Supports

Author: Edmund A. Laport

The majority of radio applications in the temperate zones use wood poles for supports because of their availability and relatively low cost, ease of working, and satisfactory durability and strength. Cedar and chestnut poles give long life without protective treatment, and termite damage is relatively infrequent. Other natural round poles may be of many kinds that are locally available. Square-sawed timbers from 5 up to 8 inches square are often used and of course have a neater appearance. The part set in the ground can be dipped in creosote for rot protection and the aerial part of the pole painted. Sometimes the entire pole is creosote-dipped. The pole can be set directly in the ground or in concrete.

A line of poles should be surveyed with a transit and set in a neat line.

Where there are differences in grade level, the pole heights may be varied to keep the line straight in a vertical plane if best appearance is desired. Otherwise the line level can follow the grade with constant wire height above the ground.

FIG. 4.76. Details of a coaxial feeder, as built for Station WJZ, designed for a transmission capacity of 2,000 kilowatts. The conical porcelain insulator supporting the inner conductor at each 20-foot section joint is assembled to the flange on the outer pipe before the next section is bolted on. A flux-grading flange for the ends of the feeder is shown at right. The line was designed for full rating without pressurizing. (Photograph courtesy of National Broadcasting Company.)

A small change in the direction of the line may be made at a suspension-type pole where the suspension insulators hang at an angle, but when the direction of the line changes more than 45 degrees, the corner pole should be of the strain type and guyed on the outside to withstand the side stress of the bend. Where a sharp bend is wanted, it is best to make the bend over a distance of two or three spans if there is space enough and there is no interference with other lines or structures. An example is shown in Fig. 4.22.

The recommended depths of setting for poles are shown in Table 4.4.

Guy anchors in soil should be set at the same depth. In rock, a guy

anchor can be set in a drilled hole with molten lead retaining an eyebolt.