RE: [SI-LIST] : Trace Impedance Selection

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From: Ron Miller (rmiller@Brocade.COM)
Date: Mon May 08 2000 - 13:56:15 PDT


TO: Don Degroot@boulder.nist.gov

In discussing EMI the following conversation came up.

Can you shed some light on this discussion?

Ron Miller

One of my thoughts below:

IMPEDANCE OF FREE SPACE VS HEIGHT NOT CONSTANT

I have seen dipole antenna impedances versus height above the ground
which go from zero ohms at ground level to about 400 ohms at 1/4 wavelength
and back again to low impedance(~50 ohms) at 2/4, and 400 again at 3/4 and
ripple like that with height until they meet an assymptote at 377 ohms.

From these impedance graphs I surmise that the near field impedance
requires a more complex model which would probably come from transmission
line equations. For our EMI case these equations would likely take the form
of
a coupling structure to the 377 ohms of free space.

> -----Original Message-----
> From: Doug McKean
> Sent: Monday, May 08, 2000 10:15 AM
> To: si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : Trace Impedance Selection
>
> 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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