A COMPACT TRIANGULAR PATCH BANDPASS FILTER James R. Kelly and Rob D. Seager Wireless Communications Research Group, Department of Electronic and Electrical Engineering, Loughborough University, Loughborough, Leicestershire, England, LE11 3TU; Corresponding author:
[email protected] Received 22 June 2008 ABSTRACT: This article describes an effective mechanism for miniaturizing a microstrip triangular patch resonator. Measurement results show that the technique can yield ⬃56% size reduction. The article also provides design equations which can be used to determine the size of discontinuity required in order to achieve a specific resonant frequency. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 566 –570, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24099 Key words: triangular patch resonator; microstrip filters; microstrip resonators; dual-mode resonators; miniturization 1. INTRODUCTION
The microstrip triangular patch is a valuable circuit element that has been used to produce circulators and patch antennas [1], as well as bandpass and bandstop filters. Patch resonators offer several important advantages in comparison with their linear counterparts. This includes support for dual-mode operation as well as higher power handling capability and reduced conduction losses. Their disadvantages include increased radiation loss and larger surface area [2, pp. 100]. Although there are several microstrip patch resonators, within the literature, that incorporate triangular sections [3] relatively few are wholly triangular [4 –10]. For this reason, there is considerable scope for producing new and exciting research in this area. There are six basic topologies for bandpass/bandstop filters based around the triangular patch resonator [11, pp. 24]. In 2000 Hong and Lancaster [8] presented an important triangular patch bandpass filter, to the microwave community. The filter is based around an isosceles triangular patch resonator. Although larger than some of the others in the literature, this resonator is very useful because it supports dual-mode operation. Section II of this paper builds on the achievements of Hong and Lancaster by presenting a more compact version of their filter. It was also discovered that one could miniaturize the filter’s resonators by utilizing Chauraya’s slit-slot discontinuity (A. Chauraya, private communications, 2002). An explanation for the miniaturization effect is provided towards the end of the section. Section III presents a series of polynomial equations which connect the dimensions of the discontinuity to the centre frequency of a 3-pole filter. The range and accuracy of these equations is reported in Section IV.
modes is excited. Hong and Lancaster designate the fundamental degenerate modes by the numbers 1 and 2. To operate the filter within mode 2, the input and output feed-lines should be tap connected to the slanted sides of the first and third resonators [this design will be referred to as the HL design, see Fig. 1(a)]. The reader will note that the tap connected feed-lines, which appears in this design, are positioned closer to the base of each triangle than the apex. In addition, resonator 2 is displaced, along the x-axis from its neighbors, thus creating a large coupling gap. In the modified design, announced here, [see Fig. 1(b)] the patches are aligned. Additionally the feed-lines are repositioned so that they are coaxial with the vertical midline. Microwave measurements for this filter reveal that its centre frequency is 3.94 GHz. This is 18.76% lower than that of the HL Design (⬃4.85 GHz). By introducing a novel slit-slot discontinuity [12, 13] into the base edge of each triangular patch, it was possible to reduce the filter’s centre frequency, for a fixed set of patch dimensions [see Fig. 1(c)]. Figure 2 defines the terminology used to describe the different geometrical features of this discontinuity. The resonators of the reference filter incorporated 0.25 mm (long) by 0.5 mm (deep) slits and 2 mm2 slots (nominal dimensions). For the purpose of this study, the substrate parameters as well as the dimensions of each resonator were identical to those used by Hong and Lancaster. To be specific, the filters were fabricated on a 1.27 mm thick Rogers RT/duroid substrate having with a relative dielectric constant of 10.8. The base (b) and height (h) of each triangle was 10 mm and 15 mm, respectively. This was necessary to build upon the foundation of knowledge laid down by Hong and Lancaster, and to facilitate comparison with their results. Adjacent triangular patches, in the new design, were separated by a coupling gap of 0.5 mm. The dimensions of the slit-slot discontinuity control the degree of resonator miniaturization. The following example serves to illustrate the high degree of miniaturization that can be achieved. One of the filters fabricated as part of the parametric study (described below) incorporated resonators loaded with 0.25 mm (long) by 0.5 mm (deep) slits and 5 mm2 slots (nominal dimensions). Measurement results suggest a centre frequency of only 2.15 GHz, for this filter.
2. FILTER STRUCTURE, DIMENSIONS AND THEORY OF OPERATION
When a patch resonator is shaped into the form of an equilateral triangle it supports a pair of fundamental degenerate modes [9, 10]. These modes become separated in frequency when: (1) a triangular portion of the apex is removed to leave a stub, or (2) the patch is reshaped into an isosceles form. Under such conditions the modes are described as split degenerate. In 2000 Hong and Lancaster [8] demonstrated practical designs for a pair of microstrip bandpass filters, which utilized an isosceles triangle as their resonant element. Each of the resonators within these filters contribute just a single transmission pole to the frequency response of the complete filter. The topology of the filter and the configuration of the feed-lines determine which of the two
566
Figure 1 (a) The HL design, (b) the new design, and (c) loading resonators within the new design by a slit-slot discontinuity
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 2, February 2009
DOI 10.1002/mop
This value of centre frequency is 45.43% lower than that for the design shown in Figure 1(b), and 55.67% lower than that of the HL design. The research discussed in this section relied heavily upon full-wave computer simulation conducted using Micro-Stripes, Version 6.1, from CST. 2.1. The Mechanism for Resonator Miniaturization Consider an undisturbed isosceles triangular patch resonator which is fed using tap-coupled lines attached to the centre of each slated side. Under these conditions the patch exhibits a resonance across each slanted side, as well as the base edge. The introduction of a slot discontinuity interrupts the resonant path along the base edge of the patch. To be specific the path length is extended, because it must now meander around the top and bottom of the slot. The insertion of a combined slit-slot discontinuity reduces the resonant frequency still further. This occurs because the slit-slot discontinuity prohibits half-wave resonance across the base edge of the triangle. This resonance is replaced by one taking a complex meandering path around the periphery of the slot. 3. DERIVATION OF SYNTHESIS EQUATIONS
A parametric study was conducted to provide empirical data from which to derive the synthesis equations. However, this was not a full parametric study because the dimensions of the patch (b ⫽ 10, h ⫽ 15), as well as the substrate parameters (h ⫽ 1.27 mm, r ⫽ 10.8) remained fixed. The following geometrical features were included within the parametric study: slot length, slot width, slit length, and slit depth. All (except one) of the linear dimensions, used to describe the discontinuity, were (nominally) identical, at any one time, to those of the reference filter. When studying the effect of the slit length, for example, three filters were manufactured. The only difference between each of these three filters was the length of the slit. Table 1 lists various performance parameters for filters within the parametric study. In this table, the symbol “fc” denotes the centre frequency of a 3-pole filter [such as that shown in Fig. 1(c)], and “GD” is the shorthand for the group delay. 3.1. Percentage Bandwidth Basic filter theory says that the bandwidth of a coupled resonator bandpass filter will increase in proportion to the strength of coupling between adjacent resonators. For this particular filter structure, the slit
TABLE 1 Performance Parameters for Filters within the Parametric Study (from Measurement Data) Dimension (mm) fc (GHz) 3 dB BW (%) 10 dB BW (%) GD Ripple (ns) Ref. filter [Fig. 1(c)] N/A 3.438 Slot width 0.5 3.844 2.5 3.269 6.5 1.9153 Slot length 1 3.574 3 3.314 5 3.080 8 2.396 9 2.260 Slit length 0.5 3.436 0.75 3.434 1 3.434 Slit depth 0.25 3.511 0.75 3.340 1.5 3.072 3.5 2.348 Coupling gap 0.2 3.407 0.3 3.432 0.7 3.441
15.300
21.299
38.302
15.383 14.512 15.973
22.984 22.083 26.203
26.639 29.396 14.903
15.677 15.455 15.181 16.642 16.126
21.543 21.782 21.543 22.667 21.451
35.071 41.021 19.701 10.754 9.076
15.408 15.516 15.718
21.511 21.723 21.724
16.985 19.607 73.969
15.469 15.439 14.661 15.080
22.616 21.718 21.151 23.279
31.046 14.824 14.900 16.338
21.692 18.929 12.688
29.461 25.839 19.682
14.747 15.491 33.853
depth can also be used to control the bandwidth (however, the coupling gap has a far stronger effect). To be more specific, an increase in the slit depth (from 0.25 mm to 3 mm) affects a simultaneous reduction in both centre frequency and percentage bandwidth. The bandwidth and centre frequency of the filter are virtually unaffected by changes in the slit length. This is important because it means that small fabrication errors, affecting this parameter, will have a minimal effect on the filter performance. 3.2. Centre Frequency The centre frequency of the filter may be controlled by any one of three different geometrical features: the slit depth, the slot width or length. The slot length and width have a considerable effect on the centre frequency, but a relatively small effect on the percentage bandwidth. Varying the slot length and width (between the minimum and maximum values considered in this study) alters the bandwidth by just 9% and 14%, respectively. No simple equation has yet been found which relates the percentage bandwidth to changes in the slot width and slot length. Three polynomial synthesis Eqs. (1)–(3) were derived from the empirical data contained in Table 1. Each equation was obtained by curve fitting to a graph produced using nine data points. The function of these equations is to determine the slot length (l ), slot width (w ), and slit depth (d ) required to synthesize a 3-pole filter exhibiting a particular centre frequency (fc ). l ⫽ ⫺ 2.5653f 2c ⫹ 8.8886fc ⫹ 1.8264
(1)
w ⫽ ⫺ 0.1199f 2c ⫺ 2.4936fc ⫹ 11.846
(2)
d ⫽ ⫺ 2.8926f c ⫹ 10.331
(3)
4. RANGE AND ACCURACY OF THE SYNTHESIS EQUATIONS Figure 2
Terms used connection with the novel discontinuity
DOI 10.1002/mop
Table 2 specifies upper and lower limits on the values of filter centre frequency which can be achieved by altering the dimensions
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 2, February 2009
567
TABLE 2 Upper and Lower Limits on the Filter Centre Frequency Centre Frequency (GHz) Feature
Minimum
Maximum
0 1.8 1.9
3.6 3.9 3.5
Slot length Slot width Slit depth
of the slit-slot discontinuity within each resonator. For centre frequencies above and below the stated values, the synthesis equations generate unphysical results (i.e. negative values). In practice, the achievable range of centre frequency values will also be restricted by the limitations of the p.c.b. fabrication facilities and by space constraints within the resonator. The sensitivity of each centre frequency synthesis equation was assessed quantitatively (see Table 3). This was achieved by inserting an initial value for the centre frequency into each equation. This value was then incremented by a fixed percentage (10%). On each occasion, the resulting change in feature size was noted, and expressed as a percentage (see column 2 of Table 3). These results clearly indicate that the sensitivity of each equation varies as a function of the initial value of centre frequency. At low frequencies, a 10% shift in the centre frequency is affected by a relatively small percentage change in the feature size. If the nominal feature size is already small then this percentage will be comparable with the fabrication tolerances, associated with p.c.b. production. The results, therefore suggest that filters, intended for operation at low frequencies, should not incorporate small features. For higher values of centre frequency, the equations clearly become increasingly insensitive.
Figure 3 Measurement results for two filters in the slit depth study
5. EXPERIMENTAL RESULTS
Figures 3– 6 present a selection of microwave measurement results, taken using a HP8753D Vector Network Analyzer. In certain filter applications, it is important to ensure linear phase response within the passband. For this reason, Figure 4(a) shows the transmission phase response of a filter within the slit depth section of the parametric study. From inspection of this figure, one can clearly see that the filter affords a reasonably linear phase response, throughout the majority of the passband. This response is typical of that exhibited by filters in other parts of the parametric study. Several simulations were also conducted to determine the accuracy with which a filter could be modeled. Figure 6 compares simulation and measurement results for filters within the slot length study. There is a good standard of agreement between these
TABLE 3
The Sensitivity of the Filter Synthesis Equations
Initial Centre Frequency (GHz) Slot width 1 2 3 Slot length 1 2 3 Slit depth 1 2 3
568
Change in Predicted Feature Size (%) 2.97 9.40 29.66 4.12 4.04 40.37 3.89 12.73 52.49
Figure 4 Measurement results for a filter incorporating a 0.25 mm deep slit. (a) The amplitude and phase of S21, (b) The amplitude and group delay of S21
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 2, February 2009
DOI 10.1002/mop
results, which indicates that the modified filter may readily be simulated for the purpose of engineering research and design. Many simulation packages (including Micro-Stripes) discretise the geometry according rectangular or triangular mesh cells. This leads to a type of quantization error known as staircasing. Any edge that is curved or does not run parallel to a co-ordinate axis will suffer staircasing approximation. In this particular situation the slanted sides of the triangle will suffer staircasing. Small cells were used within this region in order to minimize the size of this error. Measurements suggest that the dimensions of the fabricated filters are, on average, 53.8 m smaller than desired. 6. CONCLUSION
Figure 5 The scattering parameters associated with two filters in the slot length study
This article demonstrates a new technique for reducing the size of a microstrip triangular patch resonator. This central idea is to insert a novel reactive slit-slot discontinuity into the base edge of the triangle. This discontinuity extends the path length for the resonant current, thereby reducing the resonant frequency for a fixed set of patch dimensions. The article uses measurement results to show that one can reduce the dimensions of the resonator by ⬃56% in comparison with predecessors. The article also explains how different features of the discontinuities effect the filter’s frequency response. Finally, a series of simple polynomial equations were derived so that the engineer can determine the dimensions of the discontinuity which are required to yield a particular value of filter centre frequency. The range and accuracy of these equations is quantified. ACKNOWLEDGMENTS
The authors wish to express their gratitude to the following companies and organizations: (1) Flomerics for the use of their MicroStripes code. Micro-Stripes is now owned and marketed by CST; (2) Leicester Circuits UK Limited, for manufacturing our filters accurately and free-of-charge; (3) Rogers Corporation for donating microwave substrate materials; and (4) The UK Engineering and Physical Sciences Research Council (EPSRC) for providing financial support for the first author’s studentship.
REFERENCES
Figure 6 The comparison between scattering parameters obtained through simulation and measurement for filters incorporating: (a) 1 mm long slots, and (b) 5 mm long slots
DOI 10.1002/mop
1. L. Liu, S. Zhu, and R. Langley, Dual-band triangular patch antenna with modified ground plane, Electron Lett 43 (2007), 1169 –1170. 2. J. Hong and M. Lancaster, Microstrip filters for RF/microwave applications, Wiley, New York, NY, 2001. 3. J.J. Yu, S.T. Chew, M.S. Leong, and B.L. Ooi, New class of microstrip miniaturised filter using triangular stub, Electron Lett 37 (2001), 1169 –1170. 4. X. Wang and Y. Li, New microstrip triangular patch resonators filters with two transmission zeros, in proceedings of Asia-Pacific conference on environmental electromagnetics, 2003, pp. 364 –368. 5. S. Chaimool, S. Kerdsumang, and P. Akkaraekthalin, A novel microstrip bandpass filter using triangular open-loop resonators, in 9th AsiaPacific Conference on Communications, 2, 2003, pp. 788 –791. 6. B. Strassner and K. Chang, Novel cymbal filter for DC-block and bandpass applications, Microwave Opt Technol Lett 28 (2001), 33–34. 7. B. Strassner and K. Chang, New wide-band DC-block cymbal bandpass filter, IEEE Trans Microwave Theory Tech 50 (2002), 1431–1432. 8. J. Hong and M. Lancaster, Microstrip triangular patch resonator filters, IEEE MTT-S Int Microwave Symp, Boston, MA, Digest 1 (2000), 331–334. 9. J. Hong and S. Li, Dual-mode microstrip triangular patch resonators and filters, IEEE MTT-S Int Microwave Symp, Philadelphia, PA, Digest 3 (2003), 1901–1904. 10. J. Hong and S. Li, Theory and experiment of dual-mode microstrip triangular patch resonators and filters, IEEE Trans Microwave Theory Tech 52 (2004), 1237–1243.
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 2, February 2009
569
11. T. Okoshi, Planar Circuits—for microwaves and lightwaves, SpringerVerlag Series in Electrophysics, Berlin, New York, 18, 1985. 12. J.R. Kelly, A. Chauraya, R.D. Seager, and J.C. Vardaxoglou, Metallodielectric electromagnetic bandgap (MEBG) lumped element equivalent circuit, in PREP poster proceedings, 2004, pp. 97–98. 13. J.R. Kelly, A. Chauraya, R.D. Seager, and J.C. Vardaxoglou, Metallodielectric electromagnetic bandgap (MEBG) lumped element equivalent circuit, in Proceedings of ARMMS, 2004. © 2008 Wiley Periodicals, Inc.
ULTRA-WIDEBAND MIMO ANTENNA WITH ENHANCED ISOLATION Shun-Yun Lin and Hong-Ren Huang Department of Electronics Engineering, Cheng Shiu University Kaohsiung, Taiwan 833, Republic of China; Corresponding author:
[email protected] Received 25 June 2008 ABSTRACT: This article presents a four-element multi-input multioutput (MIMO) antenna design. To enhance the mutual isolations for ultra-wideband operation in 2– 6 GHz, a circular slot antenna with a stepped ground plane was used as an element of the MIMO antenna. Such fabrications generate nonplanar connections and discontinuous interfaces between the elements and the system ground planes. This strategy effectively decreases the mutual coupling and provides 10 dB enhancements in isolation characteristics. Both theoretical and practical investigations on the adequate information for understanding the proposed antenna’s characteristics are implemented and examined experimentally. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 570 –573, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24091
proposed antenna were theoretically started by the commercial microwave simulation software Ansoft HFSS 10.0 and SEMCAD 13.2. To confirm the simulation results, practical prototypes were implemented and examined experimentally. 2. ANTENNA DESIGNS
The element of the proposed MIMO antenna for UWB communication system is shown in Figure 1(a). As the former study [5], it was a circular slot with radius, R, and fed by a 50-⍀ CPW line through a circular patch. The feed patch was with a radius r and an offset between the lowest border of the slot and the lowest border of the patch. The proper radii (r and R) and offset ensure the impedance matching between the feed and the slot over the desired operating band. In this study, the circumference of the slot was about one wavelength at the lowest operating frequency (2 GHz) and the offset was optimally selected to be 0.2 mm. In experiments, the element can be easily conducted by line-cutting or stamping a single metal plate (a 0.1-mm copper plate was used in this study). As integrating into the RF system, one edge of the element ground plane, which was opposite to the feeding edge, was bent and vertically connected to the system ground plane. The vertical portion with height h introduced a nonplanar connection between the element and the associated RF circuitry. As shown in
50Ȼ CPW-line feeding point
Unit: mm
r
R
50
h 52
Key words: slot antenna; MIMO antenna; UWB communication system
Y
(a) 1. INTRODUCTION
For a multiantenna system, more than two antennas operated at the same band to achieve the requirements of high transmission speed and high channel capacity is named multiple-input multiple-output (MIMO) system. When the MIMO system is implemented, the independence of each element from the others has been an important subject to ensure desired performances. In addition to raise spatial separation and software approaches, diversity polarization [1], diversity pattern [2], spatial filter [3], and shielding [4] are the common strategies to enhance the mutual isolations of the multiantenna system. The functions of the mentioned strategies are dispersive and unsuitable for ultra-wideband communication systems except for the last one. However, an additional metal box or wall is needed usually to prevent the fringing field from interfering each other. In this article, we investigate a four-element MIMO antenna with enhanced mutual isolations to meet the requirements of WiMax 802.16e (2– 6 GHz). In the proposed antenna, each element was connected to the system ground plane through a vertical portion. Such fabrications generate nonplanar connections and discontinuous interfaces between the elements and system ground plane. As any one element is excited, the induced currents at the other unexcited ports are reduced by the discontinuous interfaces, and the mutual couplings are effectively lowered. Experimental results indicate that the mutual isolations of the proposed four-element MIMO antenna are remarkably enhanced by more than 10 dB compared with the conventional type. Also, the proposed antenna achieves more than 25 dB isolations over the entire ultra-wide operation band. Investigations into the adequate information for understanding the characteristics of the
570
X
#4
3
d d
system ground plane
#2 #1
(b) Figure 1 The configuration of the proposed MIMO antenna. (a) The element, (b) whole structure. The system ground plane was printed on an FR4 substrate with thickness 0.8 m and permittivity 4.4
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 2, February 2009
DOI 10.1002/mop
