A design of a planar dual-mode filter is proposed and developed for satellite and wireless communication systems. The novelty of the proposed structure consists of replacing simple diagonal design with a starlike one. This offers the ability of controlling the central frequency and the bandwidth. The filter was implemented on Rogers substrate with 10.8 dielectric constant. The proposed filter structure is 37% smaller in size in comparison with traditional dual mode filters.
© 2005 Optical Society of America
A planar dual-mode filter was proposed by I. Wolff  in 1972, and has received great attention in the design of high-quality filters. That filter was achieved by coupling two degenerate modes into a geometrically symmetrical resonator with asymmetrical feed lines. When a perturbation element is added, while the geometrical symmetry is preserved, two transmission zeros can be obtained for the ring dual mode filters . In 1999, Zhu, Wecowski and Wu  applied the principle of coupling of two degenerate modes to a square patch resonator with two unequal length diagonally crossed slots. This design is referred to as the traditional design in this paper. In 2000, Cassinese et al.  proposed a new square patch resonator design with diagonally crossing slots and additional symmetrical transverse cuts on each side of the patch.
This paper proposes a novel dual-mode microstrip square patch filter, which exhibits an elliptic response in frequency band 800–1900 MHz intended for mobile and satellite communication. A narrow band, dual-mode cross-slotted filter in a compact size has been manufactured and tested. Both lossless and lossy cases were considered. The new filter was fabricated on a Rogers Panel (RT6010) having thickness of 0.635mm and a relative dielectric constant 10.8. The filter was designed and simulated using a full-wave EM simulator .
2. Filter design and characterization
Figure 1(a) shows the traditional design; in Fig. 1(b) – 1(d) we show our new designs, in order of their increasing complexity. The perpendicular feeding lines excite two degenerate modes. These modes correspond to the and in a square patch resonator, where the z axis is perpendicular to the ground plane . The coupling between modes was achieved by introducing a differential length ΔL = L1 - L2, where L1 and L2 are the lengths of the diagonal slots as shown in Fig. 1. For most of our designs, we selected the external coupling gap of d=0.1 mm, which can be easily manufactured by an inexpensive photolithographic technique. A weak mode coupling was achieved for ΔL= 0.05 mm. As the differential length ΔL was increased, a stronger coupling of the degenerate modes was obtained, as shown in Fig. 2(a) and Fig. 2(b).
When undercoupled, no filter response could be obtained. The simulations confirmed the coupling strengthening with the ΔL increase from 0.15 mm to 0.25 mm . For a further increase of δL, the filter exhibits high in-band ripple and low return loss, as shown in Fig. 2.
We next introduced a vertical cross of length L3 into the preliminary dual-mode filter structure, centred at the intersection of L1 and L2 slots. The layout of this design is shown in Fig. 1(b). A significant shift in the central frequency of about 130 MHz was found with the introduction of the L3 cuts. The frequency response is given in Fig. 3. When vertical cross cuts were added, the central frequency shifted towards lower frequency, the bandwidth became narrower, and the return loss increased. In the design L1 =25.75 mm, L2=25.50 mm and L3=19.60 mm, the central frequency shifted from 1.15 GHz to 1.02 GHz.
For the next design, we introduced two fringe slots on each side of the preliminary square patch design. The length of the fringe slots is L4, and were positioned a distance L5 from the corner of the square. The layout of this design is shown in Fig. 1(c). Adding fringe slots with L4=4.50 mm and L5=6.70 mm, led to shifting the central frequency from 1.15 GHz to 0.98 GHz, to narrowing bandwidth and to increasing the return loss.
The final design is shown in Fig. 1(d). It introduces vertical cross and fringe slots to the earlier cross-slotted square patch. With both vertical cross and fringe slots, the central frequency is lowered at least 25% and the bandwidth decreased at least 65%. Comparing the designs from Figs. 3 and 4 one can see, (Fig. 3), that the effect of the fringe slots on the resonant frequency and bandwidth is greater than that of the vertical cross. Figure 3 and Table 1 show a comparison between the traditional design, and the series of our progressively more complex designs. One easily observes that the proposed designs have much better rejection compared to the traditional one. All these designs have been implemented using Rogers substrates. The measurement for the design with fringe slots [Fig. 1(c)] is shown in Fig 4. We note that the central frequency is shifted to 0.98 GHz and the bandwidth is 11.64 MHz. We also simulated another comparison on the square patch size of different designs. Table 2 shows the comparison of all the designs mentioned in Figs. 1(a) to 1(d) with same resonant frequency. From Table 2, one can easily observe the proposed designs have much smaller patch size requirement.
An improved design of planar dual-mode filter was proposed, manufactured, tested and measured. It is based on overlapping several cross- and star- designs. The control of the resonant frequency and bandwidth has been studied and its performance can be reported. The final proposed design is 37 % smaller in size in comparison with traditional layout, and it provides transmission zeros on both sides of the bandwidth. It is expected that the new design will offer a useful option in the high temperature superconductor (HTS) technology.
References and links
1 . I. Wolff , “ Microstrip bandpass filter using degenerated modes of a microstrip ring resonator ,” Electron. Lett. 8 , 163 – 164 ( 1972 ). [CrossRef]
2 . M. Guglielmi and G. Gatti , “ Experimental investigation of dual-mode microstrip ring resonators ,” in Proceedings of 20th European Microwave Conference, ( Budapest , 1990 ), pp. 901 – 906 .
3 . L. Zhu , P.-M. Wecowski , and K. Wu , “ New planar dual-mode filter using cross-slotted patch resonator for simultaneous size and loss reduction ,” IEEE Trans. Microwave Theory Tech 47 , 650 – 654 ( 1999 ). [CrossRef]
4 . A. Cassinese , F. Palomba , G. Pica , A. Andreone , and G. Panariello , “ Dual mode cross-slotted filters realized with superconducting films ,” App. Phys. Lett. 77 , 4407 – 4409 ( 2000 ). [CrossRef]
5 . Sonnet Software , Inc. Sonnet Suites User’s Guide, 9.0 ed ., ( Sonnet Software, Inc., North Syracuse, NY , 2003 ).
6 . J. S. Hong and M. J. Lancaster , “ Bandpass characteristics of new dual-mode microstrip square loop resonators ,” Electron. Lett. 31 , 891 – 892 ( 1995 ). [CrossRef]