[SI-LIST] Re: Question on EMI radiated power
- From: "Charles Hill" <chuck@xxxxxxxxxxx>
- To: <Andrew.Burnside@xxxxxxxxxxxxxxxxxxx>, <a.ingraham@xxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Sun, 6 Nov 2005 12:21:10 -0700
Andrew,
We were discussing the feedpoint impedance of an antenna which is a complex
scalar quantity and how a portion of that, the radiation resistance,
represents the power transfer.
When dealing with coupled structures a matrix quantity is used which has the
off-diagonal terms of mutual impedance. The phase angle of the mutual
impedance relates to the phase shift in the coupling. The feedpoint
impedance of a coupled structure has to be computed from the matrix along
with the self-impedances of the coupled elements; the feedpoint impedance is
not a diagonal term of the matrix. So even with a coupled structure, the
radiation resistance is a portion of the feedpoint impedance which
represents the radiated power and it is always real valued.
If it helps, consider another case of coupled inductors--a transformer. The
only power that is transferred is a portion of that where the input voltage
and input current are in phase--right?
Charles
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Andrew Burnside
Sent: Sunday, November 06, 2005 4:44 AM
To: chuck@xxxxxxxxxxx; a.ingraham@xxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Question on EMI radiated power
Chuck
:My interpretation of a reactive component is energy storage in the near
:field surrounding the antenna, and of course there will be no power
transfer
:due to this. This is completely consistent with energy storage in
::capacitors and inductors. Therefore, there is no such thing as a complex
::radiation impedance.
However, the reactive component may relate to any (partial) mutual
inductance/capcitance there is to other structures in the near field.
Therefore, we don't have an ideal antenna due to the near field loading.
When considering EMI, we may well be within the near field range.
If we consider digital circuits only for a moment, then the majority of the
emissions are likely to be due to clocks / and current dumped down the power
supply network. Yes, there may be some direct emissions from interconnects,
but these are usually fairly small compared to the PSN, as we aren't talking
power electronics here.
If we are considering the far field only, then yes it is possible to get
away with a resistance only if we are only interested in power transfer.
However, I would have thought though that the emission phase is becoming
signficant, considering the rise times of some of the signals present.
Regards
Andrew
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