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The relation between loss resistance and conductivity for monopole antennas over a ground plane is derived. Its accuracy was confirmed by good agreement between it and finite element and method of moments simulations. This relation may be used to calculate conductivity of any material that radiates. A Wheeler Cap system was setup. System losses were determined by equating the loss resistance of a 70 mm long × 1.17 mm diameter copper monopole at resonance with that predicted using the published conductivity for this material. The loss resistance of a 70 mm long × 2 mm wide × 2 mm thick monopole antenna manufactured from an aerospace grade carbon fibre reinforced plastic (CFRP) laminate, IM7/977-3 prepreg tape with a [0 45 90-45]2s stacking sequence, was measured then corrected for system loss. The conductivity of the CFRP laminate was calculated to be 51,000 S/m at 980 MHz.