This method of leading in the rat-tail is the only one to be recommended if the transmitter is one-quarter kilowatt or over in power.
Fig. 21. High-tension Cable and Insulator.
The lead-in should be anchored just outside of the window so as to relieve the glass pane and the tube from all strain. Pirelli cable or the high-tension cable which is used for the secondary wiring of an automobile is the best conductor to use for the aerial in the interior of a building. The way to lead it over the ceiling is to support it on a porcelain cleat similar to that shown in Fig. 21.
Many are under the erroneous impression that four times the length of the aerial is the wave length which the station will emit. This is only at the best a very rough approximation, for many undeterminable factors such as the nature and location of surrounding objects, trees, etc., so affect the capacity and inductance of the aerial that the wave length must be determined empirically after the aerial is in operation.
The standard wave length of the United States Navy for ship installations is 425 meters. An inverted L aerial, calculated before erection to have a wave length as near as possible to this, has the following dimensions: Four horizontal stranded phosphor bronze wires (7 strands No. 20 B. S.) each 160 feet long and spaced 5 feet apart, four vertical wires 85 feet long and a 35-foot rat-tail.
It is always desirable that the wave length should be as long as possible, for the waves will then travel farther and are not absorbed to such an extent by trees, etc. The absorption due to trees is said to vary as the fourth power of the frequency.
It is sometimes very convenient to calculate the strain on insulators or masts caused by a horizontal antenna. This is easily found by the following equation:
P equals L² x W/8S
where P is the required strain in lbs., W the weight in lbs. per foot of aerial, L the length of the aerial and S the sag of the wire in feet.