RE: [SI-LIST] : Trace Impedance Selection

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From: Mary (mary@advocate.net)
Date: Sat May 06 2000 - 11:39:42 PDT


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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