Multimode and Wideband Printed Loop Antenna Based on Degraded Split-Ring Resonators

In this paper, a compound multimode printed loop antenna based on degraded spilt-ring resonators (SRRs) for broadband wireless applications is proposed. The proposed antenna consists of an outer split ring and one inner closed ring with a big difference in size, which can be considered as degraded SRRs. By virtue of the orthogonal radiating mode of two rings, the compound multimode loop antenna can achieve multi-band resonances. Collaborating with the two coupled rings, a parasitic strip is placed close to the feed gap of the inner ring, which not only induces a new resonant frequency at upper band but also greatly improves the impedance matching in all high-frequency bands. The proposed antenna not only covers the first split ring mode at 816 MHz but also exhibits a broadband property covering from 2.3 to 4.6 GHz (reflection coefficient <inline-formula> <tex-math notation="LaTeX">$|S_{11}| < -10~\textrm {dB}, 67\% $ </tex-math></inline-formula> fractional bandwidth). The antenna operational principle and physical mechanism are analyzed by carrying out the studies of mode analysis and surface current distribution. To demonstrate the effectiveness of the proposed antenna, an antenna prototype is fabricated and tested. It is numerically and experimentally proved that broadband and multimode characteristics, stable radiation patterns with a peak gain of 1.2 and 5.1 dBi in the low- and high-frequency bands respectively, and more than 80% efficiency can be achieved. With merits of a compact size of <inline-formula> <tex-math notation="LaTeX">$0.12\lambda _{0}\times 0.11\lambda _{0}$ </tex-math></inline-formula> (<inline-formula> <tex-math notation="LaTeX">$\lambda _{0}$ </tex-math></inline-formula> is the wavelength at the lowest operating frequency), uniplanar and simple printed structure and wide bandwidth characteristics make it suitable for a wide range of wireless applications.