Re: [SI-LIST] : Trace Impedance Selection

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From: Doug McKean (dmckean@corp.auspex.com)
Date: Mon May 08 2000 - 10:15:13 PDT


Couple of thoughts about this ...

First, even though the overall impedance may
be equal, the components of permittivity and
permeability of both mediums must be equal.
Otherwise, boundary conditions will be encountered.

Once the wave is launched from the trace (no
matter what the impedance is) into space, it
is there that your equal power transfer occurs
as it self-propogates through space. But this
isn't the primary mechanism by which a trace
causes radiation.

Second, a lower impedance may mean more current
driving the trace, thus causing more radiation.
Why I'm suggesting this is - in order for radiation
to occur, one must somehow end up with the wave equation.
In order to get there from charge movement in a medium,
do a Laplacian on Maxwell and you end with charge
acceleration as the mechanism for the final wave
equation, i.e. emissions. One of the bricks in
antenna design.

When there's no charge acceleration, i.e. DC, no
radiation, i.e. no launching of a wave into forever.
Thus, the reason why I'm toying around with this idea
is that since the lower impedance of a trace might
allow for higher charge acceleration, it should thus
cause more radiation.

One of the reasons why I hesitantly claim bends in
traces can increase emissions. Maxwell demands it
IF the discussion above regarding acceleration turns
toward the changing direction of a *vector field* such
as TE wave. To the extent that bends in traces cause
a problem for us ... ? < chuckle > Well, that's
grounds for another long thread. And now I'm
digressing.

But again, I'm not very sure about all this.
And sorry for the lengthy thread.

- Doug McKean

Vinu Arumugham wrote:
>
> 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

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