Re: [SI-LIST] : why was 50ohm impedance chosen by engneers?

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From: jan.vercammen.jv1@belgium.agfa.com
Date: Wed Jan 17 2001 - 00:34:51 PST


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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