why 50 ohm ?

This is a question asked frequently. There is quite a bit of information available on the internet.

Here are a few things to read in answer to your question:

h**p://www.reed-electronics.com/ednmag/index.asp?layout=article&articleid=CA60931

h**p://www.epanorama.net/documents/wiring/cable_impedance.html

h**p://www.trompeter.com/assets/product/PDF/wiring.pdf

(*=t above)

50ohm is a compromise between 30ohm and 75ohm.

50 ohm is the best compromise between attenuation and power handling.
In fact at 30 ohm you have the max power handling instead at 77 ohm you have the min attenuation (that is important for TV cable).

Regards.

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