It is very common in textbooks and papers written before about 1990 to substitute the approximate value 120 π ohms for Z 0. This is equivalent to taking the speed of light c to be precisely 3 × 10 m/s in conjunction with the then-current definition of μ 0 as 4 π × 10 H /m. For example, Cheng 1989 states that the radiation resistance of a Hertzian dipole is