 At Dundee , Lewis raised the issue and two solutions to improving the range were considered . The Mk . IIIA consisted of a set of minor changes to the transmitter and receiver with the goal of reducing the minimum range to about 800 feet ( 240 m ) . Lewis ' own solution was the Mk . IIIB , which used a second transmitter that broadcast a signal that mixed with the main one to cancel it out during the end of the pulse . He believed this would reduce the minimum range to only 600 feet ( 180 m ) . Two copies of the IIIA entered tests in May 1940 and demonstrated little improvement , with the range reduced to only 950 feet ( 290 m ) , but at the cost of significantly reduced maximum range of only 8 @,@ 500 feet ( 2 @.@ 6 km ) . Tests of the IIIB waited while the AI team moved from St Athan to Worth Matravers in May , and were eventually overtaken by events . Development of both models was cancelled in June 1940 . 
