DESIGN OF L-BAND SPIRO HELICAL ANTENNA

ABSTRACT

A novel antenna made of a spiral wire wrapped into a larger helical shape is introduced. The geometry of this antenna, which is a doubly helical structure, is fully described by five independent parameters, including two radial dimensions, two pitch angles, and the number of turns. Radiation properties of this antenna are examined both theoretically and experimentally. The Numerical Electromagnetic Code (NEC-2) is used to obtain simulation results. A large number of cases with different radii, pitch angles, and number of turns are investigated. Results for far-field patterns, gain, axial ratio, and bandwidth are presented. The influence of parameters on radiation properties is examined. Several prototypes of the antenna were constructed and tested using an outdoor antenna range. Measured far-field patterns are presented over a wide range of frequencies. The measured and computed radiation patterns are in good agreement.

The results of this study indicate that the proposed antenna provides circular polarization and high gain over a wide frequency range. For example, when the number of turns is 10, a gain of 11-14 dB, a bore sight axial ratio of less than 3 dB, and a half-power beam width of about 40 degrees are achieved over a 30% bandwidth. The side-lobe level for most cases examined is better than 10 dB below the main beam. A unique advantage of this antenna is its much smaller size compared with a conventional helical antenna made of straight wire shaped into a helix. Having about the same radiation characteristics, including gain, circular polarization, bandwidth, and side-lobe level, this new antenna occupies a volume more than 2.5 to 3 times smaller than the conventional helix. This reduction in size, which in turn may imply smaller weight and lower packaging and manufacturing costs, makes the proposed antenna very appealing to many communications and aerospace applications.