I want to design an x-band corrugated waveguide feed at the central frequency of 10 GHz.
If the ridge radius is the same as the smooth waveguide radius (2*r1 = 23.83 mm) then based on the balanced condition, the slot radius of aperture (r0) will be:
Sm(x1, x0) = 0
x1 = k*r1, x0 = k*r0
(r1 = internal radius of waveguide
r0 = radius of slot)
[Ref : http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5251443&tag=1
Clarricoats, P.J.B., Saha, P.K., "Propagation and radiation behaviour of corrugated feeds. Part 1: Corrugated-waveguide feed," Electrical Engineers, Proceedings of the Institution of, 1971, 118 , Issue: 9, 1167 - 1176 ]
2*r0 = 41.53 mm.
I assumed 'b' (groove width) and 'c' (groove separation or space harmonic period) to be 3 and 4 mm.
Between the smooth waveguide and corrugated one there is 5-steps transition. The first slot depth is half wavelength and changes smoothly to corrugated waveguide.
The problem is that when I simulate the structure with CST software. The TE11 mode is converted to a mode which is not HE11. Indeed, It looks like a EH mode.
I have no idea how I can solve this problem. In most of the literature, they explained the method of design when the aperture is large relative to wavelength. However in my case, the aperture is smaller than the wavelength.
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