It is very common in textbooks and papers written before about 1990 to substitute the approximate value 120π ohms for Z 0 {\displaystyle Z_{0}} . This is equivalent to taking the speed of light c 0 {\displaystyle c_{0}} to be 7008300000000000000♠3×108 m/s in conjunction with the current definition of μ 0 {\displaystyle \mu _{0}} . For example, Cheng 1989 states[2] that the radiation resistance of a Hertzian dipole is