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

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From: Sainath Nimmagadda (sainath@lsil.com)
Date: Fri Jun 01 2001 - 13:44:03 PDT


Hassan,

The antenna effect scenario is interesting. For simplicity, assuming a 90 deg
(curved) bend of uniform width, what is the maximum change in flux density B
as it turns around the bend?

Sainath

Hassan Ali wrote:

> > -----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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