Because I can get a cheap PVC case which is otherwise suitabel for
weatherproofing the antenna. And because I have seen reported (I have
not tried it myself) that such material does not get heated in a
microwave oven.

This is a good, first-order test. But, as warned, different pvc manu's give
different results. It wasn't made for s-band RF. You must test it thoroughly
without extrapolating. Some is almost OK; others are really poor. Sure ain't
kidex.

73,
Chip N1IR