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can generate a wide stopband with four attenuation poles by modifying the dimension parameters li, for i = 1, 2, 3, 4. As shown in Figure 4, the resonator has a wide stopband from 5. 13 to 8. 25 GHz with a return loss lower than 13 dB and has little effect at the low frequency range. The dimensions include: d = 3 mm, s = 0.2 mm, c = 1 mm, h = 1 mm, w = 2.73 mm, l1 = 8 mm, l2 = 7 mm, l3 =

5. 8 mm and l4 = 4 mm. The attenuation poles are generated by the quarter-wavelength open-circuited stubs and spurlines. For the resonator, the resonant frequencies ƒ1 = 6. 65 GHz, ƒ2 = 8.03 GHz, ƒ3 =

5. 81 GHz and ƒ4 = 6. 38 GHz are corresponding to l1, l2, l3 and l4. It demonstrates that the proposed resonator could be effectively applied for out of band suppression by controlling the dimensions l1, according to different spurious responses.

Open-LOOp ResOnatOR
Bandpass FiLteR

Figure 5 shows the layout of a conventional open-loop resonator filter, which is made up of two square open loop λ/2 resonators witha 0° feed structure. The electric coupling between resonators is used to design the filter with a pair of attenuation poles for sharp skirt. For the 0° feed folded open loop λ/2 BPF, which has a 0° difference between the electric delays of the upper and lower paths, and where θ1 and θ2 are the electrical lengths of the upper and lower path, the transmission matrix of the two paths can be expressed as

s Fig. 5 Layout of a conventional BPF (a) and of the proposed BPF with spurlines and stubs at the I/O feed lines (b).

l2 l1

l´1

l3 e

l´ 3

l´2

l´ 4

(a)

(b)

Z0 ;1

Z0 ;2

f
m
gb
a
w
t
l4

Z0 ;2

Z0 ;1

s Fig. 6 Full-wave simulated performance of the conventional BPF and the proposed BPF.

;S11;

CONVENTIONAL BPF

PROPOSED BPF

s Fig. 7 Simulated current distribution of the BPF at the resonant frequency with different input phase: (a) θ = 0° and (b) θ = 90°. INPUT INPUT 0 dB –2 dB – 4 dB – 6 dB – 8 dB – 10 dB – 12 dB – 14 dB – 16 dB – 18 dB – 20 dB – 22 dB – 24 dB – 26 dB – 28 dB – 30 dB – 32 dB – 34 dB – 36 dB – 38 dB – 40 dB OUTPUT OUTPUT (a) (b)