Pete mate. Not wanting to disrupt your line of thought on this issue and believe me it's making my head hurt as well. But from my recollection a Faraday Cage needs to be grounded for it to function. In other words if your Schurzen is mounted on plastic and not "earthed" then it won't behave like a Faraday Cage. So your loss of signal range is due to other factors.
Try re-locating the aerial coil to an area within your tank that is away from electro-magnetic forces.

Let me know how you get on.

All the best mate,

Nick.