planar forMs of UlTra-wideband (UWB) antennas can be integrated between the radio-frequency (RF) front-end circuitry and the radiating structure. They can therefore be implemented using microstrip technology. At California's San Diego State University, Sunil Kumar Rajgopal and Satish Kumar Sharma presented a UWB microstrip antenna that combines a pentagon-shaped slot, feed line, and pentagon stub to obtain a 124-percent impedance bandwidth from 2.65 to 11.30 GHz. In doing so, it exceeds the UWB requirement of 110 percent from 3.10 to 10.60 GHz.
The antenna uses a 50-x-80-mm groundplane, which covers only the top 20 mm or 25 percent of the groundplane length. The researchers considered three antenna design variations using the straight and rotated feed lines on two different substrates: a straight feed line on Rogers' RT/Duroid 5880 substrate, a tilted feed line on that same substrate, and a tilted feed line on FR-4 substrate. For all three designs, the substrate material's thickness was 1.58 mm. The antenna is fed using a 50-O coaxial SMA connector, which is connected to a 50-O microstrip transmission feed line. The reflection coefficient results for all three designs were obtained using Ansoft Designer simulations.
The feed lines of two designs were rotated by 15 deg. Those designs exhibited enhanced bandwidth compared to the design that used the straight feed line. The simulation of the second design, which provided the maximum bandwidth, showed that near omni-directional radiation patterns can be obtained. Yet they deteriorate toward the higherfrequency end. These variations in radiation pattern are attributed to the irregular pentagon shapes of both the slot and the stub as well as its effective electrical dimension variation with the frequency. See "Investigations on Ultrawideband Pentagon Shape Microstrip Slot Antenna for Wireless Communications," IEEE Transactions On Antennas And Propagation, May 2009, p. 1353.