Fig. 17. United Wireless Portable Outfit.
Fig. 17 illustrates the portable wireless telegraph set manufactured by the United Wireless Telegraph Company for army service and exploring expeditions or isolated camps. The aerial and the mast can be unloaded, erected, and all parts be ready for operation in fifteen minutes. The mast is made of interchangeable wooden sections. The current for the transmitter is furnished by a portable storage battery. The whole outfit is capable of furnishing efficient service for distances of 25 to 30 miles.
CHAPTER III. AERIALS AND EARTH CONNECTIONS.
The aerial or antenna ordinarily consists of a number of wires elevated in the air to emit or intercept the Hertzian waves. In fitting up a wireless station the location and erection of an aerial are of prime importance, and the successful reception and transmission of wireless messages will depend largely upon its condition.
A few years ago the wireless antenna consisted of a metal plate high in the air and having a wire suspended from it, but to-day usually exists in one of the forms illustrated in Plate II.
The higher an aerial is placed above the surface of the earth, the wider will be its electrostatic field, and consequently more powerful electrical waves will be developed. But after a height of 180-200 feet is attained, the engineering difficulties and the expenses increase so rapidly that few stations exceed it. Other things being equal, the increased range in transmitting varies as the square of the height of the radiating wires. For example, a 25-foot aerial capable of transmitting one mile theoretically will send waves 16 miles if made 100 feet high. The actual ratio is often greater, but much is dependent upon the many meteorological conditions.
After the limit in a vertical direction has been reached, the only remaining possibilities are to increase the surface and spread out horizontally.
The flat top aerials are used on shipboard or wherever it is an advantage to suspend the wires between two masts.
They are especially recommended for amateur use, since they need not be so high as the other aerials, to be efficient. The flat top aerials are directive, that is, they receive or radiate waves better in certain directions. The bent or inverted L type is one of these and exhibits a preference for waves coming from a direction opposite to that in which its free end points. This directive action of an inverted L antenna may be somewhat lessened if the leads are taken off at the center and it is made a T aerial. This is the most common form of flat top aerial in use on ships.
The inverted U type is not used extensively because the two opposite leads or rat-tails make a centrally located operating room necessary. The loop aerial is used by the United Wireless Company, in both their ship and land stations. This type of aerial is well adapted to long waves and close tuning.