Transmission lines are a very important part of a communications system. They carry RF signals from one place to another. Transmission lines are used to interconnect antennas and receivers/transmitters, interconnect computers in a LAN, and distribute cable TV signals through a town. We need to study transmission lines, because the flow of RF through a transmission line is very different from the flow of DC along a pair of wires. It is possible for an transmission line that is open-circuited at one end to look like a short circuit at the other end and in general, the impedance one measures at the input of a transmission line is dependent not only on the load placed at the far end of the line, but also on the electrical length of the line. It is these unusual properties that make transmission lines more than just a pair of wires.
TYPES OF TRANSMISSION LINES
The simplest type of transmission line consists of two conductors separated by a small distance. This type of transmission line is called parallel line, open wire line, or ladder line (because the spacers used to separate the two wires look like rungs of a ladder).
Another type of two wire transmission line consists of two parallel wires embedded in an insulating material (typically polyethylene). This type of transmission line is called twin-lead, and is very similar to open wire line.
Another type of two wire line is called unshielded twisted pair. (UTP). It consists of two wires twisted around one another. The most common type of UTP is CAT5 network cable. It consists of 4 UTP’s in a common sheath. In addition to the properties discussed in the next section, CAT5 has two important properties peculiar to multi-pair transmission lines:
NEXT – near end cross talk – this is a measure of how much RF is coupled from one UTP to another within the cable.
Delay skew – difference in RF signal transit time between the fastest and slowest UTP’s
A fourth type of transmission line is shielded pair. It consists of two conductors insulated from each other and surrounded by a shield made of metal braid. The braided shield prevents RF from being radiated by the conductors as well as stray RF from entering the transmission line.
A fifth type is coaxial cable (coax). Coaxial Cable consists of a wire placed at the center of an outer cylindrical conductor. The two conductors are insulated from one another by a dielectric that may be air, dry nitrogen or a solid dielectric such as polyethylene or teflon. The outer cylindrical conductor is normally covered with an insulating jacket to protect it from the environment. The outer conductor may be flexible or stiff, depending on the application.
RIGID COAXIAL CABLE
FLEXIBLE COAXIAL CABLE
Yet another type of transmission line is a hollow metal tube, known as waveguide. In a waveguide the RF travels as a guided electromagnetic wave, rather than as RF current, as is the case for the other types of transmission lines. The dimensions of the waveguide must be on the order of one wavelength of the guided RF wave, which limits application of waveguides to frequencies in the UHF and microwave region.
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