O.K. I am not sure that can follow your ideas.
What makes you think that radiated energy from TX will deflect from ...let's say burried coin in the ground??? Even if deflect, what makes you sure that you will receive it from long distance, also what makes you think that you'll be awared about received signal and its origin??
I am trying to say that whole principle here is wrong! Not gonna work! Never worked and it never will work.....ever never! Trust me on that!
O.K. you can detect coin with this method from ...let's say few mm..and nothing more! Whole idea is nonsence! Forget that! Wasted time!
If you still want to try....who's stoping you? Me? Of course not!
Good luck wasting your time!