Radio Broadcast (May-Oct 1922)

RADIO BROADCAST 147 Screw Eye Ln Insulator screw Eye No. 14 Copper Antenna Wire 75 to 150 Ft. long, 25 to 50 Ft. high Insulating F.ntrance Bushing Fig. 5. Typical antenna for receiving from amateur and broadcasting stations pressure at the lower end of the hose will be maintained constant. A smaller shower, however, will not keep the funnel full, and if the pipe pressure were to be maintained, a larger funnel, such as that shown in Figure 2 would be required. When a radio antenna is located near to a radio transmitter, the waves striking it are comparatively strong. These strong waves will give good results on a small antenna. They correspond to the heavy shower maintaining the pipe pressure using a small funnel in the water analogy. However, when the radio receiving antenna is at considerable distance from the transmitter, the waves are relatively weak and they miust be intercepted by a longer antenna to maintain the electrical voltage, just as it was found necessary to use a larger funnel to maintain pipe pressure in a light shower. Another determining factor in the pressure at the bottom of the pipe in Figures 1 and 2, is the height of the funnel above the end of the hose. The higher the funnel, the greater the pressure. Similarly, the radio antenna should be placed high enough above the ground or the signal strength will be reduced. A frequent mistake that is made in the erection of the antenna of a receiving station is in thinking that the only prerequisite is to get the wire or wires as high as possible regardless of any other conditions. In fact there is a tendency on the part of people living in tail apartment houses to feel that if they place their antenna on the roof, regardless of how close it comes to other objects it will function properly. The fallacy in this reasoning will be evident when it is realized that the electrical pressure on the antenna (one of the factors in determining the loudness of the signal) depends on how far the antenna is above grounded objects. The frame of apartment houses is generally of steel. This steel frame work rests on the ground. Current is therefore free to flow up the steel frame and thus bring the ground pressure nearer to the antenna. This can be better understood by referring to Figure 3 and 4. If the lower end of the rubber tube is made to approach the funnel, the water pressure at the end immediately decreases. Similarly when a radio receiving antenna is brought near to a steel frame apartment or to trees, or if the vertical wire connecting the antenna wire to the radio set (commonly called the lead-in) is run down an elevator shaft or through a conduit duct, the electrical pressure to ground is greatly diminished with a corresponding decrease in signal strength. The erection of an antenna in such a way that wires come close to metallic objects resting on the ground is therefore equivalent to placing the antenna very close to the ground. Screw Eye Tie Wire Insulator Connectgi:, Lead-in Antenna Wire Screw Eye' Insulator Tie Wire Fig. 6. Details of antenna construction