Waveguide-to-waveguide coupling issue and query
I am puzzled about the level of aperture coupling between two waveguides. I have a long straight section of WR90 rectangular waveguide (operating at approx. 10 GHz). I want to couple the 2nd harmonic (20 GHz) and the 3rd harmonic (30 GHz) waves from this WR90 waveguide. So, to couple the 2nd harmonic frequency, I insert a slightly smaller waveguide stub (cut-off at around 16 GHz) on the top wall of the WR90 waveguide forming a T-junction or stub. I was able to obtain 5 dB coupling.
For the 3rd harmonic, I start with new WR90 waveguide and repeat the procedure, this time the waveguide stub is slightly smaller than for the 2nd harmonic, such that the cut off frequency is greater than 20 GHz (approx. at 26 GHz). In this case, I was only managing about 1 dB of coupling. I wanted to get more out of this 1 dB by studying the effect of the aperture size, but after a long-winded of computational analysis, I concluded that I could not obtain better than 1 dB, 1.5 dB perhaps just managed. However, certainly not 5 dB from the 2nd harmonic stub.
Further investigation reveals that the higher frequency I tried to couple, the less coupling I will get.
I would be grateful if you could give me some suggestion on why this is. It seems it is heavily dependent on the size ratios between the waveguide stub and the WR90. What is the fundamentals that I am not aware of? How might one enhance the coupling at higher frequencies if the main waveguide is fixed?
Thank you in advance.
I think the problem is not so easy as you expect.
First, you have to "couple" a signal source into your WR-90 waveguide. The first harmonic, 10 GHz, is the "dominant" mode, the higher harmonics are higher modes, and those are more difficult to inject or take off.
To separate the harmonics, the easiest way is to use narrower waveguide sections as high-pass filters, with a cut-off frequency selected so that lower frequencies are rejected.
I do not recollect if there is a good book on multimode waveguides; in textbooks various modes are described but not the devices to inject or take off the harmonic power. Google may help in your search.
Higher-mode waveguides are often used in "beam-guides" to feed large antennas with a high power; such waveguides offer a low loss.