I recently had a rather unusual idea: to use the QO-100 repeater satellite located in GEO orbit above Africa, operated by Shenzhen Tonglian. The uplink frequency is 2.4 GHz, and the downlink frequency is 10.4895 GHz. I will elaborate on my idea below, but please note that these are just my speculations and should be considered for reference only.
The position of Shenzhen is at a negative angle, approximately -7 degrees, as shown in the figure below:
My first thought was the atmospheric refraction effect. Under normal conditions, this effect is not significant – at a certain altitude, the propagation distance of radio waves is approximately 4/3 of the line-of-sight distance, which means it's 1/3 longer than the line-of-sight distance. Anomalous propagation in the troposphere can also cause refraction of microwaves, depending on the rate of change of the atmospheric refraction coefficient with altitude. As the altitude increases, if the refractive index decreases more rapidly, the radio waves will be bent towards the ground more, and if the gradient is sufficient, the radio waves can be reflected back to the surface to form a waveguide. This waveguide can also occur in the upper atmosphere, not necessarily on the ground. These significant refractions usually only occur during tropospheric propagation. There is a critical gradient value that causes the degree of bending of the radio wave to be equal to the curvature of the Earth's surface, at which point the radio wave can continue to propagate. Of course, this critical value does not exist everywhere, so the radio wave must spread out somewhere (this is not important here, but it helps to understand the atmospheric refraction effect).
Because the atmosphere is very thin above 10-20 km, with small refractive indices and minimal gradients, and it's not the troposphere, I believe that radio waves are largely unaffected (and can be considered to propagate in a straight line). Microwaves are also not affected by the ionosphere. Most radio wave refraction occurs below 10-20 km. Therefore, trying to cross several hundred kilometers to reach QO-100 would be quite difficult. If strong tropospheric ducting were present at these locations, I'm not sure if it would be useful.
The second method I considered was aircraft reflection, which should have a higher probability. Currently, the recorded aircraft reflection distance in Europe is over 900 kilometers, and this is essentially the theoretical limit. The location from Shenzhen to an angle of 0 degrees is also approximately the same distance. Initially, I used the Desmos mathematical graphing software to create a high-precision model and study the possibility of atmospheric refraction. I found that the sighting lines for Shenzhen and the 0-degree angle locations intersected at a height of over 10 km above the surface, which corresponds to the altitude of high-flying aircraft. The exact location is above Yulin City, where there are no flight routes except for those in the western and northern outskirts of Yulin, but the difference is not too great. With a little atmospheric refraction, the aircraft should be able to fly at this altitude or deviate slightly. However, I am unsure whether this method will work. The microwaves would have to pass through the aircraft reflection and then cross over thousands of kilometers. I consulted some information, and it was found that aircraft reflection has a net gain, and there is a similar principle for lunar reflection. You can see the image below.
There are some hills in Shenzhen that face west and have unobstructed views, so it should be fine. The larger the parabolic antenna, the better, as it provides higher gain. I'm planning to make my own foldable one for hiking. The main issue is whether I can actually get a signal. I hope it won't be completely impossible, but there should still be some chance, haha. If the large parabolic antenna and radio don't work out, then I can use them for EME (Earth-Moon Emergency) communication, which would also be useful. Right now, I'm planning to first build the parabolic antenna to see if I can receive a signal from QO100.
Currently: I'm planning to build a parabolic reflector with a diameter of 4 meters, a focal length of 4 meters. I intend to apply aluminum foil tape to a large plastic film, and use aluminum alloy or PVC for support on the back side. I haven't decided whether it will be mounted vertically or horizontally (I saw a post from an expert saying that vertical mounting with a low angle is not effective, but I also read an article from abroad that said vertical mounting results in poor signal quality, but the reason isn't the parabolic reflector itself, but rather that tightening the screws securing the feed point causes it to deviate?).
These are my various ideas; what are your thoughts on them?
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