Contents I. Introduction
Slot-ring antennas, also called coplanar waveguide patches [1], are the magnetic dual of the microstrip ring antenna [2] and are usually one guided-wavelength in perimeter at the first resonant frequency. These antennas have been used in many applications such as millimeter-wave receivers, transmitters, and imaging arrays, as well as aircraft landing systems and automotive anti-collision systems [3]. Slot-ring antennas are an attractive alternative for their compact geometry, potential for polarization diversity, and capability to transmit and receive on both sides of the ground. plane. In some cases, active elements and variable capacitors have been used to change the antenna characteristics. For example, in [4] a varactor loaded slot ring antenna that was able to switch between linear, left-hand circular and right-hand circular polarizations was reported. Results showed that the antenna in circular polarization mode presented a 30% bandwidth around 2.4 GHz. In this work, a tunable and multi-band rectangular slot-ring antenna configuration is presented
