[roberts]

"It will be interesting to hear your ideas about the two separate detection curcuits.."

It is quite question. I guess only original designer knows right answer.
My presumption would be that second reciver - direct receiver with ferrite rod, actually have unusuall role here. If those coils are fully resonant, than weak signals would not be enough to disturb balance. Therefore some extra "disturbing factor" must be added in overall circuitry - i guess direct receiver is there cose of that..But than again, if i am right, why using fully resonant coils???