I'm a little stumped.
I'm sure I'm missing something obvious
but I just can't see it.

Stupid questions:

1 - Are you using a short test jumper cable or adapter connector between the
test instrument and the end of the coax? Could the test lead or connector be
at fault?

2 - Is there any possibility of a STRONG signal coming back down the cable
to stuff up your test instrument readings? Do you live near a source of RF,
such as local broadcast radio?

3 - Is there any possibility both feeder cables are defective? Perhaps you
are using coaxial cable not meant for this purpose? TV coax, or LAN cable?

4 - Have you wiped out your neighbours TV viewing or something? Checked the
cables for sabotage (pins or nails)?

5 - Have you climbed the mast and checked the antenna directly at the feed
point, without a feeder cable?

You have probably thought of all these, but I had to ask. Regards Harry