Re: [SI-LIST] : Trace Impedance Selection

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From: Vinu Arumugham (vinu@cisco.com)
Date: Fri May 05 2000 - 17:55:52 PDT


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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