Design of Low-Cost Probe-Fed Microstrip Antennas

Abstract

The concept of microstrip radiators, introduced by Deschamps in 1953, remained dormant until the 1970s when low-profile antennas were required for an emerging generation of missiles (James & Hall, 1989; Garg et al., 2001; Volakis, 2007). Since then, but mainly over the last three decades, the international antenna community has devoted much effort to theoretical and experimental research on this kind of radiator (Lee & Chen, 1997). Currently, low-loss RF laminates are used in their fabrication and many of their inherent limitations have been overcome (Garg et al., 2001). On the other hand, low-cost solutions are in demand now that both market and technology are ready for mass production (Gardelli et al., 2004). Recently, the design of single-fed circularly-polarized (CP) microstrip antennas manufactured with FR4 substrate was reported (Niroojazi & Azarmanesh, 2004). Unfortunately, the use of low-cost FR4 as the substrate introduces some additional complexity on the antenna design. This is due to the inaccuracy of the FR4 relative permittivity and its high loss tangent (around 0.02). Variations in the FR4 electrical permittivity can shift the operating frequency and the high loss tangent dramatically affects the antenna axial ratio and gain, resulting in poor radiation efficiency. To increase the efficiency, microstrip antenna on moderately thick substrate must be designed. However, the technique used to compensate for the probe inductance, when the patch is fed by a coaxial probe (a known practical way to feed microstrip antennas), still relies on the designer’s expertise. For instance, a series capacitor, which may be constructed in several ways, has been utilized to neutralize this inductance (Hall, 1987; Alexander, 1989; Dahele et al., 1989; Vandenbosch & Van de Capelle, 1994; Nascimento et al., 2006), or the probe geometry has been modified (Haskins & Dahele, 1998; Teng et al., 2001; Chang & Wong, 2001; Tzeng et al., 2005). Unfortunately, due to their complexity, many such techniques are not suitable when the antennas are series-produced in an assembly line. To overcome some of the abovementioned issues, two efficient techniques for designing low-cost probe-fed microstrip antennas are proposed. Using only their intrinsic characteristics, linearlyand circularly-polarized microstrip antennas can now be designed without the need for any external matching network. Limitations of the proposed approach will also be discussed. The chapter is organized as follows: Section 2 covers the design of linearly-polarized microstrip antennas; results obtained with the new approach are compared with those using the standard design technique. Circularly-polarized antennas