Dielectric in circular waveguide

[mauloftin]

Hi, can you help me ?

I have dielectic in circular waveguide, can you tell me, how can I do matching for wideband good return loss ? Have some book, article or theory ?

Thank you very much

 

[sergio mariotti]

Could You be more specific?
Some techniques exists but depend on the function of the dielectric.
Is the dilectric an illuminator? Or is it used for size reduction?.
Generally speaking in order to reduce the discontinuity, the way ti match the transition from air to dielectric are:
1) smoot transition
2) stepped transition , 2 or more steps lambda/4 long each.

 

[Jacky Loh]

hello sergio mariotti

could you give me an example about the two methods? thx

 

[mauloftin]

Hi,
thank for your answer. It is for dielectric feed for parabolic antenna on the end of waveguide. Smoot transition is not good, and stepped - i dont know exactly, how can I find the best design ? And lambda/4 is it lambda in the waveguide or in the space or in the waveguide in dielectric (the diameter is the same as waveguide with "vacuum") ? A tried 3 stepped, but the find diameters is not so easy.

 

[sergio mariotti]

well, usually when We speech about wavelength (λ) We mean wavelength of a radiation flowing inside it's proper media. So we speech about λg inside a dielectric.
But since λ/4 steps gives often a large enough bandwidth, if You make a mistake and confuse λ and λg and the εr is small enough, the error should be not excessive.

From a mechanical point of view, the steps may be done by the help of a lathe machine.

From a EM point of view , a CAD like CST may help you on the design.
The CAD is not strictly necessary in order to design the transition between circular air waveguide to circular dielectric waveguide. The following should work without any CAD design:
3 or more steps λg/4 long each, linear spaced.

But I think it isn't enough. Because also the transition between dielectric feeder and free space should be taken in account.