problem of open end coaxial cable

[airforce000]

Hi:

Various explainations here can all be calculated from Maxwell equation, and the equation itself can not be further proved ,it's actually the arithmatic result from many actual experiments.

From certain point of view, it's part of the "rules" in this universe discovered so far. IMO,we can never know the real "root cause" but only can "understand/explain/use" the factor in our own ways.

Regards

 

[zorro]

The mode supported by the infinely-wide waveguide is a TEM planar wave confined in space. When this waveguide stops, from this point you have (in far field) a cylindrical wave (not omnidirectional). In directions different to those contained in the plane parallel to the plates, the E field has not the same orientation.

Why? Could you explain much in detail.

Imagine that the two parallel plates are equidistant to the xy plane (one at each side).
Suppose that in the space between them a planar wave propagates in TEM mode parallel to the x-axis. E-field is parallel to z-axis; H-field is parallel to y-axis. The plates end at x=0.
By simmetry, the distribution on fields is the same in all the planes parallel to the xz plane, so wothout loss of generalty wi will consider the fields in the xz plane.
In this plane, in free apace at far field there are beams propagating from the origin in all directions. Both E- and H-fields must be normal to the direction of propagation and perpendicular between them. H-field is parallel to the y-axis. Then, E-field is in the xz plane but it is not parallel to z-axis like inside the waveguide.
I hope it is clear.

Regards

Z

 

[xuexucheng]

Thank you very much again!
I have understood most.
However,

in free apace at far field there are beams propagating from the origin in all directions.
Z

Why this happen? why the wave can not propagating forward only in x direction?

Happy new year!

 

[zorro]

Of course there is a beam propagating along the x-axis from the origin (x,z)=(0,0).
But there are also other beams in other directions. They all pass by the origin because it is the source of energy, and electromagnetic energy propagates following straight lines.
Regards

Z