Radiation characteristics of a conformal spiral antenna

The radiation characteristics of a conformal spiral antenna positioned on a finite-length conducting cylinder are numerically analyzed using the moment method. The effects of cylinder length on the radiation field are discussed. It is found that when the cylinder length is more than twice the free space wavelength, the current induced on the conducting cylinder is attenuated significantly near the end of the cylinder and the radiation field converges. It is also found that the current on the back side of the conducting cylinder affects the front-to-back (FB) ratio of the radiation field and the gain in the front direction. An FB ratio of 22 dB is obtained when the radius of the conducting cylinder is half of the wavelength. The front gain is 1 dB lower compared to the gain (8.7 dBi) of a planar spiral antenna on a planar conductor. The conformal spiral radiates a circularly polarized wave with an axial ratio of less than 3 dB over an 18% bandwidth. As an application, a conformal array antenna for omnidirectional radiation is realized by using seven spirals around the conducting cylinder. An axial ratio of less than 3 dB over all directions is obtained with a main polarization gain variation of less than 1.2 dB.