Investigation of minimum cavity height of small EBG-resonator antennas for maximum directivity

By employing a highly-reflecting surface (HRS) with negative reflection phase as the superstrate and a PEC ground plane (its reflection phase is 180°), the cavity height (d) can be smaller than ¿₀/4. This paper investigates the minimum cavity height of the small EBG-resonator antennas (42 mm × 42 mm × d) with PEC side walls, operating at 12 GHz, to achieve the maximum direcitivity (15 dB). The results show that, down to a subwavelength value of ¿₀/24 = 1.04 mm, the maximum directivity can still be obtained with slightly changed directivity bandwidth (about 11.6%). Lower than 1.04 mm, the peak directivity at 12 GHz begins to decrease and the directivity bandwidth is narrowed significantly. A 1.04 mm cavity height makes the thickness of the EBG-resonator antenna comparable to, or even better than stacked microstrip antennas.