With the introduction of rotating antennae on
the transmitters and receivers used for CHL
and Naval radars it was now possible to build
an oscilloscope that displayed a trace which
swept a radius from the centre of the screen
and rotated about the centre in time with the
rotating aerial. The phosphors used to coat
the screen were especially chosen to produce
a long 'afterglow', so that each trace remained
visible to be refreshed with each cycle of revolution.
All that was needed now was to draw on (or
overlay) this special screen with the map of
the area scaled to coincide with the known radar
range. The radial time base display developed
by G.W.A.Dummer was tested and eventually the
was born and within a year was made compact
enough to be installed in aircraft as well as
used for the original ground-based applications.
By mid-1940 CHL was rapidly evolving from a
defensive instrument into a potent offensive
tool that gradually gave the RAF pilots a tactical
advantage over the attacking aircraft.