[SI-LIST] Re: Help Explaining Microstrip

  • From: "Mick Zhou" <XZhou@xxxxxxxx>
  • To: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>, <jeff.loyer@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Wed, 7 Nov 2007 11:24:33 -0500

Well, it is proven the volume charge density must be zero. So this is
totally different from his intuitive explanation based on charge density
movement. Physicists have tried so hard to deign measurements to detect
the second order (v/c)^2 effect if it does exist.  It is good we can
feel it.=20

As I mentioned, I cannot go beyond this point although it is a very
interesting topic.

Anyway, thanks,

Mick

-----Original Message-----
From: Grasso, Charles [mailto:Charles.Grasso@xxxxxxxxxxxx]=20
Sent: Wednesday, November 07, 2007 11:03 AM
To: Mick Zhou; jeff.loyer@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: Help Explaining Microstrip

Surely not! I though Eric Bogatins clear and consise
explanation was exactly on point.=20


Best Regards
Charles Grasso
Compliance Engineer
Echostar Communications Corp.
Tel: 303-706-5467
Fax: 303-799-6222
Cell: 303-204-2974
Pager/Short Message: 3032042974@xxxxxxxxx
Email: charles.grasso@xxxxxxxxxxxx

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Mick Zhou
Sent: Wednesday, November 07, 2007 8:29 AM
To: jeff.loyer@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Help Explaining Microstrip

All,

After I read some other arguments, I did a little research. Seems we
have to leave this to physicists/theorists who really understand
relativity etc., since it is much more complex than what we think in the
frame of Maxwell theory in electrical engineering. It is related to a
long debate in physics. It is not settled down as I can tell. If
somebody knows it is settled down, please share with us.

Here is just one example (available online),

Longitudinal electrodynamic forces
---and their possible technological applications
By Lars Johansson

You can follow the references. I will stop here.
Hope this helps.

Xingling (Mick) Zhou=3D20
Staff Device Engineer=3D20
Phone: 336-678-8942=3D20
Fax: 336-678-0415=3D20
E-mail: xzhou@xxxxxxxx=3D20

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Mick Zhou
Sent: Tuesday, October 23, 2007 4:40 PM
To: jeff.loyer@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Help Explaining Microstrip


Jeff,

To me, you have brought two different questions.

1. Why DC resistance is not influnced by the adjecent currents?=3D3D20
        The force you are talking about is the total force between too
loops Sure, if you place the two loops in free-space, you will see they
pysically move. This is exacly what Ampere etc observerd 200 hundred
years ago.
        R is different. It is about how the E-field is distributed
inside the conductors. For DC it is uniform for microstrip lines.  We
can think of infinite skin depth as well. Another assumption we use is
the uniform sources in this line case.
        We may be confused by the force on the charges at micro level.
They are not free charges. The force is not strong enough to force the
charge move to the surface. In skin effect, it is the E-field that
distributes toward surface only at high frequencies. Equivalent, R
increases.=3D3D20
=3D3D09
2. Why the micostrip and its ground do not repulse each other?
        I think it is a misunderstanding of "current".  There is no
isolated current elements although sometimes we take advantage of the
simplifications.  The microstrip and its ground form a loop together. It
should be considered as one current loop. There is a clear explanation
in Jackson's Classical Electrodynamics, Sect.5.2, (3rd Ed., 1998). This
is at macro-level,Biot and Savart law which is "Ampere's observations
about forces between current-carrying loops".
        It will be interesting to understand the self-interaction within
one loop at micro-level with Lorentz Force if careful enough. We have to
consider statistic effects. This is far beyond what this society is
concerned.=3D3D20

Regards,

Mick


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Loyer, Jeff
Sent: Tuesday, October 23, 2007 2:10 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Help Explaining Microstrip

I've been thinking (and reading a bit) about this, so thought I'd throw
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.=3D3D3D20

 V+ -------------------------------
                                  |
                                  |
                                  |
 -----<<<<<<<<<<<<<<<<<<<<<--------
 |
 ----->>>>>>>>>>>>>>>>>>>>>--------
                                  |
                                  |
                                  |                    =3D3D3D20
 V- -------------------------------

Assuming the "<" and ">" sections are close together, they will repulse
following the formula: F =3D3D3D3D 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). =3D3D3D20

Why aren't the DC currents influenced by the repulsive force? =3D3D3D20

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...
Start with the current we're talking about causing the repulsion: DC.  I
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?

Thank you in advance.

Regards,

Paul Levin
Senior Principal Engineer
Xyratex


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