In article ,
jim writes:

inductive loops eh... could you describe how they work please? - are

A sort of up-side down metal detector, i.e. the metal detector
is buried in the ground and detects metal passing overhead.
Google is you want to know more.

they DIY-able?

Yes, but possibly not legally. I don't know what the radio licence
requirements are, but whatever, you're unlikely to have the tools
necessary to prove they meet the radio licence requirements (as
they are radio transmitters).

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]