Aircraft Obstruction Lighting

Author: Edmund A. Laport

Grounded supporting towers do not require any special isolation means, and the conduits for the lights are run directly up the tower from ground. When the tower is the radiator and is series-fed, as is common practice, the lighting circuits must pass by the insulated base and the antenna coupling circuits in such a manner as to appear as very high reactance at the operating radio frequencies.

There are three more or less standard methods for conducting lighting power for the tower past the antenna base.

1. The use of a toroidal tower-lighting transformer. The primary is toroidally wound on a circular iron core, and the secondary is an ordinary winding. The latter is cross-linked with the primary like two links of a chain. The primary is attached to the ground side of the tower base, and the secondary is attached to the lower part of the tower proper, above the insulator. The primary and secondary are thus separated by a large air space. At radio frequency this transformer acts as a simple capacitance in parallel with the base insulator.

2. Forming the power wires into an inductance having a sufficiently high impedance to have negligible effect on the feed-point impedance of the antenna. This lighting choke is formed with the two wires of the lighting circuit wound in parallel and is designed to have a sufficiently high Q not to cause excessive power loss.

3. Using a motor generator, the motor located on the ground side, and driving the generator, at tower potential and located on the tower proper, through a coupling shaft of dielectric material.

Aircraft obstruction-lighting requirements are dictated by regulations that vary somewhat in different countries. These specify the power of lamps to be used, the heights of lamps, the number to be used at each specified level, and the use of flashing beacons at the top and at intermediate levels in special cases. The specifications are sometimes based on the proximity of the tower to airports or airways and on the tower height. In some countries, towers below certain heights or towers located outside of practical flying areas are not required to be lighted. A high tower with several levels of double lamps and a large flasher

FIG. 2.34. Radio-tower-lighting system.

beacon at the top and at an intermediate level requires, in some cases, 2 kilowatts or more of lighting power.

The changing of lamps on towers is usually inconvenient, and the frequency of changing is minimized by operating the lamps at 5 to 10 percent below rated voltage. This reduces the light intensity slightly but increases the life of incandescent lamps tremendously. Beacon lamps

are usually required in duplicate, with automatic change-over in case of failure of one.

A circuit diagram of a tower-lighting system using a tower-lighting transformer is shown in Fig. 2.34.