[SI-LIST] Re: Help Explaining Microstrip
- From: David Instone <dave.instone@xxxxxxxxxx>
- To: "Si-List (E-mail)" <si-list@xxxxxxxxxxxxx>
- Date: Thu, 25 Oct 2007 11:52:23 +0100
Dear Paul,
Well, the feeder was a 'standard' open wire feeder, two wires about 10
inches apart, so I guess the current would be in opposite directions, (as
Irecall the carrier frequency was around 15KHz). Regarding whether they
sprang apart or together, I wasn't there I can only recall what I was told,
the actual words he used were 'you could see the feeder doing this' and
demonstrating with his hands. At morse speeds the wires would be in
continual movement so telling whether they were moving apart or together
would be difficult to determine.
Regards Dave Instone +44 (0)1235 824963 OXFORD SEMICONDUCTOR LIMITED 25
MILTON PARK ABINGDON OXFORDSHIRE OX14 4SH Registered in England no 2733820
Registered Address: As above
Paul Levin wrote: Dear Dave, That demonstration is exactly my problem. Were
those two feeders carrying current in opposite directions? If so, I believe
Oersted says that they should spring apart. Were those two wires part of a
single-turn inductor? Then minimizing energy (=L*I*I/2) says minimize L
(mu0*Area), hence minimize Area, or get closer together. These two things
seem to be in opposition to each other. Regards, Paul Levin Xyratex
-----Original Message----- From: David Instone <dave.instone@xxxxxxxxxx>[1]
Sent: Oct 25, 2007 2:14 AM To: "Si-List (E-mail)" <si-list@xxxxxxxxxxxxx>[2]
Subject: [SI-LIST] Re: Help Explaining Microstrip More than 40 years ago,
oneof the members of the amateur radio group I belonged to at the time was
shown round the VLF high power transmitting station at Rugby UK. He said
thatwhat most demonstrated the power of the Tx was seeing the two wires of
the open wire feeders springing towards each other every time the morse key
was pressed. No need for a strain gauge there. Regards Dave Instone +44
(0)1235 824963 OXFORD SEMICONDUCTOR LIMITED 25 MILTON PARK ABINGDON
OXFORDSHIRE OX14 4SH Registered in England no 2733820 Registered Address: As
above Loyer, Jeff wrote: 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[3], 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
remainingdistributed thoughout their entire cross-sectional areas to
maintainthe smallest impedance (resistance, in this case). =20 Why aren't
theDC 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... 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,
aswas pointed out, the adhesion to the substrate is strong enough to keep
thetraces from separating. So: for the AC-case, very sensitive strain gauges
would detect the microstrip trace being repulsed by the ground plane, but
whythe current distributions (and subsequent impedance) aren't affected
isn'tclear to me. Still left wondering... Jeff Loyer -----Original
Message----- From: si-list-bounce@xxxxxxxxxxxxx[4]
[mailto:si-list-bounce@xxxxxxxxxxxxx[5]] 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
toexplain transmission line to non-engineers and I find myself stumbling
oversome 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
toeach other? Thank you in advance. Regards, Paul Levin Senior Principal
Engineer Xyratex
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