Chapter 71: Seepage and Blind Spots

Morning light filtered through the dorm windowpanes pasted with old newspaper and fell across the desktop. Lin Chen had already been sitting there for two hours. Principles of Measuring Instruments lay open to Appendix Three, with the supplementary outline sent by the Provincial Electronics Institute spread beside it. Error analysis for standing-wave-ratio measurement. Systematic error, random error, calibration residue. His pen moved across the graph paper, deriving the coupling formula between the magnitude of the reflection coefficient and phase drift. The scratch of nib on paper was the only sound in the dormitory. The other beds were empty; his roommates had gone to the library to claim seats, or were still asleep. He did not need quiet. He needed order.

At the third step of the derivation, his pen stopped. The ideal attenuation step in the formula was linear, but the real equipment had contact-resistance thermal drift. The competition notice explicitly said “manual setting,” which meant the mechanical play in the knob switch would introduce additional loss. He flipped back through the textbook, looking for the corresponding section. Nothing. Then he pulled out the old instrument manual Old Zhao had given him. The yellowed pages listed only basic parameters, with no error-compensation model. He fixed his eyes on the tolerance line—“±0.5 dB”—and his brow tightened slightly. This was not a textbook problem. It was the sort of trap that could be fatal in the field. If he adjusted by theoretical values alone during the hands-on portion, the spectrum analyzer’s noise floor would swallow the signal outright. He had to derive a correction coefficient himself.

The wall clock pointed to a quarter past nine. The doctor’s instructions said the dressing should be changed in three days. He set down his pen and pushed his chair back half a foot. His left foot hovered off the floor, avoiding any dampness that might still linger there. He untied the bandage he had wrapped the night before, moving very slowly. The instant the tape peeled away, it tugged at the edge of the wound, and he drew in a breath without making a sound. When the gauze came off, the flesh beneath was exposed. The scab that had been dry now showed a ring of dark red around the edges, and at the center there was a very small amount of pale yellow seepage. Not an infection—just tissue fluid. Early autumn in the southwest was hot and airless, and the dormitory had no ventilation. The wound was healing more slowly than expected. He picked up a cotton swab dipped in iodine and wiped in a spiral from the outer edge inward. The sting climbed along his nerves. He clenched his molars and waited for the liquid to evaporate.

The seepage was slight, but it meant he could not be on his feet right away. He counted the spare gauze in the drawer. Three pieces left. Half a bottle of iodine. One roll of tape. Enough to last until Wednesday. He bandaged it again, adjusted the tightness, and made sure his toes could still move slightly without affecting his gait. When he put on his shoe, he deliberately padded the forefoot of the insole with two layers of stiff cardboard to spread out the pressure. By the time he finished, it was already nine forty. The blank space in the derivation still waited on the desk. He sat back down but did not begin writing immediately. He needed to straighten out his thinking first. The source of the error was not the formula itself, but the physical connections. He closed his eyes and built a three-dimensional model of the laboratory in his mind: signal cable, BNC connector, attenuator knob, spectrum analyzer input. Every contact point was a variable. He opened his eyes and drew an equivalent circuit in the blank space on the graph paper. He treated the mechanical play as a variable resistor in series and the thermal drift as a capacitor in parallel. Once the model was simplified, the correction coefficient became two measurable parameters. He did not need a perfect solution. He only needed an approximation that would converge within ±0.2 dB.

By two in the afternoon, the derivation was finished. He checked it three times with his calculator, and the margin of error stayed within controllable bounds. He copied the correction formula onto the title page of his report and circled the key variables in red. At last, the progress bar of his theory review had been pulled to ninety percent. What remained was muscle memory. He closed the book and rubbed the back of his sore neck. His eyes landed on the white envelope at the corner of the desk. The postmark was from Qingshi Village, and the handwriting was his mother’s usual neat script. He took up the paper knife and slit open the seal. The stationery was thin, only two pages. It did not mention medicine costs. It did not mention the family’s hardship. It only said that the autumn rice was heavier than last year, that his father had put on a medicated plaster for his back pain, and that he should not panic during the competition and remember to eat a hot meal afterward. Every line was cautiously restrained. He read it twice, folded the paper along its original creases, and tucked it into the inner compartment of his canvas bag, against his copy of Principles of Measuring Instruments. It was not heavy, but it gave enough weight to keep the bottom steady.

At six in the evening, a long line had formed at the canteen window. He bought two steamed buns and a plate of shredded potatoes for eighty cents. On the way back to the dormitory, the wind was stronger than yesterday, and the clouds hung low. It was going to rain. He quickened his pace, but the rhythm of thirty-centimeter steps did not break. Back in the dormitory, he checked the canvas bag first. The attenuator was secured firmly in its insulated box, the multimeter battery had enough charge, and the graph paper was sealed in a waterproof plastic bag. He pulled open the drawer and took out the bus route map from the provincial capital to the electronics institute, marking the transfer points in red. If it rained, the dirt road by Laboratory Three would flood. His left foot could not go wading through water. He would have to detour onto the cement road after getting off the bus, walking an extra four hundred meters but avoiding the mud pits. Four hundred extra meters meant leaving twenty minutes earlier. He recalculated the time budget from the beginning. It would be enough.

He spread open his account book and crossed out the day’s spending. Balance remaining: seven yuan three. The eighteen yuan for the bus fare had already been set aside. The money for emergency medicine and dry rations was still there. He closed the account book, took the attenuator out of its insulated box, and connected it to the DC power supply. The screen lit up, and the preheating indicator began to flash. He suddenly realized a problem. The official manual for the HP8591E said that the noise floor would stabilize after fifteen minutes of preheating, but the hands-on portion of the competition allowed only twenty minutes of preparation time. If the equipment was cold when they entered, the data from the first five minutes would all be drift values. He had to finish preheating before entering, or bring a small load resistor to power it in advance. But the exam rules strictly forbade bringing a private power supply. He stared at the flashing indicator light, his fingers tapping unconsciously on the desktop. Very slowly. Once, then twice.

The wind outside struck the glass with a muffled thud. Rain began to hammer down, sparse drops at first, then quickly joining into lines. He got up to shut the window and fastened the latch. The dormitory dimmed by another degree. He returned to the desk, put the attenuator back into its box, and pulled the zipper shut. Tomorrow morning he would go to the laboratory for calibration. He had to confirm the preheating habit of that shared spectrum analyzer. If the invigilator allowed the machine to be powered on early, he could gain five minutes. If not, he would have to rely on the manual compensation formula and force his way through. He had to prepare for both paths.

He took off his outer clothes and folded them neatly at the foot of the bed. The gray shirt he would wear tomorrow had already been ironed flat, with no frayed edge at the collar. He checked his shoelaces and made sure they would not come loose after being tied. At last he looked once more at the itinerary on the desk. Wednesday. Bus departs at six in the morning. Arrive at seven thirty. Theory written exam at eight thirty. Hands-on in the afternoon. The timeline was like a taut string, every point on it locked against a margin. He lay down and closed his eyes. A faint swelling pain came from his left foot; the gauze over the seepage lay cool against his skin. There were no curves in his mind, no anxiety. Only parameters. Step size. Impedance. Matching. Error. Calibration. Margin. Four lines crossed in the dark without colliding, without tangling. Each went forward on its own.

The sound of rain grew denser. He turned over to face the wall. Tomorrow he had to go to the campus clinic to get two packets of desiccant and, while he was there, confirm the departure schedule of the earliest bus. If the timetable had changed, he would have to recalculate his stride. He slipped a hand beneath the pillow and touched the folded letter. Its edges had already been worn a little soft. He did not take it out to look, only pressed it once through the cloth. Then he withdrew his hand, and his breathing gradually steadied.

The night was still long. The road was still far. Thirty-centimeter strides. No stepping in puddles. Forward.