FROM WIRE TO COAX: THE DEVELOPMENT OF HIGH- FREQUENCY FEED LINES 

When radio technology was still in its infancy at the turn of the 20th century, the common method of "transporting high frequency" was the asymmetrical single-wire line and the symmetrical two-wire line. They were used as adapted or tuned feeder lines for a long time, both in the commercial sector and in amateur radio. 

Although W. Siemens proposed and patented a coaxial cable for low-frequency applications as early as 1884, there was still a long way to go before we could recognize what we understand by a coaxial cable today. 

It took almost half a century and numerous inventors and patents before the development of the coaxial cable and 50- Ohm technology was complete. In amateur radio, the use of coaxial feed lines only began in the 1950s and 1960s. 

Today, matched twin-lead lines are still used advantageously only for the symmetrical feeding of multi-band dipoles

HIGH-TECH IN COPPER AND PLASTIC 

A coaxial cable is axially symmetrical, but viewed against ground it is an asymmetrical feeder line. 

The outer conductor usually consists of a dense copper braiding; high- quality cables are double-shielded and have a thin copper foil underneath. The inner conductor of more flexible coaxial cables is designed as a copper strand. Coaxial cables with a larger outer diameter have a core made of one or more solid copper full wires. The insulating material between the inner and outer conductors - the dielectric - is preferably made of low-loss polyethylene (PE), either as a solid material, foamed or in a web construction with a predominantly air dielectric. The outer protection of the cable is a plastic sheath made of polyvinyl chloride (PVC), which is colored black for good UV resistance

HOW MECHANICAL PROPERTIES SHAPE ELECTRICAL BEHAVIOR

The technical data of coaxial cables are determined by their mechanical properties such as the diameter of the inner and outer conductors, the distance between them and the nature of the insulating material (the dielectric) between the inner and outer conductors. 

These factors essentially determine the electrical characteristics of the cable: the impedance, also known as the wave impedance, the frequency-dependent attenuation, and the voltage and power strength. 

Coaxial cables have a fixed characteristic impedance, which is independent of frequency and is 50 ohms for the usual cable types. 

The characteristic impedance is determined by the ratio of the inner diameter of the outer conductor to the outer diameter of the inner conductor, i.e. by the size and distance between the conductor surfaces facing each other

LITTLE LINE THEORY 

Coaxial cables are normally designed for use as a traveling wave line, which means that the current and voltage curves are the same along the entire line, i.e. no minima or maxima occur. However, this requires that the coaxial cable is terminated at both ends with its impedance of 50 ohms. The transmitter has a 50-Ohm output and the antenna and its matching also fulfill this condition in the resonance case. Only the two-sided termination of the cable with 50 Ohm ensures an optimal power transmission, in this case one speaks of power matching. Then the standing wave ratio is SWR = 1.0.

DIFFERENCE IS MADE BY THE LINE ATTENUATION

Unfortunately, the fact that every RF line has a more or less significant attenuation is unavoidable. The attenuation of a fully insulated coaxial cable is minimal at al diameter ratio of 3.6. This is the theoretically optimal value. In practice, there are slight deviations. Well-known cables such as RG58 or RG213 have diameter ratios of around 3.28. The optimum is not very pronounced, but more widely dispersed. The nature of the intervening dielectric, whose properties are characterized by the dielectric constant, is equally important. In addition to cables with a solid dielectric, there are also cables with a dielectric made of foamed insulating material or a predominant air insulation. The latter cables have lower losses than those with a solid insulation. They offer less attenuation, but unfortunately they are also more mechanically sensitive. 

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DR

N1EA