[aprssig] Why is coax 50 ohm ?

[Andrew Rich]

Is there a reason why coax is 50 ohm ?

And 75 ohm ? 

And 92 ohms ?

Is it historical or is there a scienific reason why ?
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Andrew Rich
Amateur radio callsign VK4TEC
email: vk4tec at tech-software.net
web: http://www.tech-software.net
Brisbane AUSTRALIA 

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