Complex sentence: There is a specific value of "D" r for which the loss is a minimum and this turns out to be 3.6 for the coax cable available during the war; it was rigid air-insulated pipe and remained so for some time afterwards.
Split into simpler sentences: There is a specific value of "D" r for which the loss is a minimum and this turns out to be 3.6. The coax produced during the war was rigid air-insulated pipe, and this remained the case for some time afterwards.