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The best coaxial cable impedances in high-power, high-voltage, and low-attenuation applications were experimentally determined in 1929 at Bell Laboratories to be 30, 60, and 77 Ω respectively. For an air dielectric coaxial cable with a diameter of 10 mm the attenuation is lowest at 77 ohms when calculated for 10 GHz. The curve showing the power handling maxima at 30 ohms can be found here:
CATV systems were one of the first applications for very large quantities of coaxial cable. CATV is typically using such low levels of RF power that power handling and high voltage breakdown characteristics were totally unimportant when compared to attenuation. Moreover, many CATV headends used 300 ohm folded dipole antennas to receive off the air TV signals. 75 ohm coax made a nice 4:1 balun transformer for these antennas as well as presented a nice attenuation specification. But this is a bit of a red herring: when normal dielectrics are added to the equation the best loss impedance drops down to values between 64 and 52 ohms. Details and a graph showing this effect can be found here: 30 Ω cable is more difficult to manufacture due to the much larger center conductor and the stiffness and weight it adds.
The arithmetic mean between 30 ohms and 77 ohms is 53.5; the geometric mean is 48 ohms. The selection of 50 ohms as a compromise between power handling capability and attenuation is generally cited as the reason for the number.
One reference to a paper presented by Bird Electronic Corp as to why 50 ohms was chosen can be found here:
50 Ohms works out well for other reasons, such as that it corresponds very closely to the drive impedance of a half wave dipole antenna in real environments, and provides an acceptable match to the drive impedance of quarter wave monopoles as well. 73 Ω is an exact match for a centre fed dipole aerial/antenna in free space (approximated by very high dipoles without ground reflections).
RG-62 is a 93 ohm coaxial cable, originally used in mainframe computer networks in the 1970s and early 1980s. It was the cable used to connect the terminals (IBM 3270) to the terminal cluster controllers (IBM 3274/3174). Later, some manufacturers of LAN equipment, such as Datapoint for ARCNET, adopted RG-62 as their coaxial cable standard. It has the lowest capacitance per unit length when compared to other coaxial cables of similar size. Capacitance is the enemy of square wave data transmission and is much more important than power handling or attenuation specifications in these environments.
All of the components of a coaxial system should have the same impedance to reduce internal reflections at connections between components. Such reflections increase signal loss and can result in the reflected signal reaching a receiver with a slight delay from the original. In analog video or TV systems this visual effect is commonly referred to as ghosting. (see Impedance matching)
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