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A novel dual-band filter using open loop ring resonator (OLRR) and defected stepped impedance resonator (DSIR) is presented in this paper. The OLRR forms one pass on the upper plane, and the DSIR forms the other pass on the lower plane. A proper designed feed structure is analyzed and used, which can provide desired external couplings to the two passes independently. At last, a dual-band filter with cascaded quadruplet (CQ) topology is designed and fabricated, which operates at WLAN bands 2.45/5.25 GHz. The simulated and measured results show good agreements and validate the proposed design method.

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A four square loops antenna above a finite ground plane is investigated for circular polarisation and gain enhancement. By choosing proper perimeters of the two pairs of loops, the axial ratio (≤3 dB) bandwidth can reach 16%. It is demonstrated that a new pair of loops added to the original dual-loop can increase the gain to 11.6 dBic. A broadband balun is incorporated for impedance matching. The antenna performance is confirmed by experimental results.Introduction: Loop antennas have been reported to radiate circularlypolarised (CP) waves if gaps or stubs are introduced in proper positions [1][2][3][4][5]. Compared with conventional spiral antennas, the loop antenna is simple in structure, but has low gain [2,3]. In recent researches, to improve gain, a short backfire dual-loop antenna [4] is applied to realise 11 dBic, or a dual-loop antenna array [5] is used to realise 14.6 dBic. Though it can increase gain, the antenna structure is complicated.According to the structure and theory of self-phased quadrifilar helix antennas [6] and single-fed crossed V-diploes [7], a novel structure is introduced that adds a pair of loops to the original dual-loop. By optimising the size of the two pairs of loops, it can obtain inductive impedance for one pair of loops and capacitive impedance for the other pair of loops [6]. So it can realise 908 the phase shift. The antenna can radiate CP waves. As the pair of larger loops has a minimum axial ratio (AR) point at lower frequency, and the added pair of smaller loops introduces a minimum AR point at higher frequency, the AR bandwidth can be increased to 16%. This is much wider than that in [6,7]. The gain of the antenna is improved as well, 1 to 2 dBic higher than that of the dual-loop [2, 3], and 2 dBic higher than that of self-phased quadrifilar helix antennas [6]. In this Letter, first the performance of the four square loops antenna is analysed. Then an experimental example of the four square loops antenna is presented to verify the theoretical and simulated results.