Cooperative Spectrum Sensing has been investigated in Rayleigh-fading environments over non-ideal reporting channels, where the simulation results have shown that its performance is limited by the probability of reporting errors. This paper proposes a transmit diversity scheme using Differential Space-Time coding where channel state information is not required. By regarding multiple pairs of Cognitive Radios as virtual antenna arrays in multiple clusters, Differential space-time coding is applied for the purpose of decision reporting over Rayleigh channels. Hard combination schemes are employed at the fusion center due to their minimal bandwidth requirements. Simulations results show that this method also achieves full transmit diversity, albeit with slight performance degradation in terms of power. The results also show improvements in sensing performance when compared to conventional cooperative spectrum sensing over non-ideal reporting channels.