The Lodge-Muirhead capacity aerial does not make use of a ground and is rarely seen in this country. Lately the United States Signal Corps have applied it to their balloons where an earth connection would be impossible. The upper part of the balloon is covered with a network of wires which serves as the upper aerial, and a second system of wires is suspended below the balloon to take the place of the ground. By this means they have had little difficulty in establishing successful communication between the balloons and the earth.
The pyramid aerial is the type used by Marconi in long-distance ultra-powerful stations, but is debarred from extensive installation on account of the large cost of erection.
The cage and grid aerials are of the vertical type and are excellent where a high support to elevate them can be secured. They are at present used principally by the Massie and Stone Companies.
Plate II. Aerial Systems.
The desirable feature of an aerial is a quantity known as its electrostatic capacity and is measured by the charge required to raise its potential one unit. An increase in capacity enables more energy to be accumulated in the antenna, and consequently greater radiation results. The capacity of an aerial may be increased by adding wires, but must not be carried too far or the transmitting apparatus will not be able to raise its potential sufficiently. Owing to an effect caused by mutual induction between the wires, the lines of strain are not distributed symmetrically, and the capacity will not vary directly but rather approximately as the square root of the number of wires. In order to decrease this action and use the surface most efficiently, the wires should not be placed nearer than one-fiftieth of their length and preferably farther apart.
The materials used for the insulation and suspension of an aerial must be reliable, so that in event of bad weather the station will not lose energy or be put out of working order because the aerial blew down.
Porcelain cleats or a string of porcelain insulating knobs make inexpensive insulators. The standard insulator for wireless telegraph work is the "Electrose" insulator. These are made of a molded composition, and have iron rings set firmly in the ends so that they can withstand a very heavy strain. Hard rubber is undesirable for an aerial insulator because it becomes carbonized and covered with a conducting layer.
Fig. 18. Electrose Insulators.