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

From: Chris Rokusek ([email protected])
Date: Tue Jun 05 2001 - 10:36:09 PDT

Steve,

> Chris,
>
> I don't see how you can justify the surprise introduction of a 20 ohm
> driver resistance into the discussion. That reduces the incident
> amplitude
> seen by the load at the far end similarly. Is it appropriate to
> compare a
> circuit that provides 70% (Rdvr=20, Zl=50, Rload=50), against one that
> provides 100%, (Rdvr=20, Rser=30, Zl=50, Rload=open)?

Sorry if this is a surprise, I used a 20 Ohm driver in my first reply to
this thread to which no one posted any objections.

> For your analogy of a tight "U" shaped trace, I think that model is
> great. But the radiating cross-sectional area does not increase. The
> whole point is that we are assuming TEM, and so the wave is
> polarized. The
> scalar formula you are using holds only for the perpendicular
> cross-section
> which is just the original Length * Height. Put another way, if we have
> two traces, A and B where the driver for A is on the left, and B
> is on the
> right, and they are close to each other, driving each line
> alternately will
> not increase E over what we get by driving one line alone.

Doesn't look like we are going to converge on this one. I can't convince
myself if I take 1/100th of the current and put it on 100 lines each firing
sequentially that they are not going to add in phase at some point in the
far field. Especially if I consider the extreme case where each of the
lines are electrically short such that there would be complete overlap
between firing times.

Also, no one yet has mentioned the added discontinuity in the case of series
termination whereby the wave must make a reversal? Although this is a
higher frequency effect probably on the order of the oh-so-fun-trace-corner
thread, it should also be noted.

Best Regards,

Chris Rokusek
Innoveda

>
> Regards,
>
>
> Steve.

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