2.12 Video Tutorials and Animations
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2.13 Why 50Ω ?
There are two explanations: one is more practical and one more scientific. In microwave
transmission systems of the 1930s, coaxial transmission lines were initially fabricated in England with
a standard British plumbing pipe. Using a commonly available centre conductor to this pipe led to a
50Ω characteristic impedance. According to some, that was the birthplace of the 50Ω standard.
Another explanation is that for minimum signal attenuation, the characteristic impedance of
a coaxial transmission line must be around 77Ω. For maximum power handling capability the
optimum impedance is around 30Ω (Figure 2.13-1). The arithmetic mean between 30 ohms (best
power handling) and 77 ohms (lowest loss) is 53.5, the geometric mean is 48 ohms. Thus the choice
of 50 ohms is a compromise between power handling capability and signal loss per unit length, for
air dielectric.
However there are exceptions where using a different characteristic impedance makes more sense.
For example, in long-haul cable television systems, where the signal needs to be amplified
repeatedly to overcome cable losses, attenuation is more important than power handling capability.
Accordingly, the cable TV industry uses 75Ω as a standard, requiring special test equipment, cables,
connectors and other components for their operation [3].
Figure 2.13-1 Power handling and Attenuation vs characteristic impedance. Power is normalised to its maximum value,
attenuation to its minimum value.
Further information on this topic may be found at
http://www.microwaves101.com/encyclopedia/why50ohms.cfm
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