Significance of the Ground Plane Dimensions and Slot Antennas Performance

Abstract

The main objective of this paper is to investigate how the ground plane dimensions affect three key radiation properties, which are radiated power, directivity and radiation resistance, of slot antennas etched in them. Two models described as half-wave- and fixed- slots whose dimensions are specified as (0.5 λ by 0.02 λ) and (60 mm by 2.4 mm), respectively, are employed at operating frequency of 8 GHz. Total field radiated by the slot is arrive at as the superposition of the primary field (infinite-size pattern) and edge diffracted field. This enables us to develop relevant expressions for the three investigated parameters, from which design curves are generated. It is shown that slot antennas cut in a finite-sized ground plane has profiles of its radiated power and directivity decreasing with increase in the ground plane size while the radiation resistance increases linearly with increase in the size of the ground plane, approaching an upper bound as the size of the ground plane becomes very large. The power radiated is approximately constant with the size of the ground plane, having approximate values of −51 dB and −66 dB, for the two slots specified as (0.5 λ by 0.02 λ) and (60 mm by 2.4 mm), respectively, when the side-length of the ground plane assumes a value of 7 λ or greater, at the selected operating frequency. In addition, it is revealed that the directivity decreases linearly with increasing sizes of the ground plane, for each of the two models under consideration. The radiation resistance of a (0.5 λ by 0.02 λ) slot cut in a square ground plane whose side-length is 5.5 λ is 277 Ω while that of the corresponding (60 mm by 2.4 mm), slot cut in a similar ground plane dimensions stands at 42 Ω. Values of radiation resistance of a rectangular slot cut in an infinite-sized ground plane are 387 Ω and 45 Ω for (0.5 λ by 0.02 λ) and (60 mm by 2.4 mm), slots, respectively. The value of 387 Ω obtained for the (0.5 λ by 0.02 λ) slot cut in an infinite-sized ground plane is consistent with the expectation derivable from Booker’s universal formula connecting impedances of the slot and its complimentary- dipole. Indeed, this impedance is of the same order of magnitude as that of free-space which is 377 Ω and that is a good match. This lends credence to the veracity of our formulation.