1. A system for irradiating biological tissue comprising: a. an ellipsoidal reflector 110 having foci F1 121 and F2 122 separated from each other by at least an F1-F2 separation distance of at least 5 cm; b. a source of EM radiation configured to emit diverging EM waves of sub-10 GHz EM radiation, at least some of which are reflected by the ellipsoidal reflector to form converging EM waves having one or more sub-10 GHz radiation frequency(ies); and c. a surrogate medium having an ENTRY_SURFACE via which converging EM waves enter the surrogate medium, wherein for a selected one or more of sub-10 GHz radiation frequency(ies) of the sub-10 GHz EM radiation, i. an average refractive index n AVERAGE SURROGATE — MEDIUM of the surrogate medium equals at least a minimum refractive-index threshold value that is at least 2; ii. the ENTRY_SURFACE 565 includes a power-significant portion via which an entry-surface-power-significant-fraction ESPSF equal to at least 0.1 by power of the converging waves of EM radiation at the sub-10 GHz selected frequency(ies) enter the surrogate medium; and iii. for all locations on the power-significant portion of the ENTRY_SURFACE 564 : A. a local normal line passes within an F2-tolerance-distance from F2, the F2-tolerance-distance being at most 1 cm and/or B. a local Poynting vector of converging EM waves at the selected frequency(ies) is substantially normal to the ENTRY_SURFACE at the location within an angular deviation tolerance that is at most 10 degrees.