# [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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