The performance of solar thermal power systems using cavity receivers and parabolic dishes highly depends on the effective absorption of concentrated solar radiation by cavity receivers. Correct measurement of convection losses is challenging due to non-isothermal surface temperatures and unpredictable flow conditions inside the cavity. Correct prediction of radiation losses can help to predict convection losses. Effect of increasing the area ratio of normal cavity using cylinder in cylinder arrangement to increase the surface area for heat transfer, is studied experimentally. The specially designed heaters for model cavity size using nichrome wires sheathed between ceramic sheets were used to apply the thermal load, and the heat transfer rate was observed. Experimental temperatures were used for calculating the radiation heat losses using radiosity network method. Modified cavity surface is divided in parts and radiosity values for each part is calculated by solving simultaneous equation obtained by network method, using Gauss-Seidel method. Finally, the radiation heat loss from each surface is added to get total radiation heat loss. More heat transfer area for cylinder in cylinder arrangement and with the same heat input modified cavity shows higher surface temperatures. Network representation provides a better understanding of radiative interaction between different parts of the cavity. Radiosity network method predicts more accurate results than mean radiation heat loss calculations by calculating actual radiosity values for different parts of cavity. The difference in prediction is high at lower temperatures, emissivity and reduces with increasing temperature and emissivity. Effect of inner cylinder surface temperature was studied with three different cases and found that the radiation heat losses are less affected by inner cylinder surface temperatures. Effect of aspect ratio on radiation heat losses is presented in this work. Experimental results show that proposed cavity receiver design provide double surface area for heat transfer with increased surface temperatures for same heat input and total heat loss.