[SI-LIST] Re: Help Explaining Microstrip
- From: "SILR" <silr@xxxxxxxxxxxx>
- To: <levinpa@xxxxxxxxxxxxx>, "'SI-LIST Reflector'" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 19 Oct 2007 12:34:37 -0400
Hi Paul,
You asked:
"What is the force that overcomes Oersted and Ampere and causes the trace
and return currents to be so heavily attracted to each other?"
I don't believe there are any laws of physics that will overcome what
Oersted and Ampere have observed. I think the cases that you're describing
cover different physical EM concepts which in some ways may seem to be in
conflict with one another but in every one of those cases, they will always
(for now until proven otherwise) obey the laws of physics (Maxwell's
Equations).
I have given this some thought and this is the way I see your dilemma... (I
may be wrong on this but I'm sure the list will point that out if I am...so
rest assured :-)
I believe what Oersted and Ampere observed during some of their experiments
in their time was the outcome of the Lorentz Force. This force is
described by the following eqn:
F = qE + (qv X B) :
Where
q is charge
E is E field
v is the direction of the current
B is Mag field
In a static case, you can ignore the (qv X B) term and you just have qE
which says all like charges will repel one another and vice versa.
In your case (and what Oersted and Ampere observed) where you have two
conducting wires, the (qv X B) term takes over and would cause the two wires
to attract one another if they are carrying currents in the same direction.
Conversely, they would repel if carrying currents in opposite directions.
So all is well here!
Now, what happens in your microstrip case ??????
First, I think of microstrips as the TRUE transmission lines (or TL).
Whereas in the case above for the two current carrying conductors, all I see
are two wires with DC currents in them so I do not really consider them to
be transmission lines. Rather, I view the two wires as if they are short
magnetic wires. Magnetic wires such that the North-South poles line up in
the direction of the current (N being the head of the current vector and the
S being the tail of the current vector) If you look at it this way then
you can easily see why they would attract and repel one another.
In microstrip TLs, first I think in terms of AC currents with the TLs having
both a signal current path and return current path to which they both
reference to one another. Furthermore, when I think of AC currents, I
don't really view the AC currents traveling down the TLs... rather I view
the EM field disturbance traveling down the TLs. By this way, I view the
AC currents to flow such that EM field is able to propagate down the TL.
And while doing this, the AC currents will flow such that it requires the
least amount of energy to do so. Meaning that it will find the path of the
least Impedance. For this to happen, it has to flow such that the signal
current and the return current have to be as close as possible (or coupled)
to one another. This is where you may seem to observe the opposite of what
were described by the two conducting wires above.
So in summary, I view the two DC wires to behave as if they were sort of a
pair of magnetic wires. Whereas for a microstrip TL, I view it as a single
transmission line where the EM field propagates down the line and the AC
currents are there to make that happen with the least amount of energy. I
don't view each of the signal path and the return path as separate magnetic
wires as were described for the two DC conductors. Rather, I view the two
paths of a TL making up a tightly coupled current loop designed to allow
propagation of EM fields down the line between a transmitter and a receiver.
At least this is the way I view it... Hope it helps to some degree... If
it doesn't, then I'm sure the rest of the list will help you in the end!
:-)
Thanks!
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Paul Levin
Sent: Wednesday, October 17, 2007 4:44 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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