Aerials are of numerous classes and forms, but the most prominent types can be divided into two main groups, called respectively, the "Flat-top" and "vertical" antenna.
The vertical aerials are the older form, and are usually employed for long-distance work or ultra-powerful stations. The aerials intended for transmission from Europe to America, installed by Marconi, consisted of huge inverted pyramids, supported by four heavy lattice-work towers, over 200 feet high. Vertical aerials also sometimes take the form of an umbrella, or fan, where only one supporting pole is available. Iron pipe masts may be employed for the purpose, by setting on an insulating base. The umbrella aerial is used extensively in the army and portable sets.
The flat-top aerials are gradually coming into very extended use. They are used to the exclusion of all others on shipboard. They need not be so high as a vertical type aerial in order to be as efficient. Flat-top aerials consist of a vertical portion and a nearly horizontal portion. The horizontal portion is practically useless, as far as its work in radiating waves is concerned, it being used for the purpose of increasing the capacity of the aerial. An increase in capacity in an aerial means that more energy can be stored and radiated. Flat-top aerials have the objection, however, of possessing a directive action; that is, they receive, or radiate waves, better in one direction than in the other. A flat-top aerial always receives or transmits better in the direction that the ends point than in a direction at right angles to the wires.
FIG. 16.—A diagram showing pyramid aerial.
The accompanying diagram is an illustration to show the effects of the directive action of a flat-top aerial. The black lines marked A B, and appearing very much like a little grating, represent an aerial of the inverted "L" type, looking down on it from above. B is the free end of the aerial, and A the closed end, or end to which the wires leading down to the station are attached. If a snapshot of the lines of strain produced in the ether as the waves move away from the aerial could be taken, they would appear like the curved lines in the illustration. It can be readily seen that those passing outward from the aerial in a direction opposite to that in which the free end points are the strongest, and that the radiation in that direction is the best.
FIG. 17.–A diagram illustrating the directive action of a flat-top aerial.
FIG. 18.—Aerials of the "V" and inverted "L" types.