A network analyzers is not marketed as an antenna analyzer, or even an
intentional radiator for that matter. I have a signal generator, an
amplifier and an antenna in my lab but it does not mean I can sweep the
entire LF to UHF bands at any wattage.

Then you need to get a better signal generator.


I meant "does not mean I can legally sweep the entire LF to UHF bands"


I was just looking through Part 18 and it really is unclear to me,
however
one thing was clear to me, ISM devices are prohibited for use in certain
bands. One of which is search and rescue. So now how can I test my ELT
antenna installation on my aircraft if I'm violating FCC rules?

You test it during the time window when testing ELT's is allowed.


This does not apply to the newer 406 ELT's that are currently being sold,
and which are monitored by very sensitive LEO, MEO and GEO satellites. A
band that is also recognized and protected by the ITU.