The antenna is at once both the mouth and the ear of the wireless station. Its site and arrangement will greatly determine the efficiency and range of the apparatus.
The site selected is preferably such that the aerial will not be in the immediate neighborhood of any tall objects, such as trees, smokestacks, telephone wires, etc., because such objects not only absorb an appreciable amount of energy when the station is transmitting messages, but also noticeably shield the aerial from the effects of incoming signals and limit its range.
The nature of the ground over which the waves must travel also enters into the question, and is always considered in locating a station. In gliding over the surface of the earth, the waves generate weak currents in the earth itself. If the ground is very stony or dry, these earth currents encounter considerable resistance, and the possible distance of transmission over soil of this sort is very much less than if it were moist. Moist soil and water offer very little resistance, and the difference in the results obtainable at the receiving station when the waves travel over an area of this sort is very marked.
FIG. 11.—An amateur aerial and station.
A station which can only send 100 miles over land can send messages three or four hundred miles over the ocean.
Forests exert a very decided effect upon the electric waves. Each individual tree acts as an antenna, reaching up into the air and absorbing part of the energy. The difference in the range of a station during the summer months and that of the same station in winter is considerable. In summer the trees are full of sap and, being much better conductors of electricity when in this condition, act in the capacity of innumerable aerials rising in the air, and able to absorb appreciable amounts of energy. During these same months the air becomes highly ionized, in which state the air molecules carry an electric charge, and are particularly opaque to the waves. This condition also usually exists in the presence of sunlight, the result being that the most favorable time for the wireless transmission of messages are the hours around midnight.
FIG. 12.—The Army wireless station at Fort Gibbons, Alaska, showing steel lattice work mast and aerial system.
Locality is another factor which usually receives a fair share of attention in selecting the site. Certain sections of the country, for seemingly no apparent reason, are very hard to transmit messages, either to or from. Wireless stations located on the Pacific Coast, for instance, are more efficient than those situated along the Atlantic seaboard, while those in the tropical regions are only able to send short distances in comparison to those farther north or south. Messages seem to travel better in the direction of the lines of longitude than along the lines of latitude.