The letters RDF are not an acronym, but rather a "compression of RD (Radio Detection) and DF (Direction Finding)", a term devised by Wing Commander John Andrews of the Air Staff Directorate of Operations in 1935. Andrews said "the intention is to create a name which does not immediately indicate its method of operation". In mid 1943 the US Navy term RAdio Direction And Ranging (Radar) was adopted. The only actual acronym was RPF (Radio Position Finding) used initially by the US Army.
All this was news to me until the Museum was donated the RAF History journal and contradicts much of what is on the internet.
The first manifestation of RDF was the Chain Home system, the sites of which were called Air Ministry Experimental Stations, again intended to confuse.