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Novel transmission-line structures, such as substrate-integrated-waveguide (SIW) technology, can equip antennas with high gain at high frequencies for a variety of communications applications. To demonstrate, a square ring-slot, single-layer, circularly polarized SIW antenna was designed and developed for X-band applications. Through the use of two shorted square ring-slots in the top wall of the SIW substrate, along with shorting via holes, a circularly polarized (CP) antenna was achieved. The antenna features a good radiation pattern with high gain, with measured impedance bandwidth of 6.5% and axial ratio (AR) bandwidth of 1.5%. This high-gain antenna is suitable for satellite-communications (satcom) applications at X-band frequencies.

In addition to satcom systems, CP antennas are used extensively in radar and various other communications applications. They are useful since they can resolve problems in wireless channels such as polarization mismatch generated by Faraday effects and interference generated by multipath effects. SIW technology, as first proposed by Wu,1 enables easy integration with planar circuits by replacing conventional microstrip and stripline transmission-line designs. SIW technology features lower cost than conventional waveguide, with low loss, high-power capacity, and high quality factor (Q). In fact, SIW-based CP antennas have been reported by various researchers.2-7

These designs employed different approaches for CP performance. In ref. 2, for example, a 16-element top-wall SIW slot antenna used two compounded slot pairs to obtain CP performance centered at 16 GHz, although with a usable impedance bandwidth of just 2.3%. In ref. 3, a single grounded coplanar waveguide (CPW) structure was used to feed an X-band cavity-backed crossed-slot antenna, although this structure also suffered a narrow impedance bandwidth of less than 3% and a narrow axial ratio (AR) bandwidth about 1% for an AR of less than 3 dB. In refs. 4-6, shorting via holes were used to connect the metal area bounded by the ring-slot at the top wall and the bottom wall of SIW to obtain a CP wave, so the locations of the shorting via holes and fabrication errors associated with them greatly affected the CP characteristics.

In ref. 5, the authors studied SIW cavity-backed antennas using microstrip-to-SIW and coaxial-to-SIW transitions. A good level of cross-polarization was obtained with the coaxial-to-SIW transition, with antenna gain about 7 dBi. The AR bandwidth and the impedance bandwidth were almost the same as for a circular ring-slot antenna.4 However, the antenna occupies a relatively large area, and its design is complex.

In the present report, a square ring-slot SIW CP antenna was developed for X-band applications. Two shorted strips are etched into the slot to obtain the CP wave. The design is simple and fabrication for the shorted strip is easier than for the antenna with the shorting via hole.

SIW Fashions CP X-Band Antenna, Fig. 1

Figure 1 shows the antenna’s geometrical configuration (side view on left and top view on right). The overall size of the SIW antenna is 34 × 34 mm2. It consists of three parts: the SIW structure, the square ring-slot with two shorted strips, and a coaxial feeding probe. The SIW is created by two rows of via holes and its one end shorted.

Parameter P is the distance between two via holes and d is the diameter of the via holes, while parameters W and L represent the width and length of the SIW structure, respectively. These SIW values should imply that it operates in fundamental mode, TE10. Parameters g1 and g2 are the length of the shorted strips and their width is as long as the width of the ring-slot denoted as ws.

Then, the distance between the feeding probe and the bottom of square ring-slot is denoted as s2, and it is in the center of the SIW in the y-direction. Parameter s1 is the distance between the top of square ring-slot and the shorted end , with the inner dimension of the square ring-slot as ls.

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