$6.$ Consider a dipole antenna with total end$-$to$-$end length of $4$ feet $($each leg is $1$ foot$),$ fed by current I$0=4$ A$.$ a$.$ Based on the antenna size, calculate the optimal operating frequency. b$.$ Calculate the total radiated power, Prad $($Hint: the radiation resistance of a dipole is known$)$ c$.$ Calculate the radiated power density S at R$=5$ km and $=90($broadside; maximum$),$ $=60,$ $=45,$ $=30,$ $=0($end fire$)$