Dick, I believe that your 1amp antenna current into a wire from 450 watts
input PA, shows that you have an antenna system problem somewhere. It
suggests a loss resistance of aroun 400 ohms, which is very high. I can get
one amp into an antenna with about 60 watts. You should be seeing 2.5 to 3
amps, at that power level.

I think the problem is you do not have the tuning /matching of the antenna
properly sorted out, and this is putting high voltages back to the TX and
the LPF.  LF antennas can be difficult beasts, particularly when you are
"portable" and you do not have much time. If I can ride my "hobby horse" I
suggest you make up a simple RF bridge. You can use this to check the
antenna at very low power, before firing large amounts of RF at it. With
practice you will see what inductance loading and what impedance it is at
resonance. If you try to do this with a TX and SWR indicator you can get all
sorts of false indications.  Each antenna location seems to need special
treatment and needs a very flexible approach.

You should also be aware that on a shared site, the power from other
stations may be affecting your gear. I do not suggest it happened but
received signals from an HF station nearby could shut down yout SWR
protection, I have seen it happen on HF Field-day sites.

Best of luck for your next attempt.
 Alan G3NYK