The wire connecting the antenna and the output (or input) end of the transmitter (or receiver) is called a transmission line or feeder line. The main task of transmission lines is to effectively transmit signal energy. Therefore, it should be able to transmit the signal received by the antenna to the input of the receiver with minimal loss, or transmit the signal emitted by the transmitter to the input of the transmitting antenna with minimal loss, and it should not pick up or generate stray interference signals on its own. In this way, it is required that the transmission line must be shielded or balanced. When the geometric length of a transmission line is equal to or greater than the wavelength of the transmitted signal, it is called a long transmission line.

　　Types of transmission lines

　　There are generally two types of transmission lines for ultra short wavelength bands: parallel line transmission lines and coaxial cable transmission lines (microwave transmission lines include waveguides and microstrip, etc.). Parallel transmission lines typically consist of two parallel wires. It is a symmetrical or balanced transmission line. This type of feeder has high losses and cannot be used in the UHF frequency band. The two wires of a coaxial cable transmission line are the core wire and the shielded copper mesh. Due to the grounding of the copper mesh, the two conductors are asymmetric to the ground, so they are called asymmetric or unbalanced transmission lines. Coaxial cables have a wide operating frequency range, low losses, and have a certain shielding effect on electrostatic coupling, but are powerless to interfere with magnetic fields. When using, it is important to avoid running parallel to lines with strong currents, and to avoid approaching low-frequency signal lines.

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