 The Mk . IV series operated at a frequency of about 193 megahertz ( MHz ) with a wavelength of 1 @.@ 5 metres , and offered detection ranges against large aircraft up to 20 @,@ 000 feet ( 6 @.@ 1 km ) . Considerable skill was required of the radar operator to interpret the displays of its two cathode ray tubes ( CRTs ) for the pilot . It had numerous operational limitations , including a maximum range that increased with the aircraft 's altitude and a minimum range that was barely close enough to allow the pilot to see the target . It was only with the increasing proficiency of the crews , along with the installation of new ground @-@ based radar systems dedicated to the interception task , that interception rates began to increase . These roughly doubled every month through the spring of 1941 , during the height of The Blitz . 
