2.45 Ghz High-gain Electrically Small Antenna with Composite Right/left-handed Ladder Structure

Electrically small antennas are needed to further lower the cost of wireless elements. However, the antenna gain is constrained to the Harrington limit, while the impedance matching and directivity characteristics deteriorate in electrically small antennas. Using a dipole antenna loaded with a left-handed ladder structure is a promising technique for reducing the size of antennas. In this letter, we designed a lumped-component loading left-handed antenna on the basis of the composite right/left-handed transmission line theory. A 4-unit cell left-handed dipole antenna with a size of 20 mm (0.160) was fabricated with a T-junction balun. The actual gain of the fabricated antenna was-0.01 dBi, which was about 1 dB lower than the Harrington limit and the highest ever achieved for fabricated antennas operating at around 2.45 GHz (kr = 0.502). The directivity measured in the E-plane was similar to that of typical half-wavelength dipole antennas. Introduction: Recently, there has been significant interest in researching and developing wireless communication circuits for 2.45-GHz, 24-GHz, and 60-GHz band applications. In particular, electrically small antenna technologies are needed to achieve miniaturization of wireless equipment for wireless communication and sensor networks. In recent years, several extensive attempts have been made to fabricate electrically small antennas for system-on-a-chip (SoC) or system-in-a-package (SiP) [1], [2]. However, the antenna gain is constrained to the Harrington limit, while the impedance matching and directivity characteristics deteriorate in electrically small antennas. Various methods for reducing antenna size have been reported [2-7]. Using a dipole antenna loaded with a left-handed ladder structure is a promising technique for reducing the size of antennas [4]. When parallel plate capacitors and spiral inductors are fabricated on a printed-circuit board with a thickness of a few mm, however, the sizes of the capacitors and the widths of the spiral inductors are expected to be more than 4 × 4 mm and 2 mm for achieving left-handed components operating at 2.4 GHz, and these components cannot reduce the antenna size to be smaller than a conventional dipole antenna. In contrast, lumped components of inductors and capacitors with sizes of 0.4 × 0.2 and 0.6 × 0.3 mm are commercially available. In this letter, the left-handed dipole antenna with the right/left-handed ladder structure was designed and fabricated using lumped inductors and capacitors. The highest achievable gain of an electrically small antenna that can be enclosed within a sphere of effective radius r is given by