HFSS-simulated receive pattern for the 2.5 cm log-spiral antenna when placed on the heterogeneous multilayer human head model. The receive pattern is shown for five specific radiometric sensing frequencies, i.e., 1.0, 1.1, 1.35, 1.6 and 2.0 GHz. Radiometric antenna efficiency η (%) is evaluated in terms of the ratio of the power collected from brain to the total power received by the antenna and thus intrinsically normalized.