Mu-near-zero metasurface for microstrip-fed slot antennas

A mu-near-zero metasurface as a superstrate consisting of a periodically square closed ring is proposed for gain and bandwidth enhancement of microstrip-fed slot antennas (MFSAs). The metasurface exhibits near-zero permeability (0 <mu < 1) and negative reflection phase. For a unidirectional radiation pattern, the metasurface above the MFSA is employed to reduce the back-lobe radiation (180(a similar to)) and enhance the gain at 0(a similar to) without a metallic reflector. Interestingly, if additional metasurfaces are placed on both sides of the MFSA, a low-profile, high-gain and wide-band bidirectional antenna can be obtained. The results indicate that the metasurface can also enhance the radiation characteristics, impedance bandwidth and efficiency. Both simulation and measurement of the proposed antennas with metasurface show a fractional bandwidth of 25 % and the overall thicknesses of lambda (0)/12 and lambda (0)/6 of uni- and bidirectional antennas, respectively.