 The Mk . IV was used in the front lines for only a short period . The introduction of the cavity magnetron in 1940 led to rapid progress in microwave @-@ frequency radars , which offered far greater accuracy and were effective at low altitudes . The prototype Mk . VII began to replace the Mk . IV at the end of 1941 , and the AI Mk . VIII largely relegated the Mk . IV to second @-@ line duties by 1943 . The basic electronics design , originally a television receiver , was used as the basis of the ASV Mk . II radar , Chain Home Low , AMES Type 7 , and many other radar systems throughout the war . 
