[SI-LIST] Re: TEM wave propagation and standing waves
- From: Davi Correia <davi.correia@xxxxxxxxx>
- To: "ankit.wangoo@xxxxxxxxx" <ankit.wangoo@xxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Sun, 3 Mar 2013 12:29:22 -0800 (PST)
Hello Ankit,
When you terminate your TL by a short (in EM theory they call it "perfectly
electric conductor"- PEC), your tangential electric field has to be zero. The
only way for that to happen is if your incident wave has a value of, say, +a,
your reflected wave has to have -a (the tangential component). That way, at
your short (PEC) you have a minimum (zero). That is not true for the magnetic
component. As a matter of fact, the short is where you magnetic field
(tangential) is maximum. Hence, they have to be completely out of phase. The
same argument is true if you terminate your TL with a (perfect) open, but this
time you flip who is zero and who is max.
If you want, you can also see this from a circuit point of view: if you
terminate with a short, you can not have any voltage, right? That is electric
field. But the current will be very large (maximum, in fact). That is magnetic
field. And if it is open, there is no current (zero magnetic field) but large
voltage (electric field).
If you want more info, any undergrad level EM book should have this (Ramo,
Shadiku, Pozar, etc.).
I hope it helps.
Regards,
Davi
________________________________
From: Ankit wangoo <ankit.wangoo@xxxxxxxxx>
To: si-list@xxxxxxxxxxxxx
Sent: Sunday, March 3, 2013 8:16 PM
Subject: [SI-LIST] TEM wave propagation and standing waves
Hi
We all know in transmission line structures such as co-axial
cables, strip-line and micro strip-lines(partially ,if we assume field
lines remains inside the dielectric) , electromagnetic energy flows in TEM
mode, that is electric and magnetic field are always perpendicular to each
other.
We also know that characteristics of TEM waves guided by transmission lines
are same as those for uniform plan wave propagating in an unbound
dielectric medium.
When we solve Helmholtz equation we find that Electric field can have one
of solution as E=a*e^j(wt-kz) + b*e^j(wt+kz). where first term is a forward
travelling wave and send is backward travelling wave.
From Ampere circuital law in point form , we can find that H, jw*mu**H*Þl
cross *E* . Then H some out to be in phase with E field .That means that at
a particular position and at particular time when electric field
is maximum , magnetic field will also be maximum.
however , when we study standing wave in transmission line which
is terminated by short.We find that current (magnetic field ) and Voltage
(electric field) are actually completely out of phase. when current is
max, voltage is zero and vice-versa
What can explain this difference in analysis ?
I was thinking more about this,,, standing waves are formed by two waves
and each of them electric field and magnetic field are in phase.however in
standing wave , because of reflection one of them get polarized in
different direction such that some points electric field gets cancelled and
some point magnetic field get cancelled.
however ,i am not completely convinced.Can somebody shed some light on this
or refer me to some appropriate reading material?
Thanks for your help
Ankit wangoo
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