Design of concentric helical antennas for maximum gain and isolation over wide frequency separation

Some space and ground applications require antennas and primary feeds operating over separate frequencies, covering a much wider bandwidth than that of conventional uniform helical antennas. This paper addresses the design feasibility of two concentric helices, separately fed and sharing the same ground reflector. A novel and effective matching transformer is proposed and tested. Such a compact 2.5 wavelength-long antenna provides 12 dBi gain at 760 MHz and 1290 MHz, i.e., at a 52-percent frequency separation, an axial ratio of less than 2 dB and a VSWR of less than 1.8. Detailed gain and radiation pattern analysis suggest 16-dB isolation between the two ports, at any one frequency. Such an antenna provides some mass and significant space saving, a wider bandwidth, and a higher isolation to reduce intermodulation products than single helices.