Regarding Polarization in Waveguide

[kathir1983]

Hi members, am new to antenna field.I would like to know more about polarization in waveguide.As per book,it is stated that TEM wave cannot travel in waveguide.If that is the case how come horizontal or vertical polarization can be achieved.If am wrong,please correct me.Thanks in advance.

 

[reasonableman]

I'm not expert on tranmission lines but know a little about fibre optics. A horizontally or vertically polarised wave just has the field variation in one direction (although the E and M fields are orthoganal which makes it a bit difficult to visualise, just consider one).

Maybe if you just consider it from the point of view of the wave rather than thinking about the path of propagation?

Persoanlly, I find it hard to see how a waveguide doesn't introduce some sort of polarisation...

 

[Phrak]

The electric and magnetic fields in the fundamental mode are both perpendicular to the direction down the guide. That sounds pretty transverse to me.

...So I did a little research. The fundamental mode for a waveguide is TE_{1,0}.

TEM requires that there be no magnetic or electric field components in the direction of propagation. Down the geometric center of the guide P, E and M are all perpenducular. Toward the edges, the magnetic fields have longitudinal components.

 

[kathir1983]

I will put my question clear...we are connecting the output of magnetron to DDC through rectangular waveguide.The output of magnetron is TEM wave (both magnetic & electric field perpendicular to the direction of propagation) but the rectangular waveguide dominant mode is TE10 (electric field perpendicular to the direction of propagation & not magnetic field).In such case how will be my electic field orientation.whether it will be horizontal or vertical w.r.t to direction of propagation.

 

[kathir1983]

Query Regarding Polarization

whether the electric & magnetic field should be perpendicular to direction of propagation or is it electric or magnetic field alone is perpendicular to direction of propagation (Linear polarization)

 

[Phrak]

OK. Assuming your guide is matched to the magnetron frequency so the guide is conducting in the fundamental mode, the electric field will stretch across the shorter dimension of the guide. Looking down at the wider side, the magnetic fields will obtain as regions of alternating circulation, strung along the length of the guide. In each region the magnetic field circulates, alternately clockwise, then counter clockwise. The loops of field lines get more rectangular in shape on the outside of these regions as the field lines get closer to the walls and the adjacent loops.

Still looking down on the widest side of the guide, along the longitudinal centerline the magnetic field is transverse to the Poynting vector along the length of the guide. But close enough to the walls the magnetic fields have a larger longitudinal component, than transverse, because of the looping business.

(I suppose this means the Poynting vector ducks in and out of the guide wall-I hadn't thought about it.)

So these loops of field, and electric field too, race down the guide at some velocity greater than c, that is a function of the guide width.

The same field patterns are obtained as the interferance from a multiple source array, spaced a little more than half(?) a wavelength apart--as can be shown using boundary conditions.

 

[kathir1983]

sorry...i didn't get you...

 

[Phrak]

See figure 1:2. It's close enough to correct.

http://www.swedetrack.com/waveguid.htm"

 

[Pumblechook]

I thought TEM required a centre conductor making it a coaxial cable rather than a waveguide.

 

[CarlosEJordan]

For propagation in a waveguide there will be either a transverse magnetic (TM) or transverse electric (TE) mode.

If the z-axis points down the length of the waveguide, then in a TM mode wave the magnetic field will not have a z component but the electric field will. In a TE mode wave the electric field will not have a z component but the magnetic field will.

In Griffiths' "Introduction to Electrodynamics" he shows in an interesting and surprisingly simple way how a solution for a TEM wave (transverse electromagnetic wave) does not exist for the wave equation in the case of a waveguide.

 

[Phrak]

For TEM, all the magnetic field across the wavefront is perpendicular to the direction of propagation, and all the electric field is perpendicular to the direction of propagation.

I hope this lends some clarity.