Coaxial (Concentric) Feeder

Author: Edmund A. Laport

This type of feeder (Fig. 4.19) is well-known in practice and has a long history.

Rigid construction feeder line of this type, using straight pipes as inner and outer conductors, with air dielectric except for solid dielectric spacers used to maintain concentricity, is commercially available up to a quite large size for high-power transmission. "Semirigid" line of the same general nature is so called because it is sufficiently small and ductile to be shaped into bends and turns without need for cutting or fittings. There is a third form known as "solid-dielectric concentric" or "coaxial" cable, which, in its commonest form, consists of a flexible inner conductor, embedded in a pliable plastic dielectric material, over which is a flexible sheath forming the outer conductor. There is usually a protective rubber or plastic outer covering over all. Concentric cable of large power capacity has been manufactured with air dielectric and steatite spacers and has a flexibility comparable with ordinary power cables of equivalent diameter. The mechanical details of commercially available coaxial line, cable, and related fittings and hardware are described in various manufacturers' catalogues.

Fig. 4.19

The coaxial line is theoretically ideal in that all the field is contained within the space bounded by the inside surface of the outer sheath. At radio frequencies, there is no external field, and so there is no energy lost by radiation from the feeder, unless improper termination gives rise to currents on the outside of the sheath. For the same reason, there is no pickup of external fields when used for receiving.

Mechanically this type of feeder involves many problems which compromise its electrical desirability. Within the power capacity of the various designs of cable, with and without solid dielectrics, these problems have been reduced by years of development. Their applications are much more important in the frequencies above those dealt with in this book, but they find considerable use in low-, medium-, and high-frequency practices.39 Wherever commercial coaxial cable can be used, it provides a very convenient and sometimes economical method for transmitting radio-frequency energy. The rigid form of construction is not so convenient to use, but there are cases where it is a preferred type of feeder. Expansion and contraction introduce problems of considerable complexity, as does internal moisture. A flashover inside such a line can cause great damage, and its repair entails the labor of disassembly and reassembly. If the line is buried, as is often done to reduce thermal variations or for protection from external damage, an internal failure is very troublesome.

Contraction and expansion have been equalized in many designs, but in applying rigid line one may also take measures to minimize the extremes of temperature to which the line is exposed. In the open, it is desirable to shade the line from direct sunlight (see Fig. 4.74) or to wrap it with heat-insulating material as is done with steam pipes. Burial in the earth below frost line is common practice.

Moisture penetration is prevented by sealing the line as carefully as possible and then pressurizing it with dry compressed air or with dry nitrogen. Reservoirs of hygroscopic material, such as silica gel, are sometimes affixed to the system, and the new gas forced in is also passed through this material for drying. To maintain pressure, great skill and care are needed in the assembly, whether solder or solderless fittings are used. Loss of pressure can subject the line to moisture infiltration. Once moisture gets in, it is not easily removed. Leakage of nitrogen can become a considerable item of expense when the assembly is not completely tight.

Lines are usually pressurized slightly above normal atmospheric pressure for the altitude.

The design of a coaxial feeder for large power transmission becomes a major engineering project.40 An example of a line for transmitting 2,000 kilowatts peak is shown in Figs. 4.75 and 4.76. The outer conductor has an inner diameter of 10 inches.

Since safety factor is relatively expensive, coaxial lines for high power often have small flashover margins. As a consequence, only small standing-wave ratios can be tolerated. Devices indicating or actuated by standing-wave ratios above predetermined values are sometimes used to interrupt the power source in order to prevent flashover or to minimize the damage in the event of a flashover. These devices are usually in the form of potential probes projecting through the sheath and terminated on an equipotential line in the field of the feeder. Three are used, separated one-eighth wavelength (the wavelength being that in the feeder), and they excite electronic amplifiers. These are connected differentially so that nothing happens as long as the pickups from all probes are identical, but a relay is actuated when the degree of unbalance exceeds a preset limit. Probes have been distributed along a section of line at small electrical intervals and made to actuate a set of milliam-meters so that the entire standing wave can be seen in magnitude and position,

As an unbalanced system, a coaxial line is sometimes an inconvenience when it must work from or into balanced generators or loads. Two coaxial lines may be used in a balanced system in many instances. Otherwise it is necessary to use networks or line sections to transform from balance to unbalance, and vice versa. There are several practical methods for this purpose which are discussed in Sec. 4.6. For coaxial feeders,

For solid dielectric cable,

where ε is the dielectric constant of the dielectric.