Comparison between bandgaps and bandwidths of back radiation of different narrow soft ground planes

Size reduction is one of the most important considerations for wireless personal communication system antennas. In many cases, the main problem is the reduction of ground plane size. Such reduction means an increase in antenna back radiation. EBG (electromagnetic bandgap) surfaces have been used as ground planes. Soft and hard surfaces are defined in terms of surface impedance and boundary conditions in the E- and H-planes. A soft surface behaves like a PEC in the H-plane and as a PMC in the E-plane, thereby providing ideally infinite bandgaps in both E- and H-planes. Therefore, they also reduce antenna back radiation in the E-plane. A study of different soft surface ground planes has been made. All the analyzed structures have been demonstrated to work as expected. A bandwidth study is presented. The results show an important conclusion: although the theoretical bandgap of these structures can be very large, this does not mean that for a particular application (in this case ground plane) the bandwidth is large as well. For strip-loaded surfaces with vias, it was only possible to get very low back radiation in a narrow frequency band around the best ´soft´ frequency, i.e. the start of the bandgap. This best ´soft´ frequency is slightly lower than the theoretical ´soft´ frequency.