Definition
A conductor, or pair of conductors, used to carry radio-frequency (RF) energy between a transmitter or receiver and an antenna with minimum loss. Common forms in aviation include coaxial cable and, less commonly, two-wire (parallel) lines. A transmission line is designed to match the impedance of the equipment and antenna so that energy is transferred efficiently rather than reflected back along the line.
Plain English
The special cable that carries the radio signal between the radio set and the antenna without losing much of it along the way.
Context Anchor
Seen in avionics installation, antenna checks, and radio troubleshooting, especially when discussing the cable between a radio unit and its antenna.
Derivation
From Latin trans- ('across') and mittere ('to send') — literally, the line that sends something across. In radio work, what is sent across is electrical energy at radio frequencies, from the equipment to the antenna.
Why Pilots Care
A damaged or poorly connected transmission line can drastically reduce radio range or cause weak, garbled communications, even when the radio itself tests fine. It is a common hidden cause of comm and navigation problems.
Analogy
Think of it like a hose carrying water from one place to another. If the hose is damaged or poorly connected, less water reaches the other end; with a transmission line, less signal reaches the antenna or radio.
Intuition Check
Do not read “transmission line” as a power-company wire or just any line that transmits something. In this aviation electronics context, it means the physical cable or path carrying radio signal energy between equipment and an antenna.
Example Sentence 1
The mechanic traced the weak transmitter output to a damaged transmission line running between the comm radio and the belly antenna.
Example Sentence 2
A damaged transmission line can prevent the aircraft radio from sending or receiving clearly.