Technical report | Methods of Designing and Feeding Carbon Fibre Reinforced Plastic Slotted Waveguide Antenna Arrays
This report describes the ongoing development of the Slotted Waveguide Antenna Stiffened Structure (SWASS) technology during the period 2010 to 2012. In SWASS, blade stiffeners in sandwich panels or top-hat stiffeners on skins serve the dual purpose of providing both structural reinforcement while acting as radiofrequency waveguides. Slots cut through the outer skin and into the waveguides produce slotted waveguide antenna arrays. The development and validation of a radiofrequency design methodology, based on the finite element method, for resonant slotted waveguide antenna arrays, is described. This methodology was used to design seven- slot and ten-slot arrays that were subsequently machined into waveguides manufactured from aluminium alloy and aerospace grade carbon fibre reinforced plastic (CFRP). Additionally, the performance of a plug-and-loop radiofrequency feeding method for resonant SWASS waveguides is analysed.
This report describes the ongoing development of the Slotted Waveguide Antenna Stiffened Structure (SWASS) technology during the period 2010 to 2012. In SWASS, blade stiffeners in sandwich panels or top-hat stiffeners on skins serve the dual purpose of providing both structural reinforcement while acting as radiofrequency (RF) waveguides. Slots cut through the outer skin and into the waveguides produce slotted waveguide antenna (SWA) arrays. These slots will eventually be filled with a RF transparent dielectric to restore the aerodynamic surface and prevent environmental ingress.
The first half of this report describes the development of a RF design methodology for resonant SWA arrays. The methodology uses an infinite ground plane Finite Element Method model of the SWA. The slot dimensions and location were varied iteratively until the imaginary part of admittance was zero. In this condition the slot resonated because impedance was purely resistive. This approach was validated by correlating its predictions with published data. The validated model was used to design seven- and ten-slot SWA arrays, with longitudinal slots in the broad-wall, for waveguide wall thicknesses the same as that of standard aluminium alloy waveguides, and typical aerospace grade carbon fibre reinforced plastic (CFRP) laminates. Selected designs were manufactured and antenna behaviour measured. The return loss, bandwidth and antenna pattern shape for each array with the same number of slots were similar, regardless of the waveguide material. However the gain of the CFRP antenna was much lower compared to the aluminium alloy equivalent because of significant Ohmic and dielectric losses in the CFRP. Increasing the gain of CFRP SWA would require the conductivity of the inner wall of the waveguide to be increased, which is expected to be straightforward to achieve but would be associated with increased production and through-life-support costs.
The second half of this report describes the development of a plug-and-loop RF feed that; allowed adjacent SWASS waveguides to be fed, had sufficient bandwidth for resonant SWA arrays, and required no electrical connection with, or mounting holes in, the waveguide walls. Plugs were manufactured from aluminium alloy, supported the loop and a coaxial connector, and contained an integral low-high impedance choke. They fitted snugly into the end of CFRP waveguides and were bonded in-place with a structural adhesive. The loops were manufactured from brass shim and soldered to the coaxial connector. The effect of loop dimension on RF performance was measured. These feeds had a bandwidth of 10 % and loss of 0.10 to 0.15 dB, which is considered acceptable for a first generation SWASS demonstrator.
In future the plug-and-loop RF feed will be replaced with one manufactured from CFRP. The new feed could then be incorporated into a SWASS planar array that had been designed using the methodology presented in this report