Friday.

Ban Xia hummed a tune while spreading the diagrams on the table.

The diagrams from BG4MXH detailed the key modification points for the GR3188 repeater and Icom 725. Last night, BG4MXH patiently guided her like teaching pinyin to kindergarteners, helping her understand the function of each interface. He repeated each sentence three times, and if she still didn’t understand, he’d repeat it three more times – persuading with emotion, explaining with reason, teaching with patience, guiding until comprehension. The girl studied quite seriously, taking down the English-Chinese dictionary her teacher had left behind but never used, and consulted it frequently.

On this late autumn afternoon, the weather was unusually crisp. Bird droppings covered the windowsill, and golden leaves carpeted the ground.

The temperature dropped day by day. When the girl saw more and more squirrels in the residential area, she knew another winter was approaching. But Nanjing’s winters didn’t bring snow. The small animals hadn’t yet adapted to the climate changes – they were still preparing for winter storage. Their thick winter fur, which evolved over millions of years, would make them too hot to sleep this winter.

Nanjing used to have snow, but from a certain year onward, it stopped falling.

“COR OUT… the Motorola receiver’s COR OUT in the repeater connects to the radio’s PTT.”

“RX AUDIO, where’s the RX AUDIO interface? Here it is… it connects to the radio’s MIC interface.”

“Finally, GND.”

Ban Xia wore a tank top and shorts, her hair tied in a ponytail, sitting cross-legged in her chair with electronic components scattered across the table before her.

She unplugged the microphone from the Icom 725 radio and curiously examined it up close.

This was her first time unplugging it. The Icom microphone matched the radio brand, feeling heavy in her hand. The number keys were worn, used for who knows how long, and probably as old as the radio itself. Ban Xia had told them that parts rattled inside whenever the radio was moved.

The microphone plug was cylindrical – commonly called an aviation plug, short for “aviation connector.” It was an eight-pin plug with a small hole in the center and the other small holes evenly distributed in a circle around it. Its socket was in the lower left corner of the radio’s control panel, corresponding to the plug, with eight thin metal pins inside.

Each pin had a different function and purpose. Ban Xia needed to connect the radio to the repeater, and the first link would start here.

The handheld microphone was the mouthpiece – the microphone. People hold it to speak, so normally, information enters the radio through the microphone socket.

This was the radio’s mouth.

The repeater’s receiver needed to be plugged into the radio’s mouth.

And the repeater’s transmitter would enter the radio from behind.

It was practically a radio threesome.

But Bai Zhen waved his hand: No, not a radio threesome – how could it be so lewd? The repeater’s transmitter wasn’t responsible for output; it was the receiver.

Finally forming a chain where the receiver could transmit information to the radio, and the radio could transmit information to the transmitter.

So this was a radio centipede!

To figure out where each of the eight-socket pins led, Bai Zhen and Wang Ning scoured the entire internet to find the Icom 725 amateur radio manual.

“BG4MSR, look carefully at the microphone socket. These eight pins are numbered counterclockwise from 1 to 8, with the middle one being 8. We need three interfaces: PTT, MIC, and GND,” Bai Yang explained carefully. “MIC is pin 1, PTT is pin 5, GND is pins 6 and 7, we’ll use pin 7. Got that? OVER.”

The channel was silent.

“Did you get that? OVER.”

Still silence on the channel.

“Young lady? BG4MSR? Are you there? Why aren’t you responding? OVER.”

Bai Yang’s prodding eventually irritated Ban Xia, who had to plug the microphone back in to reply irritably:

“Didn’t you tell me to unplug the microphone? How am I supposed to talk?”

Ban Xia plugged in the soldering iron.

Soldering required rosin. Ban Xia searched the entire computer city but couldn’t find any. Bai Zhen had a bright idea and told her to raid music stores, where she finally found some.

Wang Ning reminded Ban Xia to be careful with the soldering iron.

Don’t grip the metal core.

Bai Zhen asked, would anyone be that stupid? Holding the iron’s metal core?

Wang Ning replied, how could there not be? Plenty of people treat soldering irons like pens.

Ban Xia straightened the thin white wire from the repeater, then took up scissors and with a crisp “snip,” decisively cut off the PCB board from one end.

The PCB board fell like a severed head. Ban Xia stripped the white rubber insulation from the wire, separating three thin wires – red, black, and yellow.

These three wires would connect to the radio’s microphone socket.

The red wire was COR OUT, corresponding to PTT.

The yellow wire was RX AUDIO, corresponding to MIC.

The black wire was GND, corresponding to GND.

But the 725 radio’s socket had metal pins, and connecting wires to thin pins was like matching needle tips – difficult to secure together.

So Bai Zhen suggested Ban Xia find some thin copper wire, first wrapping it around the pins in spiral coils, making sure they’d stay on the thin pins without falling off, then removing the spiral and soldering it to the wires.

This created simple homemade connectors.

“These pins are too thin, hard to plug in,” Ban Xia complained. “What if my hands shake?”

“Do you have Parkinson’s?”

“What’s Parkinson’s?”

“Parkinson’s is when you’re partially paralyzed and keep shaking.”

“Really? Sounds interesting, I want to have Parkinson’s too, shaking all the time.”

Ban Xia soldered everything together, connecting the repeater receiver’s three wires to the corresponding pins in the radio’s microphone socket. Half the work was now complete.

Next came the other half.

The other half started with the radio’s rear.

It had two ports on its rear.

“Two ports! ACC1 and ACC2!”

“Right, the 725 has two ports, like I told you. We need the ACC1 port! Look at the metal nameplate on the back of the radio, it shows which one is ACC1.”

“The right one.”

“Correct, the right one.”

“This is a multi-pin port, lots of holes in this socket… one, two, three, four, five, six, seven, eight… eight small holes!”

To understand the function and purpose of these eight holes, Bai Zhen and Wang Ning searched the entire internet again – detailed information about the old Icom 725 wasn’t easy to find.

The Icom 725 radio had quite a few ports on its rear, most notably the two round ACC ports in the center. These were multi-pin sockets, each with eight spindle-shaped holes arranged like a blooming chrysanthemum. They were the opposite of the microphone socket – the microphone socket had pins for information input, while the ACC sockets had holes for data output. The holes were easier to deal with; thin copper wire could be inserted directly.

But how to insert them, and which holes to use – that was the key.

“Hmm… the radio’s SQL OUT connects to the repeater transmitter’s PTT.”

“EX SPEAKER connects to the transmitter’s MIC.”

“Finally, GND.”

Ban Xia remembered BG4MXH’s instructions.

She pulled out another thin white wire straight – each repeater only had one of these PCB-tipped connection wires for connecting the transmitter and receiver.

The GR3188 repeater’s connection wire had just been cut by Ban Xia.

That wire was now connected at one end to the receiver and at the other end to the Icom 725’s microphone socket.

So Ban Xia found another connection wire from other similar repeaters, decisively cutting off one end and separating the red, yellow, and black thin wires from under the white insulation.

She soldered a long copper wire to each of the three thin wires.

“Which hole is SQL OUT?”

“Pin 6.”

Carefully inserting the three copper wires into the radio’s rear.

Connecting this white wire with one end into the radio’s rear and the other end into the repeater’s transmitter.

With this, the radio centipede was complete.

This setup had been successfully tested nine times yesterday by Wang Ning and Bai Zhen’s team.

But they weren’t finished.

There was still one important component – the antenna.

Regarding the antenna issue, Wang Ning and Bai Zhen thought long and hard. Truth be told, these days antennas are usually bought, with parameters preset by manufacturers, ready to use right out of the box.

But Ban Xia’s era had no Taobao or JD.com – online shopping was impossible. To get an antenna, the simplest method was to make one yourself.

But Wang Ning and Bai Zhen, these two old guys, hadn’t made antennas in many years. It wasn’t that they couldn’t; it was just why bother when ready-made ones were available. What’s wrong with being lazy? So they’d hung up their tools and retired their skills.

“How to handle the antenna? Make it ourselves?” Wang Ning asked.

“Make it ourselves, what else can we do?” Bai Zhen said. “Let’s make one first and test it. If it works, we’ll teach her how to make it.”

“What about impedance and standing wave ratio?” Wang Ning asked. “She probably can’t measure those.”

“We’ll make do with what we have. As long as it’s close enough, it’ll work,” Bai Zhen shook his head. “In her era’s electromagnetic environment, even a fart could be heard worldwide.”

Theoretically, any conductor could be an antenna.

Just with varying effectiveness.

So materials for making antennas could be found anywhere – clothes hangers, Monkey King’s gold cudgel, universe ring, headband, purple gold bowl, nine-toothed rake, sky-piercing lance, green dragon crescent blade – if you could get your hands on it, anything could become an antenna.

Wang Ning and Bai Zhen chose to use coaxial cable.

“Coaxial cable! Use coaxial cable, it’s everywhere in the city – TV cables, and network cables all work.”

“Best to find 75-7, 75-5 works too!”

The coaxial cable was easier to find than the green dragon crescent blade and worked better. It was often used as an antenna feedline, but could itself serve as an antenna. Coaxial cable had a simple structure with two layers: a metal core wire in the center, surrounded by a fine metal mesh shield layer, with insulating material between the core and shield, all covered by a rubber outer jacket.

So a coaxial cable’s cross-section looked like a shooting target – a solid circle in the center, surrounded by concentric rings.

To turn the coaxial cable into an antenna, the most important step was removing the metal shield layer.

The shield layer was like a Faraday cage. The coaxial cable’s superior interference resistance compared to ordinary twisted pair came from this shield mesh. It both blocked external interference and prevented electromagnetic waves generated inside the cable from radiating outward, so this layer had to be stripped away for radio waves to spread.

“144MHz, how long should we leave?” Wang Ning asked.

Bai Zhen stared at the black cable coiled on the floor, and pondered for a few seconds, quickly calculating in his head.

“About 0.5 meters.”

The two measured out 0.5 meters from the end of the coaxial cable with a ruler, then made a circular cut at the 0.5-meter mark with a small knife.

This was a grand circumcision operation – the “foreskin” alone was half a meter long.

They peeled back the 0.5-meter length of the jacket, exposing the silver metal shield mesh underneath.

Wang Ning and Bai Zhen squatted on the floor, slowly massaging the metal shield mesh with practiced hands, loosening the soft metal wire mesh, then gradually pulling it back, turning it inside out over the cable’s rubber jacket like rolling up a sleeve.

After this was done, the basic entry-level 144MHz antenna was essentially complete.

This coaxial cable served as both feedline and antenna – the stripped portion was the antenna responsible for radiating radio waves, while the unstripped portion was the feedline for transmitting signals.

Just that simple.

Finally, Wang Ning made fine adjustments using an SWR meter.

The two had conducted nine experiments with this antenna setup yesterday. The floppy coaxial cable couldn’t stand up by itself, so Wang Ning tied it to wooden mop handles. He held two mops high outside the window, looking like he might take off at any moment, scaring passing residents into calling the police.

In 2019’s Nanjing, in the Meihua Mountain Villa complex full of power switches and electric bikes, these two seemingly crude whip antennas still stubbornly supported Wang Ning and Bai Zhen’s verbal sparring.

“Old Bai, are you done yet? My back’s about to break. I’m telling you, you’ll have to compensate my medical expenses – not a penny less than 500 yuan.”

“Come down and I’ll give it to you, guarantee I’ll burn it for you, not a penny short.”

“Get lost, if you’ve got the guts come up here, try it yourself.”

“If you’ve got the guts, come down!”

“Come up here!”

“Come down here!”

Ban Xia tied the finished coaxial cable antennas to the poles. BG4MXH had specifically reminded her to use insulating material.

Two antennas total – one for the transmitter, and one for the receiver. The girl exerted great effort to extend them through the attic skylight, setting them up on the roof.

144MHz was in the UV band, specifically the VHF band. Electromagnetic waves propagated line-of-sight, so the higher the antenna, the better.

Ban Xia asked how far it could reach.

Wang Ning shrugged; he couldn’t say for sure.

UV waves were conventionally used for short-distance communication, but there were exceptions to everything. Under the right conditions, the UV band could achieve direct frequency communication over hundreds of kilometers.

Bai Zhen said that in apocalyptic Nanjing, empty of people, the electromagnetic environment would be extremely quiet, giving a very good signal-to-noise ratio for communications.

Wang Ning said a world with only one person did make things simpler – the repeater didn’t even need CTCSS tones since only one person would be talking.

That evening, the girl charged her handheld radio fully.

A Baofeng UV9R waterproof version.

Following Bai Zhen and Wang Ning’s instructions to find a waterproof handheld radio, Ban Xia found eighteen well-preserved waterproof radios on Zhujiang Road. All the big brands – Motorola, Yaesu, and others – had fallen by the wayside. In the end, only the abundantly available hundred-yuan miracle machine, the Baofeng UV9R, still worked normally.

She turned on the Icom 725 radio.

Turned on the GR3188 analog repeater.

Ban Xia took a deep breath. She had worked on this for an entire week; success or failure would be determined now.

Gently turning the switch knob on top of the handheld radio, the LCD screen lit up.

On a night under a brilliantly starry sky, the girl left her room, climbed to the attic, sat on the roof, and began telling BG4MXH about the stars she could see.