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RADIO BROADCAST
149
Insulator
Antenna Wire 75 to 150 Ft. long, 25 to 50 Ft. high
Insulator ^^^i^ ^
Fig. 9. Antenna using tree support
At least 5 Ft.
be brought several hundred feet through the building before it reaches the receiver. The abnormally long length of wire together with its proximity to grounded objects for a great distance will cause the set to give very poor results. The proper method of installing a receiver under such adverse conditions is to place the radio receiver on the top floor, if possible, running the leads from the amplifier to the loud speaker which may be located on the first floor.
If a tree is used to support one or both ends of the antenna, the tie wire should be long enough to permit the ends of the antenna wire to clear the tree branches by at least five feet. This is illustrated in Figure 9.
In the congested municipal districts and in certain localities less densely populated, considerable trouble may be experienced from noises caused in the radio set by near-by power lines. These power lines, particularly if they carry fairly high voltages, send out weak electrical waves of the same type as those sent out by a transmitter, and these waves may cause considerable annoyances on antennas in the vicinity. Contrary to the general impression, direct current lines cause more trouble in this direction than those carryingalternating current. The reason is that the frequency of the noise picked up from an alternating current line is so low that it causes comparatively little annoyance, whereas the variations in current occurring in a direct current line caused by the generators and motors connected to it is of such a high frequency as to be very troublesome at times.
If power wires are in the vicinity where it is desired to install an antenna, the antenna wire and lead-in should be placed as far as possible from the wires. The antenna should also be run at right angles, if possible, to the power wires. These precautions will reduce interference from power sources to the lowest point. In some cases where lines exist in more than one direction from the antenna location the antenna should be placed as nearly as possible
at right angles to both lines. In such cases the best location can only be determined by trial.
At times it happens that it is practically impossible to so place the conventional type of antenna that it is free from noises picked up from power wires. Or perhaps the owner of a broadcasting receiver is so unfortunately situated as to be located near to a radio telegraph station which cannot be " tuned out." Under such circumstances the most ready recourse is to a loop antenna.
The loop antenna consists of a wooden frame supporting a multi-turn rectangular loop of wire as shown in Figure 10. It is very directional in its properties, and by turning its plane toward the broadcasting station, all broadcasting is received with maximum loudness, and signals coming from other directions are reduced. Signals coming at right angles to the loop will not be heard at all. It is thus evident that if interfering noises come from a
To Radio Receiver.
Fig. 10. Loop antenna