Circularly Polarized High Power Antenna

There are existing antenna designs that will result in the radiation of circular polarization, but not all of these are well-suited to high power applications.  The two strategies that we will consider in this effort have been used previously with HPEM sources:  1) helical antennas; and 2) the coaxial beam rotating antenna (COBRA). The axial mode helical antenna is widely utilized in satellite communications and has a number of very attractive properties [J.D. Kraus, "The Helical Antenna", Proc. of IRE, Vol. 37, 3, 1949].  The antenna has a very broad impedance bandwidth (typically 40% or more) with a high input impedance of 100 – 150 Ohms.  Then antenna generates a circularly polarized beam on-axis with excellent axial ratio when the circumference of one turn of the antenna is comparable to a wavelength.  The helix also has good directivity (10 dBi or more for 3 – 4 turns) and easily integrates with a coaxial output port.  Though it has many positive properties, using helical antennas for HPEM applications requires particular care in order to accommodate the high fields both at the feed and throughout the windings.  Compromises are needed at the coaxial output, and often the use of a pressurized radome containing SF6 or another gas is required.  However, an additional advantage of helical antennas is that they are easily arrayed, which opens the possibility of power sharing among elements.  In the event that a radome is required, ASR has extensive experience in developing high pressure radomes for helical antennas.  ASR worked with Spencer Composites to deliver 2 pressurized radomes to PEO STRI for use with the WBTS sources at WSMR. The COBRA antenna was originally proposed and demonstrated by Courtney and Baum as a reflector antenna, and was later adapted to a lens configuration.  The COBRA antenna uses a large coaxial output to feed a focused aperture antenna.  Normally the TEM mode from a coax does not radiate efficiently, but the COBRA antenna introduces a phase delay in the four quadrants of the aperture. The COBRA antenna has the advantage of very conveniently coupling to a coaxial output with arbitrarily high power; there is no additional field enhancement created by the antenna geometry.  The reflector version can be made compact through the use of a hyperboloidal sub-reflector in a Cassegrain configuration.  The impedance bandwidth of the COBRA can be very broad, but the phase delay introduced by the petaled reflector (or lens) is wavelength dependent.  So while the return loss is good, the quality of the circular polarization is compromised.  Data are limited in the literature about the polarization quality as a function of bandwidth, and we will explore that issue in Phase 1 of this program.  Operation at the two distinct frequencies (1.1 GHz and 2.8 GHz) simultaneously, will be very challenging.  However an antenna that can provide both a good input impedance and high quality circular polarization will be a major step forward.