The radial centre of three circles on the three sides of a triangle as diameter is

Circle having AB diameter passes through foot of perpendiculars of A,B on sides BC and AC(since angle in semi circle is a right angle)

Circle having BC diameter passes through foot of perpendiculars of .B,C on sides AC and AB Clearly radical axis of each pair of circles is perpendicular to a  side of  the triangle $\Rightarrow$radical axis is an altitude
 $\therefore$ radical centre is of orthocenter of the triangle