 The team received a number of Western Electric Type <unk> large acorn vacuum tubes in early 1937 . These were suitable for building transmitter units of about 20 W continual power for wavelengths of 1 to 10 m ( 300 to 30 MHz ) . Percy Hibberd built a prototype transmitter with pulses of a few hundred watts and fitted it to the Heyford in March 1937 . 
