On reducing the patch size of U-slot and L-probe wideband patch antennas

The coaxially-fed U-slot rectangular patch antenna and the L-probe fed rectangular patch antenna are two recently developed single-layer single-patch wideband microstrip patch antennas. In both cases, a second resonance is introduced near the main patch resonance, either by the U-slot or by the L-probe. The U-slot or the L-probe also introduce a capacitance which counteracts the inductance of the coaxial feed, allowing for the use of thick substrates (0.08-0.1 Λ₀) where Λ₀ is the free space wavelength. Using foam substrates (with ε_r≈1) the impedance bandwidths of these antennas operating in the fundamental mode are in the 30-40% range, with stable pattern and gain characteristics. These bandwidths are more than sufficient for most wireless communication applications. The resonant length of the fundamental mode is about half of the free space wavelength. For many applications, it is desirable to reduce the size of the patch to conserve real estate space. For this reason, there have been extensive investigations on patch size reduction techniques. One method uses microwave substrates with values of ε_r>1. Another method uses a shorting wall at the location of zero electric field so that the resonant length is halved, resulting in the quarter-wave patch. Yet another method uses a shorting pin near the feed. This introduces capacitive coupling to the patch resonance, thereby increasing the effective ε_r, and reducing the frequency, which means that, for a given resonant frequency, the patch size becomes smaller. In this paper, results of some of these investigations are presented