Antenna Patterns from Single Slots in Carbon Fibre Reinforced Plastic Waveguides

Slotted Waveguide Antenna Stiffened Structure (SWASS) is a type of conformal load-bearing antenna where top-hat cross-section stiffeners on skins, or blade stiffeners in sandwich panels, serve the dual purpose of acting both as structural stiffeners and as waveguides. Cutting slots through the outer skin and into these waveguide stiffeners creates slotted waveguide antenna arrays. Two of the key requirements for SWASS are adequate antenna performance and the capability to design SWASS antenna arrays. The first step toward this goal, and the subject of this report, was to satisfy these requirements for single slot antennas. Eight 100 mm long waveguide antennas, with internal cross-section of 22.86 mm x 10.16 mm and a single slot, were manufactured from aerospace grade carbon fibre reinforced plastic (CFRP). Two slotted waveguides antennas were also manufactured from copper rigid rectangular WR-90 waveguide. Antenna gain in the bore-sight direction from 8 to 12 GHz and the Eand Hplane antenna pattern at 10 GHz were measured for each slotted waveguide. The radiation pattern of CFRP antennas was similar to that of copper antennas, with approximately 3 dB reduction in gain. Measured behaviour correlated well with the predictions made using CST – Microwave Studio. Discrepancies of up to 2 dB occurred at frequencies above 10 GHz. It is concluded that losses due to the finite conductivity of CFRP are acceptable for first generation SWASS and simulation accuracy is sufficient for initial array design.