[SI-LIST] Re: 50 Ohm vs 75 ohm

50 ohms was originally shorthand for 51.1 ohms (just as a 2x4 piece of wood 
is not 2" x 4").  I don't remember the historical reference now, but it is 
no accident that a standard resistor value is 51.1 ohms.  These days 50 ohms 
is easier to fabricate on a PCB.  Before PCBs, 51.1 ohms was used for 
maximum power transfer and 75 ohms was used for maximum voltage transfer 
based on the dielectric materials available at the time.

Dennis


----- Original Message ----- 
From: "jen guest" <silist408@xxxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Monday, August 29, 2005 9:23 PM
Subject: [SI-LIST] 50 Ohm vs 75 ohm


> Hello SI expert,
>
> I am looking for a paper or an explanation of what the differences between
> 50Ohm and 75OHM transmission line. Why must be 50Ohm impedance matching
> instead 75ohm?
>
> Please help.
>
> Greatly thanks in advance,
>
> JEN
>
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