Now for some weird stuff!
Just to remind you, this arrangement produces 3.9 @ 2mV/Div=7.8mVp2p

Turning through 90 degrees on axis makes no difference!

But turning the axis through 90 degrees drops the hum to 1.2 @ 2mV/Div=2.4mVp2p, and reduced the hum to 30.7%

Now comes the weird stuff. When I put the steel plate next to either the source TX or the XSM TX it made no difference!
But moving it near the middle finds a null point and the hum drops dramatically to
[email protected]/div =0.5mVp2p, that's a hum reduction to 6.4%

The croc clips are to earth all the metal work.
To go back to our previous example, we now have 0.5mVp2p which is 177uVrms.
With 1.55V that is a S:N ratio of 78.8dB and with 155mV after the pad its 58.8dB, but that is with a distance of only100mm.
If we take the distance back out to 200mm, then the hum falls to 0.7mV with no screen, but after finding the best placement for the screen the hum has disappeared and only hiss is visible on the scope.

These measurements are not absolute but they are all completely relative to each other and we can learn the following from our tests.
1. The distance between the PT and the IPT should be as long as possible.
2. Screens work better when sandwiched with dissimilar metals.
3. The axis of the IPT should be 90 degrees to the axis of the PT.
4. Screens should be placed after trial positioning for lowest hum.
This is the end of the transformer tests, I'll now get on with the layout.
Best
DaveP