Rectangular Waveguide With Dipole in it

[fred3142]

Hi,

I know that to receive or inject a signal into a rectangular waveguide (I'll just call it a waveguide from now on with the assumption that I mean rectangular waveguide), you can get a dipole of some sort, and poke it into the waveguide.

What I'm wondering about is, if I got a dipole antenna (for example, I I stood an old-school mobile phone up in waveguide) and put it in the waveguide and excited it, what would happen?

As far as I know, the dipole produces TEM waves; however, the waveguide doesn't support TEM, so what will it do? I'm pretty sure it's got to transmit somehow, so does it somehow convert it into TE?

Thanks.

 

[jasonRF]

You are correctt that it will in general radiate energy into the waveguide. This is in fact one practical way to get energy into and out of a waveguide. The details of what happens is messy and depends upon excatly where you place the antenna, the size of the antenna, etc.

In general, the antenna will excite a number of the propagating waveguide modes, as well as a number of evanescent (non-propagating) modes that are localized near the antenna and store electromagnetic energy. The propagating modes determine the radiation resistance (real part of antenna input impedance), while the evanescent modes determine the radiation reactance (imaginary part of the input impedance).

You can use your intuition to get a good first guess as to where the antenna should be in order to excite, say, the TE10 mode of a rectangular waveguide. You can start by using a frequency for which only the TE10 mode propagates. Then, placing the antenna where the peak of the electric field should be (in the middle!) gets you the right location in one dimension. The other dimension (how close to one of the two "ends" of the waveguide) can dramatically impact the input impedance.

Jason

 

[fred3142]

Thanks for the clarification. I'm not sure I entirely understand, from what I understand, you're saying that in order to excite the TE10 mode, one must place the antenna in a particular location in the guide; does this mean that then only the TE10 mode will be excited (because in order to excite the other modes, one'd need to place the antenna somewhere else)?

I'm actually trying to understand this question which was asked in a previous exam (in 2009). I calculated the TE10 cutoff to be about 600MHz and the TE01 to be about 850MHz. My idea is that in all the cases (i), (ii) and (iii), the signal will reach the end of the waveguide; however, depending on the frequency, it'll split over multiple modes (so for (i), it'll travel to the end of the guide in both the TE10 and the TE01 modes). I have no answer to this question, and am just trying to understand this better.

Thanks for your help.

 

[jasonRF]

Fred3142,

This is a coursework style question so this isn't the appropriate forum. Please read the pinned thread in this "Classical Physics" forum:
https://www.physicsforums.com/showthread.php?t=89899
In any case, this is a question about cutoff and the polarization of the different modes. You should be able to take it from there. Hint: think physically.

If you have more questions please post in the "homework and coursework" forum. Thanks,

jason

 

[fred3142]

Ah okay, sorry I did not realize. Thank you nonetheless!