A monofilar spiral antenna excited through a helical wire

The radiation characteristics of a monofilar spiral antenna excited through a helical wire are investigated using the method of moments. The spiral has a circumference of more than two wavelengths (2.3/spl lambda//sub 6/, where /spl lambda//sub 6/ is the wavelength at a test frequency of 6 GHz) to obtain a tilted beam. An input impedance of approximately 50 /spl Omega/ is realized by appropriately selecting the diameter and pitch angle of the helical wire. The antenna shows a gain of approximately 8 dB relative to an isotropic source in the maximum beam direction (18/spl deg/ from the direction normal to the antenna plane) and a VSWR frequency bandwidth of approximately 12%.