Rf sniffer? Inductive loop (hearing aid)? Cant tell what frequency the loop is supposed to work at?
It looks to me that with FET source biased to +3V, that would have to be a depletion-mode J-FET. Gate should be reversed biased, so the 33K has a tiny current through it, so not sure why it is needed, except to limit gate current if a very large current is induced into the loop.
The diode detector is incorrectly wired up (there is no discharge path for the C to the left of the diode.