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Actually, there is a good reason for 75 ohm coax. If you plot cable loss
versus impedance, you will find that minimum loss occurs at 75 ohms. When
you've got miles and miles of coax strung everywhere, as cable TV companies
do, you want to minimize loss to reduce the number of booster amplifiers
needed. In addition, since 75 ohm coax has lower capacitance per unit
length, it has better high frequency response than 50 ohm cables. Best
power handling ability occurs at 30 ohms impedance. I don't know that any
industry uses a 30 ohm system impedance, but - 50 ohms is a good compromise
between minimum loss at 75 ohms and best power handling at 30 ohms. (All
this is the best I remember it, there might be a bit more to it that I have
forgotten over the years.)





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