Date: Thu Apr 20 2000 - 11:49:28 PDT
To partition the discussion...
<<Do you know if the fringing (NEAR) fields in a board NOT using 20H can
radiate DIRECTLY into the far field significantly given typical excitation
(say <500mv up to 1GHz) or must these NEAR fringing fields couple onto a
more efficient far field radiator first?>>
I'll bound your question so as to minimize possible misinterpretations of
"...in a board..." If you are referring to radiation from traces on the
surface of a PCB that are well inboard of the PCB edges, they will radiate
the same independently of any power plane setback. (You may check out the
IBM paper presented about three (four?) years ago at the IEEE EMC Conference
in San Diego, CA, for the effects of trace spacing from PCB edges.) If you
are (as I suspect) referring only to those emanations from the PCB plane
edges (without any power plane setback), YES, direct radiation will result,
and ALSO additional radiation will occur with coupling. When the interplanar
TEM wave hits the open edge, it does not see an infinite impedance. Rather,
the energy wave sees an impedance transformation from the low (transmission
line) impedance of the planar structure to the 120*PI=377 ohms of free space.
This is equivalent to a sizable impedance mismatch which will normally yield
poor energy transfer; however, the fields (and currents) "spill over" on the
copper edges of each/both planes and transfer energy (through continuing
propagation) in both directions along the edges. Some energy also folds back
on the outside surfaces of the planes. Henry Merkelo (Heads up EMC studies
at U. of Illinois, international lecturer, knows his stuff) has produced
several excellent papers over the last few years that uses software to
illustrate these effects. This added effect increases the efficiency of the
coupling to free space (as well as to other traces that may radiate more
efficiently) and results in increased levels of radiation from the PCB. The
worst case of radiation induced from this action will be when the TEM
harmonic(s) coincide with the surface/outside resonance of the PCB structure.
In that case, substantial radiation will result. This is yet another
example of why potential EMI sources are best suppressed at the source,
because there are multiple mechanisms of "leaking" RF energy from a PCB.
As an added note, the common manufacturing practice of extending the PCB
dielectric beyond ALL PCB copper layers captures more of the field lines at
the abrupt impedance interface and reduces the radiated energy somewhat (but
not nearly as much as the 20H setback).
With consideration for all the preceding SI Reflector discussions on this and
related topics, do not forget that much of this potential radiation can be
suppressed or contained by good bypassing (and/or dissipative absorption)
around the edges of the PCB. By forcing a voltage minimum (as best as good
bypassing can) at the PCB edges, the PCB resonant waves (representing the
most efficient radiating frequencies) are forced to voltage maximums on the
more lossy interior of the PCB. Any fields that try to radiate more easily
find termination somewhere on the PCB and are thus reduced. I extolled the
virtues of this phenomenon in my presentation at the EMC '98 Colloquium and
Exhibition, "PCB Design Issues at the Macroscopic Level." The same concepts
are carried forth in recent papers by Larry Smith and Istvan Novak, both at
Sun Microsystems (although on different sides of the continent).
<<As the planes move further apart from each other I expect the "far field
direct radiation" to increase but I'm curious that if planes are very
closely spaced that it is possible to radiate directly.>>
Your expectation regarding further separation of the planes is valid.
Referring to the discussion presented above, the interplanar transmission
line impedance is raised by the further separation which provides a more
efficient impedance match to the impedance of free space; hence, more energy
transfer (through radiation) will occur. A good illustration of this effect
is evidenced by the easily demonstrated (and measured) radiation from a
single microstrip. Measure the radiation at any given frequency from a 50
ohm line, then repeat the same measurement for a 100 ohm line. The Zo
increase was 2:1 or 6 dB, but the measured radiation from the 100 ohm line is
8 to 10 dB higher than the 50 ohm line. FYI, radiated emissions prediction
software (such as EMCAD1 from CKC Labs) predict this same result. For the
planar case, since the impedance mismatch is on the order of 200 to 400 to
one, I would expect a more linear correlation rather than an amplified one.
That is, a one ohm planar Zo increasing to two ohms would probably give 6 dB
increase in energy transfer.
Hope this helps.
Michael L. Conn
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