An EM radiation measuring apparatus comprises an antenna element matrix on a substrate including 1st to Lth arrays which are successively arranged in an X direction, each of the 1st to Lth arrays including antenna elements arranged in a Y direction and having an output terminal, each antenna element receiving EM radiation from a target and generating a detection signal. A signal selector responsive to X and Y selection signals, selectively supplies the detection signal from one of antenna elements to one of the output terminals. 1st to Nth signal outputting circuits, each having inputs, at least an output, and at least a signal selection circuit selectively coupling the inputs to the output terminals of which the number is less than the number of the inputs. The Nth outputting circuit supplies an output detection signal, the inputs of the 1st signal outputting circuits being connected to the output terminals respectively, wherein each of the 2nd to Nth outputting circuits has a transmission line(s) substantially extending in the X direction to connect one of the input terminals thereof to at least the signal selection circuit thereof. The signal selector comprises diodes or transistors respectively connected to the antenna elements. The transistor amplifies the detection signal.
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