Once We Lived in Nanjing

What's the key to satellite communication?

It's tracking.

Low Earth orbit satellites enter and exit quickly, circling the globe in just an hour or two, "whoosh—" they zip right over your head. The low Earth orbit Reconnaissance Star that old Zhao and his team sent to the future is only 300 kilometers high and circles the Earth in an hour. Such high-speed travel poses the same challenge to Ban Xia—the little thing is too fast to track. If it's hard to track, it's hard to aim. If you can't aim the antenna at it, then it's difficult to receive data.

"When you guys communicate with satellites, how do you track their orbits?" Bai Yang asked.

"We use software, like SatOrbit or Heavens-Above, readily available for free. They provide orbital parameters of all publicly available satellites, so you know exactly when and where they will appear," Wang Ning replied while sitting in the living room on a small stool, straightening a clothes hanger with pliers, "For something more professional, there's STK and the like. Software like that is everywhere, making satellites worldwide have no secrets from you."

Obviously, in the era of BG4MSR, these software options would no longer exist. Humanity had completely perished, there was no internet, no servers, and no one to maintain or update them.

"We can't expect her to figure out the satellite orbits on her own. How could she? We'll just tell her from here," Bai Zhen said, also sitting on a small stool in the living room, holding pliers and clipping a clothes hanger, "If we're lucky and the satellite re-enters its orbit without deviation, we'll just need to report the parameters, and she'll be able to aim quickly."

"Actually, the best solution would be to bring along a relay satellite, placing the relay satellite in a geostationary orbit at 36,000 kilometers high," Zhao Bowen joined in, also sitting with pliers in hand, working on a clothes hanger, "This way, the relay satellite would hover right above the girl's head. The Reconnaissance Star sends the data to the relay satellite, and then the relay satellite relays data back to Earth. That way, you wouldn't need to exert effort tracking it all day, otherwise, you'd develop spinal problems from holding up a Yagi Antenna on the roof, following the satellite's every move."

"So why don't you get them to bring a relay satellite then?"

With a "snap," Bai Zhen cut the clothes hanger in his hands.

"I bloody mentioned it before, but the space team cursed me out for it," Zhao Bowen muttered with a grimace, "They were like, 'Are you even human anymore? No, you're not!'"

"And the result? Did they bring the relay satellite or not?"

Wang Ning also "snapped" his clothes hanger into two.

"They did bring it, but they warned me it would greatly increase the difficulty of designing the orbit and also significantly raise the probability of the satellite failing to re-enter orbit," answered Zhao Bowen, "I said there was no other way. Can you imagine a young girl standing on the roof all day, turning in circles with a Yagi Antenna? What else would she have time to do?"

The three old men sat together, babbling on while continually working with their hands, reminiscent of village women gossiping and knitting together. Chatter and rumor spread just like this; only, in their hands weren't knitting needles and yarn, but pliers and clothes hangers, with the "snap, snap" sounds echoing continuously.

Bai Yang came home from school in the afternoon to find them torturing the clothes hangers, dismantling them as if the hangers had done them some deep personal wrong, the scene was too horrific to behold.

What in the world were they doing?

They were building a Yagi Antenna.

Undoubtedly, the Yagi Antenna was the most commonly used type of antenna among the HAM community. Compared to ordinary pole antennas and dipole antennas, the Yagi's advantages lie in its high gain and directivity—it polarizes signals in a specific direction. Put simply, while other antennas are like lightbulbs that scatter light in all directions, the Yagi Antenna is akin to a flashlight directing the beam one way.

Because of the signal polarization attributes of the Yagi Antenna, it is also commonly used by HAM operators for satellite communication. Since normal amateur radios and satellites have very low power, communication between them is like a mute talking to a deaf person—they can't hear each other across the vast distances. That is, unless the mute has mastered the Martial Arts skill Sound Force.

This Sound Force is the Yagi Antenna.

Bai Zhen placed the clothes drying rack down and picked up a slender piece of wood from the floor, likely scrap from some furniture factory, about a meter long.

"What are the uplink and downlink frequencies for that relay satellite?"

"Uplink at 144MHz, downlink at 430MHz," answered Zhao Bowen.

"U band's transmission speed is a bit slow, would be nice to have something in X band," Wang Ning pointed out.

"X-band reception gets complicated; the Reconnaissance Star communicates with the relay satellite using X-band. The relay satellite can store the data and slowly transmit it back to Earth, so there's no rush," said Zhao Bowen, picking up a tape measure from the coffee table. "How long should the reflector be?"

"Uplink at two meters wave, reflector length one meter, half the wavelength."

Bai Zhen took over the tape measure and measured out a length of one meter on the pulled-taut clothes rack: "No good, this clothes rack isn't big enough to measure out a meter, do we have a larger one?"

"My mom came out of the kitchen: "All of the clothes racks at home have already been wrecked by you guys."

"I'll have Xiao Zhu go buy a bigger one."

Wang Ning said as he dialed the phone.

That night, Xiao Zhu ran all over the supermarkets of Qinhuai District.

Straighten out a clothes rack, and you've got metal wire; Bai Zhen cut a one-meter-long piece of it to be used as the reflector for the Yagi Antenna—The construction of the Yagi is so simple, it can be made using sticks and clothes racks. That's why Bai Zhen and the others chose the Yagi; it can be made from materials found anywhere, it's easy to produce, and extremely newbie-friendly.

Wooden sticks form the spine, and five stretched clothes racks are vertically inserted into the wood in sequence, looking like a fishbone and forming a Yagi pattern; this makes a simple five-element Yagi Antenna.

At the very back, and also the longest of the clothes racks, is called the reflector, responsible for concentrating the signal like the reflector bowl in a flashlight.

The second to last clothes rack is called the driven element, the core of all antennas—it's where the signal originates. Electrical currents dance wildly on the driven element to generate electromagnetic waves.

The remaining three clothes racks are called directors; they guide the radiation direction of the waves, making them radiate in the direction the antenna is pointing, achieving targeted broadcasting.

"Will you guys teach her how to do it after you're done?" Bai Yang sat and watched the three men crafting. Making Yagi Antennas by hand is a basic skill for old HAMs, Zhao Bowen has almost forgotten how, while dad and Old Wang's skills are still sharp.

"Once we confirm this thing works, we'll have her copy one just like it over there," dad replied, "We'll get by with that."

"It's simple, she'll learn it in no time," said Zhao Bowen, "With the Yagi Antenna, she'll be able to receive signals transmitted from the satellite and pass it on to us. The plan is coming together."

"How long should the driven element be?"

"96 centimeters, or 97 centimeters, you see and cut."

"The remaining three are the same length."

"All 88 centimeters."

"Pay attention to the spacing, do we want higher gain or better frequency band characteristics?"

"Let's try them both, we'll see how it goes after we make it and test it with AO-91... also, remember to adjust the impedance."

"Just aim for 50 ohms blindly."

"Who brought the SWR meter?"

"No worries, I'll have Xiao Zhu bring one over!"

The trio finally completed a UV dual-band Yagi. When they hoisted it up and pointed it in different directions, it looked like they were carrying a machine gun—Ban Xia thought so too. Two days later, when she finished making that Yagi, she felt like swinging it was just like Lu Bu wielding his Fangtian Huaji in the Martial Arts.