Circularly Polarized Microstrip Antennas Using Single-Fed EM Coupled Ring Resonators

Microstrip antennas have been widely used for aerospace applications and in small portable wireless communication equipments because of their compactness, light-weight, low-profile, and relative ease of fabrication. The small size is an important requirement for portable applications. There are various types of microstrip patch configurations. In general, the size of a microstrip patch is about one-half effective wavelength on a dielectric at a fundamental mode resonant frequency. Changing the basic patch shape may yields a substantial size reduction. A square-ring antenna is one of the smallest circularly polarized microstrip antennas with a size of λg/4 × λg/4 [1]. Circular polarization (CP) of a microstrip antenna can usually be achieved by feeding two detuned orthogonal signals to the radiating and non-radiating edges of a square patch antenna [2]. However, this dual-feeding mechanism has disadvantages of more complex geometry, larger size, and higher loss at a feeding network compared to a single feed one. Recently, ring radiating elements, such as square and annular rings, are drawing more attentions [3-8]. When operating in TM11 mode, a ring antenna has smaller in size than that of a circular and rectangular patch [8]. Square rings, which have been proposed for linear polarization, show that the width of the ring radiating element is critical for impedance matching [1], [5]. In the case of CP, two closely degenerated resonant modes of the rings are generated by either cutting diagonal slits or inserting symmetrical perturbation strips to the rings [3], [4]. Moreover, additional impedance transformers are required to match to the 50-Ω input port. In this paper, a simple feeding method has been proposed to obtain CP in microstrip square ring antennas. Impedance transformers, add-on slits, notches and strips are not required for circularly polarized radiation.