High frequency currents permeate copper wire only about one three-hundredth of an inch, and so, in order to increase the surface and decrease the resistance, it is best to make the aerial of stranded wire. A phosphor bronze wire for this purpose which is very flexible but still does not sag or stretch, is composed of 7 strands of No. 20 B. S. gauge. Such a wire 150 feet long suspended vertically and insulated from the earth will have a capacity of from 0.0003 to 0.0004 of a microfarad.
The aerial must receive very particular attention if the station is one kilowatt or over in power. In that case stranded wire is necessary. The insulation of the aerial must be as thorough as possible, and proximity to large conductors such as smokestacks, telephone lines, etc., avoided. Rope stays and guys are advisable in order to prevent dissipation of energy. If wire stays must be used they should be divided up at frequent intervals by insulators.
Fig. 19 illustrates a guy insulator used by the United Wireless Telegraph Company. It is made of two strips of well paraffined wood separated by two porcelain knob insulators.
Fig. 19. Guy Insulator.
Copper wire is the most desirable for an aerial. Iron wire must never be used unless it is very heavily galvanized, and even then it is not to be recommended. Aluminum wire is undesirable except for kite sustained aerials. When used on an aerial and exposed to smoke and other fumes it becomes quickly coated with a layer of oxide. All connections made in aluminum wire must be soldered. This necessity may be better understood when it is explained that electro-magnets on dynamos, etc., are sometimes wound with bare aluminum wire and that the natural coating of oxide on the wire is sufficient insulation to separate the turns.
Fig. 20. Insulating Tube.
Where the aerial enters the building in which the instruments are located it must be very carefully insulated. The simplest method is to bore a hole through the wall and push a porcelain tube through it. The rat-tail or leading-in wire is then passed through and the interstices between it and the tube poured full of melted paraffin.
The best method is to bore a hole in the window pane and pass the wire through a hard rubber insulating tube. Fig. 20 shows such a tube. It is three-quarters of an inch in outside diameter and has an internal bore of three-eighths of an inch. The tube is threaded throughout its entire length. Two hard rubber flanges inch thick and 2 inches in diameter are threaded to screw on the tube. The tube is inserted in the hole in the window pane and the flanges screwed on either side. If a soft rubber washer is placed between the hard rubber flanges and the pane there will be less likelihood of cracking the glass. The leading-in wire is then passed through the tube. The hole in the window pane may be bored by using a copper tube having an external diameter equal to that of the required hole. The tube is set in a brace and used like an ordinary bit, but must be kept well smeared with emery and oil or else it will not cut.