[SI-LIST] Re: TEM wave propagation and standing waves
- From: Davi Correia <davi.correia@xxxxxxxxx>
- To: Ankit wangoo <ankit.wangoo@xxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Sun, 3 Mar 2013 21:00:16 -0800 (PST)
Ankit,
The concept of a TEM or a plane wave where E and H are in phase is for
propagating waves in an unbounded media. The concept of a standing wave is not
exactly the same thing: for a start, it is not a propagating wave, but a
combination of two, propagating in opposing direction, resulting in a standing
wave. They do not represent the same thing, you can not bring a condition valid
for one to another.
Regards,
Davi
________________________________
From: Ankit wangoo <ankit.wangoo@xxxxxxxxx>
To: Davi Correia <davi.correia@xxxxxxxxx>; si-list@xxxxxxxxxxxxx
Sent: Monday, March 4, 2013 4:09 AM
Subject: Re: [SI-LIST] TEM wave propagation and standing waves
Hi Davi
I understand these boundary conditions.From this we get result that standing
wave in transmission line whichis 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.
but how this concept get into concept we discussed about TEM wave propagation
where E and H are in phase?Is it not necessary for TEM wave to have E and H to
be in phase?
regards
Ankit
On Mon, Mar 4, 2013 at 1:59 AM, Davi Correia <davi.correia@xxxxxxxxx> wrote:
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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