[SI-LIST] Re: TEM approximation
- From: jizheng@xxxxxxxxxxx
- To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 19 Sep 2003 01:03:09 -0700
Since nobody has directly addressed your question, here is my input.
Essentailly the concept of TEM (Transverse Electro-Magnetic) refers to the EM
field (associated with a uniform transmission line) having only transverse
electric and magnetic components perpendicular to the wave propagation
direction. For instance, uniform stripline and coaxial line support TEM mode.
The field pattern at the cross-section of a TEM field can be computed as for a
static field - and thus sometimes TEM is confused with static or low frequency
concept! Acutally, TEM is the dominant propagation mode associated with some
structure such as striplines far beyond GHz range.
On the other hand, quasi-TEM concept is a very useful and widely used approach
for approximation of the wave propagation associated with structures such as
microstrip transmission lines which does not support a pure TEM mode (even if
we neglect the conductor loss). Quasi-TEM approximation basically neglects the
field components in the propagation direction (Ez, Hz). This approximation is
pretty accurate when the frequency is relatively low, which means the quasi-TEM
mode can be used up to near the frequency where a so called "higher-order" mode
can propagate. One can easily find such frequency in reference books given the
physical structure of a microstrip line.
You raised a very good question - "and what model can be used above this
frequency limit ?" People like TEM or quasi-TEM mode since transmission lines
with TEM or quasi-TEM approximation can be easily solved using telegrapher's
equation which can be easily integrated with circuit simulations. At higher
frequency (into Microwave range), a so-called "Full Wave" field analysis which
usually yields an S-parameter model needs to be used for accurate
characterization of the structure. Well, things got a little complicated if you
end up with an S-parameter model and that's why you see so many people talking
about "how to simulate an S-parameter model" and "Does S-parameter model really
represents my original structure with separate grounds " ......
Hope this helps.
Ji
> > ----- Original Message -----
> > From: "Swamy Dhoss" <swamydp@xxxxxxxxx>
> > To: <si-list@xxxxxxxxxxxxx>
> > Sent: Sunday, September 07, 2003 4:28 PM
> > Subject: [SI-LIST] TEM approximation
> >
> >
> >
> >>Hi
> >>
> >>Could anyone tell me about the concept of TEM
> >>approximation for the propagation of an electrical
> >>signal through a transmission line ?
> >>Over what range of frequencies TEM approximation is
> >>valid ? and what model can be used above this
> >>frequency limit ?
> >>
> >>Regards
> >>swamy
> >>
> >>
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