[SI-LIST] Re: Help Explaining Microstrip
- From: olaney@xxxxxxxx
- To: dave.instone@xxxxxxxxxx
- Date: Thu, 25 Oct 2007 09:55:35 -0700
If you go to the science museum in Boston, there is a fun demo where you
push a button to make some very large high voltage capacitors dump into a
solenoid inductor. Just above that, there is an aluminum disk on a
vertical pipe. When the capacitors dump, the disk shoots nearly to the
ceiling, and it's a high ceiling. So yes, I'd say the force is
repulsive. But why not get a power supply and some floppy wire and
convince yourself? An auto battery should grunt enough current to
satisfy anyone. BTW, the forces in the solenoid are in two directions.
The turns want to expand radially, while the coil wants to contract along
its axis.
Orin
On Thu, 25 Oct 2007 11:52:23 +0100 David Instone
<dave.instone@xxxxxxxxxx> writes:
> 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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