[SI-LIST] Re: Even mode, common mode, and mode conversion

  • From: "Mary" <mary@xxxxxxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Thu, 28 Feb 2002 11:28:23 -0600

Steve,

Thanks for the explanation. Everything you said seems reasonable,
but I'm still confused. The discussion so far has been about
even and odd mode "signals". By "signal" do we mean the signal
voltage, the signal current or something else? Are you saying
that the even and odd modes of the "signal" will propagate
independently while the signal voltages and currents will 
not propagate independently?

Mary

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Steve Corey
Sent: Thursday, February 28, 2002 9:41 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Even mode, common mode, and mode conversion



Mary -- thanks for bringing me back to the earlier question.  Because 
each mode has its own characteristic impedance, each has a different 
relationship between its voltage and current waves.  As a result, the 
responses to the even and odd modes do not add up to zero on the 
quiescent conductor, and this is where the crosstalk comes from. 
Because modal analysis takes all the coupling into account, it properly 
predicts crosstalk.  And of course, if there's negligible coupling, the 
modal impedances are effectively the same, and the modal 
responses essentially cancel out for zero crosstalk.

I also want to emphasize as a more general point that any set of 
inputs/responses, eigenmodes or not, have to propagate independently 
from one another, due to linearity. Otherwise the system is nonlinear 
and superposition does not hold, and all of our jobs just got a lot harder.

Finally, I was looking over my previous post (I suppose I should have 
done that before sending it out...) and I spotted a couple of errors 
which I would like to correct.  First, as I stated above, in the 
presence of coupling, each mode has its own unique characteristic 
impedance.  Second, if the dielectric is homogeneous -- even if it is 
lossy -- (as long as the conductors are lossless) then every mode shares 
the same propagation constant.  Inhomogeneity and/or lossy conductors 
cause splitting of the modal propagation constants.  Sorry if I created 
any confusion there.

   -- Steve

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