john_morris_uk
Well-Known Member
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The 50 Ohms is not coincidence - it is a round number halfway between the maximum power dissipation (30 Ohms) and minimum attenuation (77 Ohms).
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With the greatest respect, this sentence makes no sense AT ALL.
Attenuation of a transmission line is not a direct function of its impedence. You can have low loss lines of a variety of impedences.
Loss is a function of the conductivity and the dielectric medium of the transmission line.
Successful power dissipation occurs when the output stage, transmission line and antenna all have matching impedences.
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My reference would disagree with you (Lee T.H., "Planar Microwave Devices", Cambridge University Press, Cambridge 2004. pp. 71-73)
Power Handling:
For any given co-axial transmission line, there exists a breakdown voltage - to increase this you increase the diameter of the outer conductor which alters the characteristic impedance. There is a ratio of inner to outer diameters which give maximum breakdown voltage. This ratio is maximum at 30 ohms. Smith (of Smith chart fame received a patent for this result)
Attenuation:
is independent of conductor size, as you say. However, the resistance per unit length is caused by the skin effect. To reduce loss, you need a bigger centre conductor, which reduces the characteristic impedance, the optimal ratio of inner to outer diameters occurs at 77 Ohms.
That's why Transmitting eqpt is at 50 Ohms (halfway between max power handling and min attenuation) and Receiving eqpt is at 75 Ohms.
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I can see that this is true at microwave frequencies (I was a microwave RF engineer in a previous life) but you will also know that the skin effect is more marked at UHF and microwave frequencies. I don't believe that your argument holds so clearly for VHF.
However, despite my previous RF background, I had not heard the argument for 50 Ohms coax expressed in these terms before, so I will check the reference with interest.
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The 50 Ohms is not coincidence - it is a round number halfway between the maximum power dissipation (30 Ohms) and minimum attenuation (77 Ohms).
[/ QUOTE ]
With the greatest respect, this sentence makes no sense AT ALL.
Attenuation of a transmission line is not a direct function of its impedence. You can have low loss lines of a variety of impedences.
Loss is a function of the conductivity and the dielectric medium of the transmission line.
Successful power dissipation occurs when the output stage, transmission line and antenna all have matching impedences.
[/ QUOTE ]
My reference would disagree with you (Lee T.H., "Planar Microwave Devices", Cambridge University Press, Cambridge 2004. pp. 71-73)
Power Handling:
For any given co-axial transmission line, there exists a breakdown voltage - to increase this you increase the diameter of the outer conductor which alters the characteristic impedance. There is a ratio of inner to outer diameters which give maximum breakdown voltage. This ratio is maximum at 30 ohms. Smith (of Smith chart fame received a patent for this result)
Attenuation:
is independent of conductor size, as you say. However, the resistance per unit length is caused by the skin effect. To reduce loss, you need a bigger centre conductor, which reduces the characteristic impedance, the optimal ratio of inner to outer diameters occurs at 77 Ohms.
That's why Transmitting eqpt is at 50 Ohms (halfway between max power handling and min attenuation) and Receiving eqpt is at 75 Ohms.
[/ QUOTE ]
I can see that this is true at microwave frequencies (I was a microwave RF engineer in a previous life) but you will also know that the skin effect is more marked at UHF and microwave frequencies. I don't believe that your argument holds so clearly for VHF.
However, despite my previous RF background, I had not heard the argument for 50 Ohms coax expressed in these terms before, so I will check the reference with interest.
