X-band circular ring-slot antenna embedded in single-layered SIW for circular polarisation D. Kim, J.W. Lee, C.S. Cho and T.K. Lee A circularly-polarised antenna which has a novel feeding structure like a substrate integrated waveguide, a cavity-backed resonator and a con- ventional ring-slot antenna is proposed for right-handed circular polar- isation (RHCP) in a single-layered fabrication. The CP generation and broadband impedance matching characteristics are accomplished by using a simple shorting via between the top patch and bottom ground plane and inserting inductive via arrays at the input port, respectively. A broadband impedance bandwidth of 18.74% and a RHCP axial ratio of 2.3% have been obtained under the condition of less than VSWR 2:1 and axial ratio 23 dB, respectively. Introduction: Substrate integrated waveguide (SIW) technology of low- loss transmission characteristic has recently become widespread in the design of many passive and active devices at microwave and millimetre-wave. An advantage is that it can be easily integrated with planar circuits by replacing the conventional microstrip and strip line. Basically, there are various techniques using feeding network and differ- ent shapes of radiating elements for cross-polarisation (CP) generation in a ring-slot antenna. For example, a stabilised CP has been generated by introducing a hybrid coupler with the same magnitudes and 908 phase differences at two ports [1]. However, as expected, the feeding network is not so simple. As other methods to generate CP, such as the L-shaped microstrip (series microstrip) feeding structure and a shorted section between the annular slot and ground plane, have been introduced [2, 3], a result has been undesirable radiation owing to the feeding network and reduced radiation efficiency caused by back radiation in the opposite side of the main direction. In this Letter, to overcome problems such as feeding loss, undesirable radiation, and reduced efficiency, a novel SIW- based and cavity-backed ring-slot antenna unified with a SIW feeding network having low-loss and broadband impedance matching character- istics is proposed for right-handed circular polarisation (RHCP) generation. y y z z A A′ A′ A′ W a b A′ W microstrip-to-SIW transition x y z A x y z A x h annular slot annular slot upper plate upper plate lower plate lower plate x A d 2 d 2 d 1 d 1 s 2 s 2 R 2 R 2 R 1 R 1 j j o o φ φ a 1 d 3 d 3 s 3 s 3 h 4 h 4 L s 1 s 1 h 3 h 3 h 1 h 2 a 3 a 2 a 1 h Fig. 1 Proposed antenna configuration a Without transition b With transition for measurement Antenna configuration: We considered the proposed cavity-backed cir- cular ring-slot antenna using SIW technology as shown in Fig. 1. The proposed antenna is mainly composed of three parts: a SIW-based cavity-backed resonator and a ring-slot antenna; horizontal via arrays and a SIW-based rectangular waveguide for a broadband input impe- dance matching and feeding network, respectively, and microstrip-to- SIW transition for measurement. The total occupied area is 25 (W) Â 38 (L) mm 2 in Fig. 1b with RT/Duroid 5880 substrate which has thick- ness (h) 1.57 mm, relative permittivity 2.2, and loss tangent 0.0009. The optimised parameter values for generating RHCP are listed in Table 1. Table 1: Design parameter values of proposed antenna Parameter a 1 a 2 a 3 d 1 d 2 d 3 h h 1 h 2 h 3 Value (mm) 16.8 10 4.9 1 1.2 0.9 1.57 3.2 10 4.43 Parameter h 4 j L R 1 R 2 s 1 s 2 s 3 W f Value (mm and deg) 9 7.8 38 5 6 0.8 1.1 0.3 25 296.58 The width (a 1 ) of the feeding waveguide has been determined from the centre frequency, 10 GHz in the X-band and the cutoff frequency 6.3 GHz of the fundamental TE 10 mode. In particular, it can be assumed [4] that the leakage from the side walls of the feeding wave- guide working as perfect electric walls is small enough to be neglected. Experimental results: A measured return loss of the proposed antenna is shown in Fig. 2, which shows good agreement between data and the simulated result using commercially available software (CST MWS). There is also a photograph of the fabricated antenna. It can be evaluated from Fig. 2 that the input impedance bandwidths are 12.86% from 9.46 to 10.76 GHz and 18.74% from 9.38 to 11.32 GHz for simulation and measured results, respectively. From Fig. 3, which shows the gain and axial ratio (AR), it is seen that the proposed antenna has 5.75 dBic as maximum gain within 8.875 to 10.875 GHz, a very stable 3 dB gain bandwidth of 1.4 GHz, and axial ratio bandwidth of 2.3% from 10.3 to 10.54 GHz for less than 23 dB. The measured radiation patterns in two orthogonal cutting planes shown in Fig. 4 imply that the proposed antenna satisfies the RHCP generation with a lower cross-polarisation at the boresight direction. 7 –40 –35 –30 –25 –20 –15 –10 –5 0 8 9 10 frequency, GHz simulated result experimental result return loss, dB 11 12 13 Fig. 2 Simulated and experimental return loss characteristics with photo- graph of fabricated antenna 8.75 9.00 9.25 9.50 9.75 10.00 frequency, GHz simulated gain simulated axial ratio experimental gain experimental axial ratio 10.25 10.50 10.75 11.00 0 –5 –10 –15 –20 axial ratio, dB gain, dBic 8 4 0 –4 –8 Fig. 3 Gain and axial ratio characteristics against frequency ELECTRONICS LETTERS 18th June 2009 Vol. 45 No. 13 Authorized licensed use limited to: Korea Aerospace University. Downloaded on January 14, 2010 at 23:59 from IEEE Xplore. Restrictions apply.