Radio Antennae
The widespread idea that 50 Ω and 75 Ω nominal impedances are connected with the input impedance of various antennae is, in fact, a myth. It is true, however, that several common antennae are easily matched to these cables. A quarter wavelength monopole has an impedance of 36.5 Ω, and a half wavelength dipole has an impedance of 72 Ω. A half-wavelength folded dipole, commonly seen on television antennae, on the other hand, has an impedance four times that of a dipole, that is 288 Ω. The 0.5λ dipole and the 0.5λ folded dipole are commonly taken as having nominal impedances of 75 Ω and 300 Ω respectively.
Read more about this topic: Nominal Impedance
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