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The Green Bank Telescope (GBT) at the National Radio Astronomy Observatory (NRAO) site in Green Bank, WV employs a paraboloidal main reflector with a 100-meter unblocked aperture. The half angle subtended by the main reflector from prime focus is 39°. This design requires a highly directional prime focus feed. The receiver plan calls for five frequency bands, each with a bandwidth of approximately 30%. The bands are 290-395 MHz, 385-520 MHz, 510-690 MHz, 680-920 MHz and 910-1230 MHz. The upper two bands use single-mode corrugated horns, while at the lower bands, waveguide feeds become excessively large. The short-backfire antenna (SBA) [1] is the choice of feed for the lower bands, as it has the required beam shape. A design by Ohmori et al. [2] is the basis for the design presented in this paper. The SBA of Ohmori has a return loss of -14 dB over 20% bandwidth. However, radiation pattern is shown at only one frequency. The current design shows small variation in beamwidth as a function of frequency, good circular symmetry of the beam and return loss better than -13 dB over the entire 30% band. A concept of a dual-band version of the SBA at 342/800 MHz has been tested. Such a feed would facilitate pulsar observations at the two bands simultaneously. INITIAL DESIGN:The SBA conceived by Ehrenspeck [1] consists of a large (main) planar reflector 2λ in diameter with a rim of height 0.25λ and a small reflector, 0.4λ in diameter, separated by approximately 0.5λ. The antenna is excited between the two reflectors and it acts as an open resonant cavity radiating along the axis outward from the large reflector. The radiation pattern has a taper of -14.7 dB at 39° at the design frequency, with sidelobes below -20 dB. The impedance match bandwidth is about 5% for a return loss of -14 dB. Ohmori et al. presented an improved version of the SBA in 1983. By changing the main reflector from a planar to conical shape and by adding a second small reflector, they increased the impedance bandwidth to about 20%. Based on this design, a SBA was fabricated with dimensions given in Figure 1 for the 385-520 MHz band. The two small reflectors are 0.46λ and 0.41λ in diameter, with the 0.46λ reflector located at 0.67λ from the main reflector. A dipole fed by a dual-line balun was used to excite the SBA. The dipole is about 0.5λ at 450 MHz, with a length to diameter ratio of 15. MODIFIED SBA AND MEASURED RESULTS:Measured far-field patterns are shown in Figure 2. The beam is narrow at 520 MHz and has a sidelobe at -7 dB in the H-plane of the antenna. Analysis shows that reducing the diameters of the two small reflectors to 0.41λ and 0.36λ, respectively, improved patterns at the upper end of the band. Measured patterns at 520 MHz show the sidelobe level at -12 dB in the H-plane. Further analysis resulted in the choice of a conical shape for the 0.41λ diameter reflector. Patterns shown in Figure 3 reveal less variant beams, good match between the principal planes and sidelobe levels below -15 dB. In order to improve the VSWR response of the S...

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