50Ω

btty038

50Ω [Copy link]

original:

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 50 characteristic impedance. According to some, that was the birthplace of the 50 standard. Another explanation is that for minimum signal attenuation, the characteristic impedance of a coaxial transmission line must be around 77. For maximum power handling capability the optimum impedance is around 30 (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 75 as a standard, requiring special test equipment, cables, connectors and other components for their operation [3].

Translation:

There are two explanations: one more practical and the other more scientific. In the microwave transmission systems of the 1930s, coaxial transmission lines were first manufactured in the UK using standard British pipes. Using a common center conductor to this pipe resulted in a 50Ω characteristic impedance. According to some, this is the birthplace of the 50Ω standard. Another explanation is that in order to minimize signal attenuation, the characteristic impedance of the coaxial transmission line must be around 77Ω. For maximum power handling capability, the optimal impedance is about 30Ω (Figure 2.13-1). The arithmetic mean between 30 ohms (best power handling) and 77 ohms (lowest loss) is 53.5, and the geometric mean is 48 ohms. Therefore, for the air medium, the choice of 50 ohms is a compromise between power handling capability and signal loss per unit length. However, there are exceptions where it makes more sense to use a different characteristic impedance. For example, in long-distance cable TV systems, the signal needs to be repeatedly amplified to overcome cable losses, so attenuation is more important than power handling capability. Therefore, the cable TV industry uses 75Ω as the standard, which requires special test equipment, cables, connectors, and other components to operate [3].

Power handling and attenuation vs. characteristic impedance. Power is normalized to maximum value and attenuation is normalized to minimum value.

Fred_1977

Learned it!

se7ens

It seems that the explanation in impedance matching is to maximize the transmission power.

Optimum power and lowest loss impedance is a little hard to explain

btty038

se7ens posted on 2022-2-15 10:52 It seems that the explanation in impedance matching is to maximize the transmission power and minimize the loss impedance, which is a bit difficult to explain

His explanation is that the sentence you said above is a comprehensive one. Matching is to ensure the integrity of the signal without damage to the transmission. Low loss and high power transmission are from two dimensions. As for the picture above, the impedance of 50Ω is finally obtained.