[SI-LIST] Comment on Radiation from Bends
- From: "Christian Schuster" <cschuste@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 30 Apr 2002 14:14:42 -0400
Dear Si-List, dear Jan,
I would like to comment on the issue on radiation from bends. Pleas find my
statements inserted in your text:
> I have done TDR measurements and the conclusion is, as everybody elses
> conclusion, that the bends are very hard to see, except at very high
frequencies.
> The radiation from bends is harder but it has been studied numerically
and
> has been measured!
Even if "high frequencies" is very relative I agree. I think it can be
safely stated that radiation is a third order effect, whereas the
discontinuity in
ch. impedance is second order (i.e. setting in at much lower frequencies).
For designers this means that it will be quite safe to approximate bend
discontinuities by purely reactive networks (e.g. the L-C-L T-network)
for all frequencies of interest.
> results have been reported in the IEICE transactions (Japanese IEEE) and
> later in the IEEE trans on EMC. The
> paper in the IEICE is publically available:
> http://search.ieice.org/2001/pdf/e84-b_9_2604.pdf
I have not read the paper (I will though) but I have done some FDTD
simulations on bends which
might be interesting for you:
Christian Schuster and Wolfgang Fichtner,
"Parasitic Modes on Printed Circuit Boards and Their Effects on EMC and
Signal Integrity",
IEEE Tr. EMC, vol. 43, no. 4, pp. 416-425, November 2001.
The paper discusses bends only in the context of surface wave excitation so
it will not really cover
all aspects of the phenomenon. However, what I found was the following:
TM0 surface waves have zero cutoff on grounded dielectric planes and may be
therefore
present for all frequencies. However, for "low frequencies" surface waves
behave as a "quasi plane wave"
with most of their energy in the air above the substrate. For "high
frequencies" the waves, i.e. their energy
becomes "trapped" in the substrate and their behavior is changed
(essentially they "slow down" and
they are sometimes called slow-wave modes then). In this regime bends can
radiate very effectively
into surface waves increasing radiation losses drastically. For a 0.6 mm
microstrip on 0.6 mm
alumina this becomes true for frequencies >= 50 GHz (well beyond what
usually is considered ).
The bad thing about is that the energy once injected into surface waves
remains in the substrate
(which should then be better considered as a sort of waveguide) and can
couple nicely into nearby
traces. The good thing about is that the critical frequencies are usually
"relatively high" and that you can push
them even higher by using lower DC's or thinner subtrates.
> Because most microstrip lines have very small h/lambda the radiation from
bends is very small. Actually the
> increase for practical tracks on PCBs is negligible and likely less than
1% (or even smaller), even for high
> frequencies.
I can confirm that h/lambda is an important parameter (h = substrate
height).
In terms of h/lambda I found a factor of about 1/3 or 1/4 critical, i.e. in
my simulations an h >=
lambda/4 gave me radiation into surface waves.
> To make things easier assume that the radiated power is emitted
isotropically than the
> far-field electric field is proportional to square root of the radiated
power. So a bend produces an
> electric field that is about 5% higher or only 0.4dB. Again, for most
practical stuff the 10% used here
> is much to high.
Although my FDTD simulations included free-space radiation I cannot make
any conclusions
about their importance or behavior. Likewise I have no idea how the
publication mentioned deals
with surface-wave radiation. However, there are publications which sort
free-space and surface-wave
radiation out..
I hope this is useful for you.
Regards,
Christian Schuster
IBM
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