Microstrip-fed dual-frequency annular-slot antenna loaded by split-ring-slot

A compact dual-band annular-slot antenna loaded by a concentric split-ring-slot is presented. A stepped microstrip feedline enables the control of the coupling and provides good matching. The annular-slot is connected to the split-ring-slot by a rectangular slot, which increases the surface of the current path, thus notably reducing the resonant frequency for a given size. The embedded split-ring-slot structure allows many resonant modes to be realised. By tuning the key parameters, these operating modes and their bandwidths can be controlled. A wide bandwidth can be realised for either the lower band, upper band or both bands simultaneously, depending on the application. Measured results show that the bandwidths in the region of 45-15% and 32--8.4%, can be provided for the lower and upper bands, respectively. For the case where a wideband response is required for both bands, it is shown that 26 and 32% can be realised. A 30%miniaturisation is also achieved compared with conventional annular ring slot antennas.