A loop antenna with loop radius of $0\mathrm{.}4$m is made of copper wire radius of $4$mm$.$ If the loop radiates at $6$MHz and carries a current of $50$A$,$ find $($sigma $=5\mathrm{.}8*10^7$ S$)$ a$)$ The radiation resistance loop b$)$ the power radiated c$)$ the radiation efficiency