ABSTRACT

A compact narrowband non-degenerate dual-mode microstrip filter with square shape cuts is presented. The structure is developed by loading the conventional non-degenerate dual-mode resonator by open circuit stubs at two opposite corners. The filter bandwidth is controlled by only decreasing the higher cutoff frequency of the conventional type. With Square shape cuts, return loss is improved. A 20% fractional bandwidth filter is designed and implemented on FR4 material with 4.4 dielectric constant and 1.6 mm thickness at center frequency of 1.5 GHz with passband of 1.3 GHz to 1.6 GHz. Analysis has been achieved using the IE3D simulator. Experimental results do agree with simulations.

1. Introduction

Now-a-days compact microwave filters are widely used in various wireless communication applications. Dualmode resonators have been used for such purposes. Each of dual-mode resonators act as a doubly tuned resonant circuit and therefore the number of resonators required for a given filter is reduced by half, resulting in a compact configuration. Dual-mode microstrip resonators have the advantages of low profile, simple fabrication, ease of integration in addition to low cost. The first microstrip dual-mode filter was presented by Wolff [1] in 1972. Degenerate modes based filters have been investigated in various topologies such as square patch [2], circular patch, triangular patch, square loop [3], circular ring [4] and meander shape [5]. Square and circular patches structures have negligible conductor loss but suffer from higher radiation loss. However, square loop and circular ring structures have less radiation loss but suffer from higher conductor loss, especially for thin strip conductors [2]. Degenerate dual mode filters have usually narrow bandwidth of (< %5). Filters with higher bandwidth up to 25% have been investigated using non-degenerate dual-mode structure [6–7].

2. Proposed Structure and Modes of Operation

The fields within a square patch resonator can be expanded by the TM^z_mn0 modes [2], where ‘z’ is perpendicular to the ground plane. The two fundamental degenerate modes correspond to TM^z₁₀₀ and TM^z₀₁₀ and the first higher order mode correspond to TM^z₁₁₀ [6]. These three modes can be excited simultaneously by a square shape resonator with feed lines, as shown in Figure 1. The simulated response is shown in Figure 2.

The etching of slots in square patch resonator as shown in Figure 3 decreases the resonance frequencies of the

three modes but the resonance frequency of the mode TM^z₁₁₀ decreases faster. Therefore, band pass filter behavior can be obtained [7]. This is shown in Figure 4. The resultant size and bandwidth decreases as the slot length increases. The square patch has a length W, while the slots have equal lengths L and width S. The physical dimensions of the simulated patch are W = 45.6 mm, L = 9 mm and S = 3 mm. Denoting f₁ as the resonance frequency of the degenerate modes TM^z₁₀₀ and TM^z₀₁₀ and f₂ as the resonance frequency of the mode TM^z₁₁₀. The effect of the slots length L on the resonance frequencies f₁ and f₂ for the patch is that the two resonance frequencies, f₁ and f₂, decrease as L increases [7]. The difference f₂ - f₁ can be used as first approximation of the possible bandwidth of the filter. For L = 9 mm, a fractional bandwidth of about 33% can be obtained using the given parameters.

Based on this design configuration, it is difficult to achieve bandwidth less than this value. However, loading the patch by open circuit stubs as shown in Figure 5 will decrease the resonance frequency f₂ of the mode TM^z₁₁₀ and approximately maintains the resonance frequency of the degenerate modes constant. Therefore, band pass filters with fractional bandwidth less than 25% can be achieved.

The layout of the filter with the stubs of width 1 mm and length 12 mm can be seen in Figure 5. The subsequent effect on the resonance frequencies f₁ and f₂ is

shown in Figure 6. This analysis is carried out using the moments method IE3D simulator, on a conducting patch of W = 45.6 mm on a substrate of dielectric constant 4.4, with height 1.6 mm. The slot length and width used are 9 mm and 3 mm, respectively. These parameters have been chosen to fix f₁ at 1.3 GHz.

As described in the previous section, almost no effect is observed on the resonance frequency of the first two degenerate modes TM^z₁₀₀ and TM^z₀₁₀. The resonance frequency of these modes f₁ is almost constant and equal to 1.3 GHz for stubs length of 0 to 18 mm. However, the first higher order mode TM^z₁₁₀ is highly affected and its resonance frequency f₂ decreases. This variation allows the design of narrow band filter, with careful control of its bandwidth. Bandwidth selection can be obtained by first choosing f₁ and then finding the appropriate stub lengths for a specific value of f₂.

3. Filter Design Parameters

For the proposed narrowband band pass filter the design parameters are:

Dielectric Constant = 4.4Height of Substrate = 1.6 mmCorresponding length of the Square patch, W = 45.6 mmCorresponding width of the slots, S = 3mmCorresponding length of the slots, L = 9mmCorresponding width of the stubs, W_s = 1mmCorresponding length of the stubs, L_s = 16mmThe layout of the non-degenerate dual-mode filter with stubs of length 16 mm is shown in Figure 7 and Figure 8 shows the fractional bandwidth obtained is about 20% in

the passband 1.3 GHz -1.6 GHz and the return loss is found to be 15.11dB. The layout as shown in Figure 9, is obtained after etching one square lattice of dimension of 6 mm × 6 mm, on the center of the conventional nondegenerate dual mode filter. The corresponding return loss of the bandpass filter at the center frequency is found to be improved and becomes 22.56 dB. The fabricated layout is shown in Figure 10. The simulated and measured results are shown in Figures 11, 12.

Now, with additional eight small square lattices of dimension 2mm × 2mm etched as shown in Figure 13, the frequency response can further be improved as shown in Figures 14, 15. The return loss now is 26.12 dB. Figure 16 shows the fabricated layout of the non-degenerate dual mode filter with eight square lattices.

Figure 17 shows the layout of non-degenerate dual mode filter with a carpet of square lattices of very small dimension 1mm × 1mm. It further improves the return loss to 30.06 dB which is shown in Figure 18.