RE: [SI-LIST] : Re: Antenna Problem on the Board

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From: Hassan Ali (hali@lanterncom.com)
Date: Fri Jun 01 2001 - 12:50:54 PDT


> -----Original Message-----
> From: Doug McKean [mailto:dmckean@corp.auspex.com]
> Sent: Friday, June 01, 2001 11:57 AM
> To: Signal Integrity
> Subject: Re: [SI-LIST] : Re: Antenna Problem on the Board
>
>
> Prasad,
>
> Yes, and thanks. Again, this is only my opinion.
>
> When you apply the vector definition of acceleration to
> include directional change of a vector field such as an
> electric or magnetic field, then you should get emission.

True! The antenna effect (especially the folded-dipole antenna mode) results
when the bend introduces a voltage drop (V) across it. This is true if the
bend creates an inductance, L, since (V = L dI/dt). Remember,

Inductance, L = Magnetic Flux /Current
Magnetic Flux = integral (Flux Density). dS , where dS is elemental
surface area (of the bend).

Flux Density (B) is a vector and changes around the bend. 90 degrees bend
gives the maximum change in B hence the maximum inductance, and hence the
maximum voltage drop, and hence the maximum (possible) radiation.

> Aside from all this theoretical talk, how this results in
> actual practice, I haven't seen an increase in emissions
> to be significant.

Probably true. There are so many factors. One is whether the PCB track
satisfies the conditions of being an efficient antenna.

> Now, concerning bends in traces with regard to crosstalk,
> I think I can say there is concern.

But again the question is, how big is the concern? All these claims have to
be qualified - one of these days ;-) I have some doubts on the claims that
the bend effect is only of concern for very high frequencies (above 20GHz).
In as far as we are worried about PCB emissions at frequencies lower than 1
GHz, then we should make sure that there are no situations where the bends
could indeed radiate substantially at those low frequencies. Some research
needs to be done here (if not yet done).

Hassan.

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