Arm out the window states the following:
"For E to work, you need radar, don't you?"
No you don't. From my experience in the USA (which is limited) much of the Class E airspace below 6,000' has no radar at all. Remember in the USA that all their Class E corridors go down to 1,200' AGL and the Class E airspace goes down to 700' at the non-tower airports. Obviously no country has radar to this level right across.
There is a myth propagated everywhere that you needs radar for Class E. This is certainly not the fact. Someone else might be able to explain.