Re: [SI-LIST] : Trace Impedance Selection

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From: Neven Pischl (npischl@cisco.com)
Date: Sun May 07 2000 - 09:12:31 PDT


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
> >
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