Electromagnetic Bandgap Microstrip to Parallel-strip Balun in Ultrawideband Applications

Electromagnetic bandgaps (EBG) have emerged as a new technology which makes possible the design of more compact filters. In last years, some studies have been carried out regarding planar transmission lines such as microstrip, or coplanar, showing very promising results. Parallel-strip lines have also received a lot of interest during last years, since it has been proven that it is possible to design antennas for UWB applications in this kind of technology. The baluns which are commonly used for feeding these structures show extremely large bandwidths, which may be not necessary, or even harmful taking into account spurious, noise or coexistence with other systems. In this paper, the design of a new UWB balun with filter capabilities is presented, using bandgaps so as to reduce this bandwidth in excess.