Effect of dimension, spacing, periodicity and shape of RIS on resonant frequency and bandwidth of 2 × 2 antenna array

This paper presents the effect of dimensions, spacing, periodicity and shapes of Reactive Impedance Surface (RIS) elements on resonating frequency of a suspended 2 × 2 MSA antenna array designed on a RIS loaded FR4 substrate. RIS consists of 13 × 13 square patch array with each patch of side 4mm and 1mm inter-element spacing. Elements of MSA array are fed through a feed line network which is fed by a 50Ω coaxial probe. Bandwidth improvement depends on the optimum electromagnetic coupling between the resonant frequency of RIS and MSA array. As RIS element dimension is decreased with periodicity of RIS elements <; 0.1λ
₀
, resonating frequency increases marginally. However, when periodicity of RIS elements become > 0.1λ
₀
with increase in the dimensions of RIS element, resonating frequency decreases considerably. Bandwidth decreases in both the cases and optimum bandwidth is obtained with RIS periodicity of 0.1λ
₀
. With constant dimensions of RIS elements, as inter-element spacing increases, periodicity also increases and capacitance decreases therefore resonating frequency increases. Resonating frequency of RIS also depends on shape of RIS element. In rectangular RIS elements with unequal periodicity in orthogonal direction, when dimensions are increased in direction perpendicular to E field, resonant frequency decreases and bandwidth increases while when dimensions are increased in direction parallel to E field, resonant frequency and bandwidth decrease significantly. RIS with hexagonal and circular shape elements are also analyzed. Resonating frequency increases with decrease in area of RIS element as capacitance decreases in hexagonal and circular RIS elements as compared to square elements. The optimum proposed structure is designed, fabricated and tested. The measured results agree with simulation results. The antenna offers impedance bandwidth (S
₁₁
<; -9.5 dB) and 11.3 dB gain with gain variation <; 2 dB over 5.7-6.5 GHz. It also provides SLL <; -20 dB, cross polarization <; -17 dB and F/B ratio > 20 dB. Overall dimensions of antenna are 70 × 70 × 2.6mm
³
.