**From:** Neven Pischl (*npischl@cisco.com*)

**Date:** Sun May 07 2000 - 09:12:31 PDT

**Next message:**Mary: "RE: [SI-LIST] : Trace Impedance Selection"**Previous message:**Song In-myung: "[SI-LIST] : or3t125a.ibs"**In reply to:**Mary: "RE: [SI-LIST] : Trace Impedance Selection"**Next in thread:**S. Weir: "Re: [SI-LIST] : Trace Impedance Selection"

Mary,

the posts are mostly serious and not sarcastic.

An illustration of the difference between the characteristic impedance of a

T-line and intrinsic impedance is simply a microstrip. While the

characteristic impedance can be designed to be 50-Ohm, the intrinsic

impedance will dictate that everywhere where the two components of the EM

field are perpendicular (draw the field lines, and you can see it is

everywhere (for TEM-mode), the ratio of E to H is 377 Ohm (for air

dielectric).

Neven

----- Original Message -----

From: Mary <mary@advocate.net>

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

Sent: Saturday, May 06, 2000 11:39 AM

Subject: RE: [SI-LIST] : Trace Impedance Selection

*>
*

*> I hesitate to respond because I'm not sure if you were being sarcastic.
*

*> I have not been following this newsgroup long enough to judge the tone
*

*> of the posts. However, if your question was serious, here is my answer.
*

*>
*

*> For plane wave propagation, the ratio of the electric field strength to
*

*> the magnetic field strength will be equal to the intrinsic impedance of
*

*> the medium. 377 ohms is the intrinsic impedance of free space, so the
*

*> ratio of E to H will be 377 ohms whether the wave is radiated or guided
*

*> by an air-dielectric transmission line.
*

*>
*

*> Transmitters and receivers don't care what the ratio of the field
*

*> strengths are, they deal with voltages and currents. The ratio of
*

*> voltage to current on a transmission line depends on the characteristic
*

*> impedance. Characteristic impedance is not the same as intrinsic
*

*> impedance. Characteristic impedance depends on the geometry of the
*

*> conductors.
*

*>
*

*> Neither the characteristic impedance of a transmission line nor the
*

*> intrinsic impedance of free space look like a resistance between a
*

*> transmitter and a receiver. That is what is wrong with the scenario
*

*> you suggested.
*

*>
*

*> A simple example, that might help to illustrate this point would be a
*

*> 377-ohm coaxial cable. If the materials were lossless, none of the
*

*> power flowing down the cable would be radiated and all of the power
*

*> would be delivered to a matched load. (Same would be true for a
*

*> lossless coaxial cable with any other characteristic impedance.)
*

*>
*

*> I hope this is helpful. If your post was not serious, then I guess
*

*> the joke's on me. Although maybe this discussion will help others
*

*> who follow it.
*

*>
*

*> Mary
*

*>
*

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

*> From: owner-si-list@silab.eng.sun.com
*

*> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Vinu Arumugham
*

*> Sent: Friday, May 05, 2000 7:56 PM
*

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

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

*>
*

*>
*

*> If you were able to connect a transmitter to a receiver using a 377 ohm
*

*> transmission line, this line would be in parallel to the "transmission
*

*> line" between the two formed by free space. Therefore, one half the
*

*> transmitted power would go through free space and the other half through
*

*> the line. As the line impedance is lowered, more power would be
*

*> transmitted through the line and less through space.
*

*>
*

*> What's wrong with this scenario?
*

*>
*

*> Thanks,
*

*> Vinu
*

*>
*

*> Mary wrote:
*

*>
*

*> > Somone recently claimed that higher impedance transmission lines
*

*> > radiate more because their impedance is closer to the 377-ohm
*

*> > impedance of free space. This is not true. It is not possible
*

*> > to judge anything about the radiation from a transmission line
*

*> > based on the value of its characteristic impedance.
*

*> >
*

*> > Characteristic impedance is the ratio of voltage to current in a
*

*> > forward traveling wave. The ratio of electric to magnetic field
*

*> > strength in a free-space transmission line is approximately
*

*> > 377 ohms regardless of what the characteristic impedance is.
*

*> > Even if you were to build a transmission line with a 377-ohm
*

*> > characteristic impedance, there is no reason to believe it would
*

*> > radiate any better or worse than a 300-ohm or a 400-ohm line.
*

*> >
*

*> > Mary
*

*> >
*

*> > **** To unsubscribe from si-list or si-list-digest: send e-mail to
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*> > majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
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*> > ****
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*>
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*>
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**Next message:**Mary: "RE: [SI-LIST] : Trace Impedance Selection"**Previous message:**Song In-myung: "[SI-LIST] : or3t125a.ibs"**In reply to:**Mary: "RE: [SI-LIST] : Trace Impedance Selection"**Next in thread:**S. Weir: "Re: [SI-LIST] : Trace Impedance Selection"

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