John Smith I wrote:
I have been searching the internet, it seems to me someone would have:

1) Constructed an airtight room, filled it with a gas such as neon and
placed an antenna in such a room. The power/freq to the antenna could
be varied and "patterns" observed in how the rf was exciting the gas
into "glowing" in the visible range while observing any visible patterns
in such.

Might be fun, but I don't know about useful. And, whatever antenna you
want to 'see', just scale it down to fit inside a bell jar.

2) Constructed gas insulated coax enclosed in a glass or plastic tube
with sufficient gap in the outer conductor where observations could be
made to see if "reflections" or other observable anomalies, and voltage
and current observed.

I've found a slotted line works fine to 'see' a transmission line.