I’ve been experimenting with building a waveguide antenna for ads-b reception, and while it’s been a fun project, it’s also been frustrating. It seems that some of the popular online antenna calculators are intended for only wifi frequencies around 2.4mhz, and even though they accept the input of frequencies in other bands, some of the resulting output appears to be incorrect. In fairness, the calculators are presented in the context of building a wifi antenna and don’t claim to be suitable for other frequencies, however I’m fairly certain that I’m not the only one to incorrectly assume that they could be used for other, non-wifi, frequencies.
The problem appears to be with the calculation of the guide wavelength, which ultimately affects the distance of the probe from the enclosed end of the tube.
Based on available materials, I decided to build a ‘can’ antenna 196mm in diameter and 447mm in length. Anticipating that my metalworking ‘precision’ might necessitate some provision for adjustment, I built a movable probe and started testing.
Using the popular calculator at https://www.changpuak.ch/electronics/cantenna.php, and entering the frequency (1090 mhz) and diameter (196mm) the distance from the closed end to the probe was given as 33.0725mm, and the guide wavelength was given as 132.29mm.
On the first attempt, I built a probe adjustment range of 20-90mm from the enclosed end. While the antenna DID receive signals using those dimensions, the results were far less sensitive and much less directional than I expected.
Reading on other sites (linked below) convinced me that guide wavelength is always longer than freespace wavelength, and since freespace wavelength at 1090mhz is around 275mm, the 132mm guide wavelength and 33mm probe position must have been incorrect. Later testing of the calculator showed that regardless of the frequency entered, for any given can-diameter the guide wavelength remained the same.
After building a second can with a 40-170mm adjustment range, and using an N1201SA impedance analyser (calibrated open, short, load) to check the S11 return loss and vswr, the best results were with the probe at about 122mm from the enclosed end of the can. With the addition of a couple of carefully placed tuning screws, vswr was 1.024 and s11 was -38.28, both of which seemed reasonable.
Recently, I mustered up the courage to work through the intimidating-looking formula to determine the guide wavelength and eventually, where the probe SHOULD be located. Using the formula from Microwaves 101, for a 196mm can, the guide wavelength is 483.46mm and distance from the probe to the closed end of the can is 120.87mm, pretty close to my 122mm.
The results to date have been more directional, but still with a lot of side and back lobe reception. Now that I have reasonable measurements, next up is to roll a new can and properly attach the enclosed end, and fine tune further.
The following calculator appears to handle more than just wifi frequencies. http://www.wikarekare.org/Antenna/WaveguideCan.html 2
References to guide wavelength.
https://www.microwaves101.com/encyclopedias/waveguide-mathematics
www.w1ghz.org/QEX/circular_wg.pdf 1
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