Andy wrote:

I just pulled out the PL259, so just the canter pin on the PL259 on the
canter fed dipole was connected
and the noise increased with the signal and then dropped when the PL259 was
screwed back in. I had the same on the Cobwebb.

When I did this on the MA5B the 10, 12, 15 and 17 meter bands did the same ,
on 20 meters it was different. When I pulled out the PL259 so just the
center pin of the PL259 was connected the noise dropped and with the PL259
screwed in
fully the noise went up. The opposite to what I thought would happen after
trying this out on my other antennas.

Andy

Sorry, but my previous comments pertained to the MA5V (vertical).

Having just assembled a TH-3, I can only suggest that perhaps you have
an open trap or two. Did you check the electrical continuity along each
of the elements after assembling them? What is the SWR on 20?

ac6xg