Conformal load-bearing antenna structures, which afford load-bearing structures with radar capability, are a promising technology to reduce weight and drag of air vehicles. This article presents an investigation of the mechanical and electromagnetic performance of slot log-spiral antenna in carbon-fibre composite structures. Compared with traditional rectangular slots, equiangular slot spiral antenna is found to offer broader bandwidth and better mechanical strength. Through experimental testing and finite element analyses, a new tip design is proposed that can significantly reduce the stress concentration of the non-load-bearing log-spiral antenna. Results of mechanical tests also show that the compressive strength of a carbon-fibre composite plate featuring a spiral slot is comparable with that pertinent to a plate with a circular hole of the same size. Filling the slots with epoxy resin can further enhance the compressive strength.