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

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From: Doug McKean (dmckean@corp.auspex.com)
Date: Fri May 25 2001 - 16:18:57 PDT


subramanya wrote:
>
>
> Hello SI Gurus,
> I have a basic question here. We all know
> that "Right angled traces are never used
> on PCBs" as the act as antennas (better
> antennas than the curved traces). In an
> article I read that this radiation is due
> not to the frequency of the signal , but
> to it's rise time. Could someone please
> explain me this theory ?

The answer is yes and no.

Yes, because theoretically, bending a conductor
will make it radiate. This comes from the
Laplacian taken from Maxwell which shows that
acceleration of charge is required for radiation.

But, what some people forget is that this is in
a vectorial sense as well. So, by changing the
direction of an accelerating charge, radiation
should happen. Fractal antennas make use of this
effect.

I have actually tried to measure the effects
of bending various lengths of wire and measuring
the radiated effects. But because of too many
variables, I can't conclusively say that bending
causes any sort of increase in radiation.
Maybe I should revisit that.

Now, how much does the bending make? I don't know.
I've personally not seen anything in my lab or
anyone's lab which would make me positively
state never use right angles. In my opinion,
as far as radiation effects go, I don't think
they really cause that much of a difference.

Now, as far as crosstalk goes, there can be a
differential-to-common mode transfer mechanism
going on. Now I've seem many a discussion try
to reduce the right angle to things such as
impedance changes, capacitance changes, but
I think they fall short.

- Doug McKean

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