One interesting thing about clean up of radio shacks and moving equipment around is that you suddenly learn something is going on you really didn't know about.

Aces high E probe (L400b)  has always had the best or concurrently equal performance of all the arrays here but for the past 24 hours it had been down a dB or two s/n wise

Thinking it was a decoupling issue I checked feeders, performance of amp, splitters etc - and it wasn't till I realized I had put the die cast aluminum fully sealed box which houses the power injector and cap/transformer atop my computer that if I moved the box over the pressed steel lid of the computer we were getting induced magnetic noise coupled into the box causing a reduction of s/n.

 I could actually hear the difference on the R75 as I slide the box over the top of the computer lid - the noise wasn't coming in common mode on coax or via the power injector cables.

The box had to be laying flat atop the computer press steel cover to induce noise  - move it just 1 cm above and the coupling noise stopped.

So - lesson learned that Die Cast Aluminum boxes aren't perfect at magnetically induced fields - its not  shielded Mu metal!

Keep any coupling box or perhaps not perfectly screened/matched cables (mine didn't couple), away from sources of potential mag coupling noise generators - 

...and don't clean up the shack as your radio performance will likely suffer (just kidding!)

Laurence KL7L WE2XPQ