Exactly speaking, the antenna is made up of coax cable with 50 ohms. So why is an antenna’s impedance always 50 ohms?
1)With the metal shielding layer
2)Good capacity of anti - jamming
3)Minimum transmission impairment
PS: Antenna , also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves, which requires the antenna should have a good impedance match.(Antenna-Wikipedia)