions of the meander line antenna. The spacing between two bends are very critical, where if the bends are too close to each other, then cross coupling will be more, which affects the polarization purity of the resultant radiation pattern. In other cases, the spacing is limited due to the available array grid space and also the polarization of the radiated field will vary with the spacing between the bends, and the spacing between the microstrip lines [5, 6]. The present meander line structure is designed with linear polarization with less cross coupling between the bends, to fit within the available array grid size.

4. CHOICE OF SUBSTRATE

Phased array antennas employ a large number of antenna elements. To realize an active phased array at the minimum cost, the radiating elements play an important role. Also, the choice of the substrate dielectric constant and thickness impacts the design in several ways, viz., the possibility of surface wave excitation with higher thickness and higher dielectric constant substrates, smaller bandwidths and lower efficiencies with higher dielectric constant materials. The loss due to surface waves can be ignored if the thickness of the substrate is less than about a tenth of free space wavelength [3]. Thus it is preferable to use an optimum thickness, low-cost substrate for containing the spurious feed radiation to minimum. In this case, a glass epoxy substrate (FR4) with dielectric constant 4.4 and thickness 3.2 mm has been selected as the

Figure 5 Measured return loss plot of meander line antenna using HP vector network analyzer

preferred substrate for the antenna element, by considering all the aforementioned factors.

5. OPTIMIZED MEANDER LINE ANTENNA ELEMENT

The meander line structure is designed on FR4 substrate with thickness 125 ml. The choice of the substrate essentially depends on the cost effectiveness of the printed antenna element for use in active phased arrays. The element is fed through an orthogonal coaxial probe. The impedance match between the feed and the meander line antenna element are being achieved by adjusting the width and length of the microstrip open end. The optimized antenna element parameters are as follows: d 0.16 g, s 0.42 g, w 0.06 g, L 0.7 g, where g is the guided wavelength in the

Figure 7 Measured radiation pattern E-theta (a) Theta 0°, (b) Theta 40°, and (c) Theta 60°

FR4 substrate at the center frequency. The feed point, length and width of the element, and the antenna substrate (thickness and dielectric constant) have all been optimized to provide a wide beamwidth as well as wide bandwidth performance. The simulation and optimization was performed with EM simulation software (IE3D) based on method of moments (MoM), and the dimensions of the optimized element is shown in Figure 2. The simulated return loss plot of the optimized element is shown in Figure 1 and the simulated radiation pattern plot in Figure 4. The radiation pattern plot shows that the 3 dB beamwidth is better than 130° at the center frequency with a cross polarization level below 20 dB.

6. FABRICATION AND TESTING OF ANTENNA ELEMENT

The antenna is etched on FR4 substrate and is fed by a coaxial connector. The element return loss has been tested using an HP vector network analyzer. The measured return loss plot of the

antenna element is shown in Figure 5. The measured impedance bandwidth is 400 MHz for 10 dB return loss. The measured E-plane and H-plane radiation pattern plots of the meander line antenna at the center and end frequencies are shown in Figure 6. The E-plane radiation pattern plots exhibits symmetry with a beamwidth better than 130°. The H-plane gives a beamwidth of better than 80°. The cross polarization is below 20 dB over the bandwidth for both E-plane and H-plane patterns. The element also gives a gain better than 5 dB over the bandwidth.

7. FABRICATION OF A LINEAR ARRAY

A linear array of 16 elements has been fabricated employing the meander line antenna elements in an E-plane array environment as shown in Figure 3. The elements are placed /2 apart to avoid grating lobes. The array has been tested with stripline feed networks. Three numbers of 1:16 way power dividers, designed in Wilkinson configuration with uniform amplitude distribution and with progressive phase shifts among the 16 output ports to scan the array corresponds to 0°, 40°, and 60°, have been fabricated. The progressive phase shifts have been achieved using stripline delay line elements at the power divider output ports. The interfacing between each of the power divider and antenna elements have been achieved through phase matched, low insertion loss cable assemblies. The array has been tested with each of these power dividers independently in an anechoic chamber. The measured radiation pattern plots at the boresight and at 40° and 60° are shown in Figure 7, for the center and end frequencies with a band width of 400 MHz. The sidelobe levels are 13 dB due to uniform amplitude excitation. The beam scanning angle is slightly different for the end and center frequencies due to the variation of the progressive phase shifts, which are realized through delay line lengths. The measured radiation pattern performance ensures the ability of the element to scan a volume of 60° in the E-plane.

8. CONCLUSION

A compact lightweight antenna element has been realized on a glass epoxy substrate utilizing the printed antenna technology. The realized printed meander line microstrip antenna element offers a wide impedance bandwidth (return loss better than 10 dB) of 400 MHz (12% in S-band frequency) and wide beamwidth of about 130° in the E-plane. The printed meander line antenna element can effectively be used to make large lightweight and low cost active antenna arrays. The element is well suited for active phased array radars with a scan angle requirement of 60°.

REFERENCES

1.I.J. Bahl and P. Bhartia, Microstrip antennas, Artech House, Dedham, Massachusetts, 1980.

2.H.Y. Wang and M.J. Lancaster, Aperture-coupled thin-film superconducting meander antennas, IEEE Trans Antennas Propag 47 (1999), 829 – 836.

3.B. Looi and C.L. Lee, A wideband mender-line probe fed patch antenna, Microwave Opt Technol Lett 37 (2003), 401-403.

4.C.M. Allen, et al., Tapered meander slot antenna for dual band personal wireless communication systems, Microwave Opt Technol Lett 36 (2005), 381-385.

5.J.R. James and P.S. Hall, Handbook of microstrip antennas, Peregrinus, London, UK, 1989.

6.P.S. Hall, Microstrip linear array with polarisation control, in IEE Proceedings, April 1983, pp 215–224.

© 2008 Wiley Periodicals, Inc.