One of my latest obsessions is building homebrew WiFi antennae. This is the first antenna I built. It is the ubiquitous circular waveguide cantenna. The probe should be 30.8mm (1.21 inch) long, and should be set 63mm (2.5 inch) from the inside of the back lid of the can.
The operating wavelength of 4.8″ shows us that we don’t have to worry that the sides of the can have ridges, because the their depth is insignificant compared to the wavelength our signal. The Guide Wavelength is the wavelength of our waveguide. The TE11/TM01 Cutoff frequencies give us the approximate upper/lower frequencies of operation for our antenna. Since Channel 1 is centered at 2412MHz and Channel 11 is centered at 2462 MHz, we have a comfortable margin. Now the interesting part is that if you wanted to tune the waveguide for the center frequency of Channel 1, you would use a probe distance of 66.04mm instead, and for Channel 11, you would use 58.52mm. What this means is that there is a whopping 5.48mm difference in the optimal probe distance between Channels 1 and 11, so if you are going to use the antenna on a fixed channel, it’s better to enter than channel number instead when running the program.
Experimenting with my calculator program, I’ve found some interesting information. As the waveguide diameter increases, the difference in optimal position for the driven element between Channels 1-11 drops. I tried upping the diameter iteratively until the TM01 cutoff frequency started to go too low to do Channel 11. From my studies, it seems that about 92mm is the optimal diameter for the waveguide if you want to try to optimize it for flattest response across Channels 1-11; this is because it minimizes the difference in the probe position between Channels 1-11 -> about 2.63mm, so the SWR curve across the WiFi band is flatter.
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