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 I don't remember the historical reference now, but it is no accident that a standard resistor value is 51.1 ohms.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 > > _________________________________________________________________ > Express yourself instantly with MSN Messenger! Download today - it's FREE! > http://messenger.msn.click-url.com/go/onm00200471ave/direct/01/ > > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > or to administer your membership from a web page, go to: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > List FAQ wiki page is located at: > http://si-list.org/wiki/wiki.pl?Si-List_FAQ > > List technical documents are available at: > http://www.si-list.org > > List archives are viewable at: > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu