A New Backfire Antenna

Measurements have shown that it is possible to increase the gain of the short backfire antenna significantly in different ways: 1.By increasing the size of the reflectors. 2.By placing extra reflectors in multiples of a half wave-length from the surface wave reflector. 3.By placing a quarter-wave ring in front of the surface wave reflector. The surface of the ring is perpendicular to the reflector and the ring has the same diameter as the reflector. The distance between the ring and the reflector giving maximum gain depends on the size and the number of reflectors. The choice of parameters (size of reflectors) was based on numerical calculations using a theory developed by Dragÿ Nielsen and Pontoppidan at this laboratory. Measurements have shown a decrease in impedance bandwidth when gain is increased. It is, however, possible to increase the gain-bandwidth product. The backward radiation is increased, too, but due to the quarter-wave ring the backlobe level is kept at an acceptable hight. The beamwidths in E- and H-plane tend to be equal with increasing gain. This is an advantage if circular polarization is wanted. The modified short backfire antenna seems to have a higher gain than any other backfire antenna of the same size (same length and diameter of the surface wave reflector).