**From:** Vinu Arumugham (*vinu@cisco.com*)

**Date:** Mon May 08 2000 - 10:24:36 PDT

**Next message:**Chris Rokusek: "RE: [SI-LIST] : May 9th Presentation: "Radiation from Edge Effects in Printed Circuit Boards (PCBs)""**Previous message:**Doug McKean: "Re: [SI-LIST] : Trace Impedance Selection"**In reply to:**Neven Pischl: "Re: [SI-LIST] : Trace Impedance Selection"**Next in thread:**Doug McKean: "Re: [SI-LIST] : Trace Impedance Selection"**Reply:**Doug McKean: "Re: [SI-LIST] : Trace Impedance Selection"**Reply:**Neven Pischl: "Re: [SI-LIST] : Trace Impedance Selection"

Can free space be modeled as a network of transmission lines, arranged in such

a manner that the impedance seen looking into any two points in space, is 377

ohm?

Thanks,

Vinu

Neven Pischl wrote:

*> Here's my $0.02.
*

*>
*

*> The scenario doesn't work because it roughly assumes that if you have a 377
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*> Ohm source, and a 377-Ohm resistor somewhere in free space, that you will
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*> have max power transfer (even without any other connection). That's what you
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*> are saying when you say that half power would go through the space (at least
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*> it seams to me so).
*

*>
*

*> It mixes the wave-guiding concept in which there is at least two-conductor
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*> structure that guides EM-waves, and it is characterized by its
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*> characteristic impedance, with the free-space EM-wave propagation in which
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*> the intrinsic impedance of the medium (not characteristic impedance of a
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*> waveguide) is 377 ohm. There is an analogy between these two ways of
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*> propagation, in terms of mathematical description, but the terms do not have
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*> the same meaning.
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*>
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*> Same as a 377 ohm line will not radiate half power to the space, when
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*> immersed into the EM field, it will not couple half power of the field
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*> (which should happen if the concept is right).
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*>
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*> It can be seen that the concept does not work also if you examine a 50-Ohm
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*> air- microstrip. If you assume the concept of splitting power between the
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*> line and the air, 50/377 of the total power which is about 0.13 (or 13%)
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*> would always be lost to the space, even in a perfectly matched 50-Ohm
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*> system. We know that it does not happen when you connect a matched load,
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*> source and a line.
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*>
*

*> Neven
*

*>
*

*> ----- Original Message -----
*

*> From: Vinu Arumugham <vinu@cisco.com>
*

*> To: <si-list@silab.eng.sun.com>
*

*> Sent: Friday, May 05, 2000 5:55 PM
*

*> Subject: Re: [SI-LIST] : Trace Impedance Selection
*

*>
*

*> > If you were able to connect a transmitter to a receiver using a 377 ohm
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*> > transmission line, this line would be in parallel to the "transmission
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*> > line" between the two formed by free space. Therefore, one half the
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*> > transmitted power would go through free space and the other half through
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*> > the line. As the line impedance is lowered, more power would be
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*> > transmitted through the line and less through space.
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*> >
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*> > What's wrong with this scenario?
*

*> >
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*> > Thanks,
*

*> > Vinu
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*> >
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*> > Mary wrote:
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*> >
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*> > > Somone recently claimed that higher impedance transmission lines
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*> > > radiate more because their impedance is closer to the 377-ohm
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*> > > impedance of free space. This is not true. It is not possible
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*> > > to judge anything about the radiation from a transmission line
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*> > > based on the value of its characteristic impedance.
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*> > >
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*> > > Characteristic impedance is the ratio of voltage to current in a
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*> > > forward traveling wave. The ratio of electric to magnetic field
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*> > > strength in a free-space transmission line is approximately
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*> > > 377 ohms regardless of what the characteristic impedance is.
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*> > > Even if you were to build a transmission line with a 377-ohm
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*> > > characteristic impedance, there is no reason to believe it would
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*> > > radiate any better or worse than a 300-ohm or a 400-ohm line.
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*> > >
*

*> > > Mary
*

*> > >
*

*> > > **** To unsubscribe from si-list or si-list-digest: send e-mail to
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*> >
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*> >
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*> >
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*> >
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*> >
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*>
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**Next message:**Chris Rokusek: "RE: [SI-LIST] : May 9th Presentation: "Radiation from Edge Effects in Printed Circuit Boards (PCBs)""**Previous message:**Doug McKean: "Re: [SI-LIST] : Trace Impedance Selection"**In reply to:**Neven Pischl: "Re: [SI-LIST] : Trace Impedance Selection"**Next in thread:**Doug McKean: "Re: [SI-LIST] : Trace Impedance Selection"**Reply:**Doug McKean: "Re: [SI-LIST] : Trace Impedance Selection"**Reply:**Neven Pischl: "Re: [SI-LIST] : Trace Impedance Selection"

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