Multiple antennas underneath a single radome are attractive for platforms with limited space. Blockage scenarios are created when multiple antennas operate under a single radome. To avoid blockage, antennas that are electromagnetically transparent at frequencies outside of their respective radiating bandwidth are required. Although a number of structures in an antenna can contribute to the blockage, this study focuses on the feed networks.
Electromagnetic simulations are used to examine the scattering behavior of the feed networks obstructing a radiating source. Corporate feed network examples are investigated and were found to have poor transparency below the design frequency(TΘΘ < -1 dB, TΦΦ < -1 dB for f < 1.3f0). Feed networks designs are presented with the aim of electromagnetic transparency. Some designs provide unequal phase to a circularly polarized antenna array and rely on a far field phase matching method. This method rotates each array element according to the prior calculated phase output. The pseudo-random orientation of the array elements results in an improved antenna array radiating axial ratio (AR = -2.18 to -1). Right hand circularly polarized (RHCP) radiation is preserved when blocked by these feed networks.
Feed networks have been designed using optimization methods and classical design techniques. A feed network incorporating several of these techniques has been built and measured for L and S band antenna arrays in a multi-band antenna system. The L band array blocks the S band array line of sight under certain pointing angles. An X band antenna is positioned underneath the L and S band antennas. The L band array attenuates the peak gain of the S band array but preserves the beamwidth. This attenuation is limited to the main beam and results in the sidelobe levels increasing with respect to the main beam under blockage. These antenna arrays yield RHCP radiation with an axial ratio better than -1.3 over the operating bandwidth and half power beamwidth, regardless of blockage conditions. The L and S band antennas provide approximately 1 dB of attenuation at X band while preserving the RHCP polarization.