I want to build a Horn antenna to increase directivity of HB100 sensor and I studied principles on an Antenna's book. I started from a void rectangular waveguide.
Since the frequency is 10.525 GHz, the greater side of the waveguide is
$$L = \frac{c}{f_{cutoff}\cdot2\cdot20\,\text{mm}} = 14.28\,\text{mm}\text.$$
It's a very small waveguide, so I ask it's possible to use a smaller cutoff frequency to make a greater waveguide? Dimensions must be about of HB100 board, 40 mm × 46.5 mm.
Second question, to match intrinsic waveguide's impedance with charateristic impedance of air is necessary to increase gradually the dimensions of the waveguide, making it like a pyramid. But, how can I determmine the minimum length that of the horn?
HB100 mechanical drawing:
Dimensions: 40 mm × 46.5 mm
Given the distribution of the antenna patches, the opening of the waveguide needs to be approximately the size of the HB100.
What do you think about a guide of 28mm x 10mm? Fundamental mode is at 5.35 GHz and first higher mode is at 10.7 GHz, that permit a singlemode propagation. The problem is to connect at HB100 a so small waveguide. I thought to connect the guide centered on the arrays, is a good solution? Distance between two elements of array is exact 28mm.
Like this:
Best Answer
I don't think this will work anywhere near as well as you expect it to.
As you already found out, no waveguide (can) exist that supports single-mode propagation at your frequency requirements and dimensions. Even the WR-112 (which in theory only goes to 10 GHz) is only 28 mm on the long side.
So lets say you use 4 waveguides, even then, I doubt you will gain anything with a horn. I suspect that the losses you will suffer because your entire system is not designed for a horn in the first place will result in your horns doing more harm than good. Keep in mind that this seems to already use a patch-antenna array, which means that it will already have a bit of directivity.
I suspect if you want to do with with a horn antenna, you should start with a transmissionline to waveguide transition.