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problem about 2 paralell planes

 
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ai.xuexi
Antenna Theory Regular


Joined: 01 Jan 2012
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PostPosted: Sun Jan 01, 2012 12:12 pm    Post subject: problem about 2 paralell planes Reply with quote

If there is a plane wave propagate(TEM mode) from left to right.
Direction of E is up down, direction of H is perpendiculor to the screen.

And I have 2 paralell conductor planes.
---------------------------------
EM--> d
---------------------------------
Then what happen? Can the wave still propagate between the planes? Can the 2 planes induce voltage? i.e. u=E*d? If it can induce voltage, why we need antenna?

Thank you
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bigSteve
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Joined: 14 Mar 2009
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PostPosted: Sun Jan 01, 2012 7:26 pm    Post subject: Reply with quote

If the plates are infinitely long (not a rectangular waveguide), then the TEM mode will propagate as you describe.

If there is an E-field between the plates, then yes there will be a voltage difference between the plates.

As to the antenna question, it's not clear what the situation is so the question can't be answered based on the information you provided.
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ai.xuexi
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PostPosted: Mon Jan 02, 2012 7:35 am    Post subject: Reply with quote

bigSteve wrote:
If the plates are infinitely long (not a rectangular waveguide), then the TEM mode will propagate as you describe.

If there is an E-field between the plates, then yes there will be a voltage difference between the plates.

As to the antenna question, it's not clear what the situation is so the question can't be answered based on the information you provided.


infinitely long ----- which direction?
As to the antenna question, I mean
-------------------------------------------------------
EM--> (the distance is d) (load is at the end). R(load)
-------------------------------------------------------
i.e. the plates will connect to a load R.
If we hope receive large voltage we can icrease d. Of course, we need match the R value to the impedance of the plates.
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bigSteve
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PostPosted: Mon Jan 02, 2012 7:17 pm    Post subject: Reply with quote

I'll use this coordinate system:
+x
|
|
|_________+z

so the wave propagates in +z, +x is the E-field direction and y is the H-field.


Ok, by infinitely long I was referring to the parallel plates as if they were a waveguide (i.e. a square cross section of metal). In this case the TEM mode would only propagate if there was infinite extent in the y-direction.

Now, if you have parallel plates with a TEM mode in there, you are basically talking about a transmission line. The goal of an antenna is to convert E- and H-fields to voltage and current waves on its transmission line. So if you already assume the TEM field exists between the parallel plates, then yes you will have the voltage and current flow, which will be transferred to the properly-impedance matched load.

This assumes you capture the TEM wave between the plates....
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ai.xuexi
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Posts: 14

PostPosted: Mon Jan 02, 2012 11:05 pm    Post subject: Reply with quote

bigSteve wrote:
I'll use this coordinate system:
+x
|
|
|_________+z

so the wave propagates in +z, +x is the E-field direction and y is the H-field.


Ok, by infinitely long I was referring to the parallel plates as if they were a waveguide (i.e. a square cross section of metal). In this case the TEM mode would only propagate if there was infinite extent in the y-direction.

Now, if you have parallel plates with a TEM mode in there, you are basically talking about a transmission line. The goal of an antenna is to convert E- and H-fields to voltage and current waves on its transmission line. So if you already assume the TEM field exists between the parallel plates, then yes you will have the voltage and current flow, which will be transferred to the properly-impedance matched load.

This assumes you capture the TEM wave between the plates....


Thank you very much!
My question is can this structure capture the TEM wave between the plates?(the plane wave incidents along z direction, E is x direction, H is y direction)

And another queston is if we have TEM in transmission line, and the transmission line is open circuit, why the TEM wave can not totally propagate foward but reflect mostly? I do not like the impedance match discuss. Can we use Maxwell equation to explanation?
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bigSteve
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PostPosted: Tue Jan 03, 2012 7:13 pm    Post subject: Reply with quote

As to whether it will capture the TEM wave: probably not. Recall that antennas satisfy reciprocity, which means that the radiation pattern and efficiency are the same whether they are in receive or transmit mode. And if you excited the structure you described, which I believe will act as a transmission line, and we know that transmission lines don't radiate much at all, then you will not get a good receive antenna out of the structure either.

For your second question, I think the wave will propagate through, because the structure won't capture much energy.
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ai.xuexi
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PostPosted: Wed Jan 04, 2012 12:10 pm    Post subject: Reply with quote

bigSteve wrote:
As to whether it will capture the TEM wave: probably not. Recall that antennas satisfy reciprocity, which means that the radiation pattern and efficiency are the same whether they are in receive or transmit mode. And if you excited the structure you described, which I believe will act as a transmission line, and we know that transmission lines don't radiate much at all, then you will not get a good receive antenna out of the structure either.

For your second question, I think the wave will propagate through, because the structure won't capture much energy.


You said "we know that transmission lines don't radiate much at all".
Why? It is open circuit. How do we explain it using Maxwell's equations?

Thanks a lot!
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