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

From: S. Weir ([email protected])
Date: Tue Jun 05 2001 - 11:29:49 PDT

Chris,

I would like to resolve this, and believe the comments below can.

At 10:36 AM 6/5/01 -0700, you wrote:
>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.

Chris, I'm sorry, but I never saw that 20 ohm driver in the discussion. I
think this is easier to analyze if we leave the driver at zero ohms for the
time being.

> > 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.

If the sources are all on the same side, I agree with you. If they are all
very close together we approximate simple vector addition.
I disagree that this is a model of the reflected wave case. See below.

>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.

Here, I think you are hinting yourself to the solution. The reflected wave
is not in-phase, it is another wave propagating in the opposite direction
and we connect an oscillator for one minute or two, our E measurement will
still be the same during the on times of the oscillator.

As the reflected wave propagates back towards the source, I returns to zero
at each point as di/dt goes through its maximum negative value w/reference
to the incident wave front. We get new spectral components along the line
based on f = 1/2*(Tflight_load - Tflight_local_point ) for each point on
the line. This effects a sort of down-chirp. The energy gets dispersed
over a frequency range.

Regards,

Steve.

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