**From:** *jan.vercammen.jv1@belgium.agfa.com*

**Date:** Wed Jan 17 2001 - 00:34:51 PST

**Next message:**Willis, Ken: "RE: [SI-LIST] : multiwire experience?"**Previous message:**ªL´Â·×: "FW: [SI-LIST] : RE: IBIS model question"**Maybe in reply to:**Yu Wang: "[SI-LIST] : why was 50ohm impedance chosen by engneers?"

hello si-list,

I would like to give some comments on (1) why 50ohm was used and on (2) the

assumption that lower resistance

transmission lines radiate less (as stated in the links of some comments).

(1) the main reason why 50ohm was chosen has probably to do with with the

practical situation at the time when the

coaxial standard was selected. For an an air dielectric a ratio of outer

diameter over inner diameter of 2.3 will give

exactly a 50ohm coaxial standard. So my guess that it has to do with harware

availabe, that is, with the diameter

of conductor tubing. Can anyone confirm this?? Is this documented somewhere??

(2) the transmission line impedance, assuming quasi-TEM operation, is defined as

the ratio of a two line integrals. The

line integral in the numerator integrates the electric field between the

conductor and the reference conductor over an arbitrary

curve. The line integral in the denominator integrates the magnetic field

around the center conductor over an arbitrary closed

curve. The line impedance does not give any information about the far-field

and, in general, one should not draw the

conclusion that a low impedance transmission line radiates less. Actually, I

associate a low impedance line with more

current and possible more radiated emission. Though much depends on the actual

driver circuits and the terminations.

What is your opinion??

regards,

Jan Vercammen

EMC/PCB Engineering Afga-Gevaert

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**Next message:**Willis, Ken: "RE: [SI-LIST] : multiwire experience?"**Previous message:**ªL´Â·×: "FW: [SI-LIST] : RE: IBIS model question"**Maybe in reply to:**Yu Wang: "[SI-LIST] : why was 50ohm impedance chosen by engneers?"

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