[SI-LIST] Re: Help Explaining Microstrip
- From: Paul Levin <levinpa@xxxxxxxxxxxxx>
- To: dave.instone@xxxxxxxxxx, "Si-List (E-mail)" <si-list@xxxxxxxxxxxxx>
- Date: Thu, 25 Oct 2007 03:01:35 -0700 (GMT-07:00)
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>
>Sent: Oct 25, 2007 2:14 AM
>To: "Si-List (E-mail)" <si-list@xxxxxxxxxxxxx>
>Subject: [SI-LIST] Re: Help Explaining Microstrip
>
>More than 40 years ago, one of 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 that what 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, 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...
>> 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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