A question about the metal detector design

[davros]

Hi i am new here and have been thinking about making a metal detector .
I noticed on reading stuff on the coils some use foil around the coils ,i was wondering why isn't the cover above the coils made of metal to reflect the rf down to the ground if its not half the rf would be wasted going both directions ?
i suppose any metal near the coils would cause a problem but i was thinking how rf is used in radio radar sonar and such .
Sorry if it sounds dumb Question as i said i am very new to metal detecting .

[Carl-NC]

First thing to learn with induction metal detectors is that they use magnetic fields, not radio waves. So radio antenna techniques don't work at all.

[Qiaozhi]

Sorry if it sounds dumb Question as i said i am very new to metal detecting .

Inside the METAL DETECTOR - Published September 2012