Abstract
In this study, two multi-band antennas and one broadband antenna using slot stepped-impedance resonators (SIRs) and coplanar waveguide (CPW) feeding have been investigated. All of the proposed antennas are fabricated on 1-mm FR4 substrates with a relative dielectric constant of 4.4 and a loss tangent of 0.02. In comparison to the conventional uniform-impedance resonator (UIR), the SIR shows a better control over spurious response. In this way, a dual-band antenna (antenna A) and a tri-band antenna (antenna B) could be designed at desired bands. Antenna A is implemented with center frequencies (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> , f√) = (1.5 GHz, 3.5 GHz), and Antenna B is realized with center frequencies (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> , f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S1</sub> , f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S2</sub> ) = (1.5 GHz, 3.5 GHz, 5.5 GHz), where f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is the fundamental frequency. The broadband antenna (antenna C) is designed by allocating the first three resonant frequencies close enough to achieve a 98.7% fractional bandwidth with a return loss of greater than 6 dB from 1.5 GHz to 4.4 GHz. Furthermore, to tackle the null radiation problem at the broadside direction of antennas A and B, parasitic slots are added near the main radiating resonators so as to achieve nearly omni-directional radiation patterns at the high-order mode frequencies.
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