# [SI-LIST] Re: Help Explaining Microstrip

• From: "Kihong Joshua Kim" <joshuakh@xxxxxxxxx>
• To: jeff.loyer@xxxxxxxxx
• Date: Tue, 23 Oct 2007 18:07:42 -0500
```Jeff,
I see your question is very viable.
Here is what I think we are in self-contradiction...

Let's review vector theory in Newton's law.
We have Lorentz' force of tangential component of ground plane in microstrip
structure due to the current flow in the signal conductor....The magnetic
field vector only has tangential component...

The thing is we ONLY have tangential component which will vector(cross)
product with qv(current direction).resulting forces only repulsive or
attractive.
-----> Don't laugh too loudly!
There is no tangential force components to make any electrons mobile along
the tangential surface except our false perception which tends our
mind-electron push towards sideways.

One more thing we should think of is boundary condition on metal surface in
ac-condition.
There is no magnetic field normal to the metal surface well-known as
boundary condition if you do path integral along the surface remaining
J(current density). This means the magnetic filed is always tangential to
ground plane. Also ending up with the conclusion that there is no force to
move electrons tangential by this magnetic filed except the electric field
built by potential difference of the circuit.

As a side-bar, there is a method to measure the electron concentration using
this type of boundary characteristics in solid-state physics so-called Hall
measurement. This method is using magnetic field normal to the metal(or
semiconductor) surface in which case it changes the current path inside the
material.

Let's do one more gedanken experiment...
What if the ground plane is not straight surface, say if it has a corrugated
surface like this.

* * * * * *
* * * * * *

*    *                           *    *
*               *                 *             *
*                         *   *  *

How will the current density look like?

Cheers,

Kihong Joshua Kim

SI in Photonics and Electronics

On 10/23/07, Loyer, Jeff <jeff.loyer@xxxxxxxxx> wrote:
>
> in my thoughts/questions...
>
> Reference: http://www.physics.upenn.edu/~uglabs/exp68_doc.pdf, among
> others
>
> Two conductors close together, carrying the same DC current (connected
> in series, resistors not shown), but in opposite directions.=20
>
> V+ -------------------------------
>                                  |
>                                  |
>                                  |
> -----<<<<<<<<<<<<<<<<<<<<<--------
> |
> ----->>>>>>>>>>>>>>>>>>>>>--------
>                                  |
>                                  |
>                                  |                    =20
> V- -------------------------------
>
> Assuming the "<" and ">" sections are close together, they will repulse
> following the formula: F =3D I^2 * (u0 * 2L)/(4 * pi * d0).
>
> But, there's no mention of the currents in the conductors being affected
> by this.  I've only heard of the currents in the conductors remaining
> distributed thoughout their entire cross-sectional areas to maintain the
> smallest impedance (resistance, in this case). =20
>
> Why aren't the DC currents influenced by the repulsive force? =20
>
> If they are influenced by the force (and the effective cross sectional
> area diminishes accordingly), the DC resistance would have to go up, yet
> I've never heard of DC resistance going up because 2 DC conductors are
> placed closed together.  What am I missing?
>
> Moving this to a PCB microstrip...
> wonder if we would measure some repulsion between microstrip traces and
> the adjacent ground, if we had small enough strain gauges.  I suspect
> not, since the current in the ground plane would be distributed
> throughout its entire area to minimize resistance.  Force that ground
> plane to be very small (such that it becomes a trace), and directly
> below the microstrip trace, and I think you would have to see repulsion.
> But again, I haven't heard of any change in current distribution due to
> the repulsive force (and, it seems that this would apply to coplanar
> traces).
>
> Now moving to AC in a PCB microstrip...
> As we move to AC, the current in the conductors distributes itself
> differently to minimize impedance - the current in the plane bunches
> under the trace.  Again, we end up with 2 conductors close together,
> carrying current in opposite directions.  I suspect the conductors must
> be repulsed, though I haven't heard of the distribution of the currents
> in the conductors being affected.  And, as was pointed out, the adhesion
> to the substrate is strong enough to keep the traces from separating.
>
> So: for the AC-case, very sensitive strain gauges would detect the
> microstrip trace being repulsed by the ground plane, but why the current
> distributions (and subsequent impedance) aren't affected isn't clear to
> me.
>
> Still left wondering...
>
> Jeff Loyer
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Paul Levin
> Sent: Wednesday, October 17, 2007 1:44 PM
> To: SI-LIST Reflector
> Subject: [SI-LIST] Help Explaining Microstrip
>
> Dear SI-LIST'ers,
>
> I'm working on a presentation to explain transmission line to
> non-engineers and I find myself stumbling over some of the basics.
> (There's nothing like explaining something to bring out all of the
> glitches in what you were sure you
> understood!)
> I'm hoping that one of you may be able to supply the missing link.
>
> Nearly two hundred years ago Oersted and Ampere figured out that if you
> have two conductors carrying current in the same direction, they would
> would to pull in close to each other whereas if you had two conductors
> carrying current in opposite directions, they would want to separate.
>
> If one were to apply just these observations to microstrip, you would
> expect to see all of the trace current bunched on the side away from the
> ground plane and the return plane current in two bunches to either side
> of the trace and as far away from the trace as possible, if not on the
> bottom.
>
> Of course, this is almost exactly opposite from what we know happens.
>
> What is the force that overcomes Oersted and Ampere and causes the trace
> and return currents to be so heavily attracted to each other?
>
>
> Regards,
>
> Paul Levin
> Senior Principal Engineer
> Xyratex
>
>
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