Bandwidth Enhancement of Low-Profile PEC-Backed Equiangular Spiral Antennas Incorporating Metallic Posts

This communication examines parasitic coupling to achieve higher bandwidth (BW) and further size reduction for low-profile PEC-backed equiangular spiral antennas. The parasitic loading is realized by metallic posts placed between the antenna plane and metal reflector. The parasitically excited metallic posts can considerably improve the antenna axial ratio at low frequencies as well as make it more rigid in construction. However, in practical applications, the low-profile equiangular spirals, albeit compact, suffer from the lack of a proper planar feed. To obviate this problem, a planar spiral-shaped microstrip feed is proposed.

I. Introduction

For a freestanding spiral antenna, it is known that bi-directional radiation with opposite polarizations is achieved. But in most applications, a unidirectional pattern is needed to block radiation on the other side. So far, some investigations have been devoted to the realization of low-profile unidirectional equiangular spiral antennas (ESAs) [1]–[3], all of which have used absorbing materials. However, the use of absorbing materials reduces the antenna efficiency. The low-profile spiral-mode microstrip (SMM) antenna has been also proposed in [4]. But a difficulty in this method is how to launch and maintain the desired spiral modes.