When erecting an aerial, it is best to fasten a pulley at the top of the supporting mast and hoist the aerial up after the pole is in position. Then in case the wires become twisted or broken they may be lowered and repaired without any difficulty.
Erection of an Aerial.—The average amateur aerial is generally from 40 to 60 feet high and supported at one end by a short pole placed on the house and at the other end by a mast set in the ground or lashed to a tree. Fig. 22 illustrates such an arrangement whereby a flat-top T aerial is supported at one end by a short pole fastened to the house and at the other end by a pole set in the ground.
Fig. 22. Flat-top T Aerial.
The flat-top loop aerial is preferred by some amateurs and it is to be recommended for receiving but is an inefficient radiator. When this type of aerial is used the two leading-in wires should be connected to a switch so that when the switch is closed they are connected. The aerial may then be used as a straight-away aerial for transmitting, and by opening the switch, as a loop aerial for receiving. This precaution is advised when a loop aerial is to be used with a low-powered induction coil as a transmitter, for otherwise there will be a loss of energy at the anchor gap.
A large aerial is of no advantage when used with a small transformer or induction coil because it cannot become properly charged.
To erect a flat-top T aerial, first select its location. If possible take advantage of two trees and lash a short pole in the top of each, so that the aerial may be raised up clear of the leaves. Another good plan is to erect a pole at each end of the house. In any case, the distance separating the poles must not be greater than three times the height above ground or the directive action of the aerial will be very pronounced. An aerial 50 to 60 feet high should have a length of from 80 to 100 feet. Stranded wire is no advantage for receiving, but must be used if the transmitter is other than a small induction coil.
Secure two spruce sticks about 2 inches in diameter and 10 feet long. Fasten an insulator 6 inches from each end of the spars and two more each 3 J feet from the ends. This arrangement will separate each of the four wires which compose the aerial by three feet.
The two spars are then laid on the ground at a distance apart equal to the desired length of the aerial. Four wires, either stranded or No. 12 B. S. gauge copper, are cut to equal lengths and fastened to the corresponding insulators. The middle of each wire is found and a long copper wire soldered to it. These four wires constitute the rat-tail or lead-in. They should be of the same length, and are not connected together until they are about to enter the building. A short rope tied to each end of the spars and fastened to the rope which passes over the pulley on the top of the pole, serves as a bridle to prevent the aerial from twisting. The aerial is then hoisted up but allowed to hang slightly slack.
There is considerable difference of opinion among experts as to whether or not the ends of the horizontal wires should be connected, and it is impossible to say with good reason which method is better. However, when erecting a flat top aerial, exercise every care to make all the wires of exactly the same length.