. The principles underlying radio communication . Right atthe ends it is almost impossible to set it in vibration. Just sothe antenna, if disconnected from earth, would be almost impos- 310 I ; I K< (MAGXETIC WAVES. slble to set In vibration if the emf. were applied at the bottomend. For successful working, the exciting apparatus wouldhave to be .joined to Inaccessible points of the wire higher is necessary, then, to make sure that the lower end of theantenna is a region where a current is large, and with a goodground this condition is satisfied. In places where the ground has
. The principles underlying radio communication . Right atthe ends it is almost impossible to set it in vibration. Just sothe antenna, if disconnected from earth, would be almost impos- 310 I ; I K< (MAGXETIC WAVES. slble to set In vibration if the emf. were applied at the bottomend. For successful working, the exciting apparatus wouldhave to be .joined to Inaccessible points of the wire higher is necessary, then, to make sure that the lower end of theantenna is a region where a current is large, and with a goodground this condition is satisfied. In places where the ground has poor conductivity (dry, rockysoil, with ground water at some considerable depth) it becomesdifficult to satisfy the above condition. In such cases a coun-terpoise antenna or earth capacity must be used. Thecounterpoise is another antenna of suitable type, supported afew feet above the ground, and insulated from it. The stationapparatus is connected to the regular antenna and the counter- Principle of theCounterpoise Fi<5 n-z. Fig. n3. Wave length «•--- Variation of the Illustrating Method Effective Resistance Q\ tnar^uUtionofan Antenna, poise, instead of to the regular antenna and earth. The useof a properly designed counterpoise is often advantageous evenwhere the earth has fairly good conductivity. As far as theantenna is concerned, the counterpoise takes the place of theground. To some extent the action of a counterpoise can beconsidered as that of two condensers in series; one antennaconsisting of the regular antenna and the counterpoise, and theother consisting of the counterpoise and the more moist layersof the earth (W, Fig. 171) deep below the surface. The coun-terpoise is usually simply a number of wires supported a fewfeet from the ground, but may be a metal screen or wires may be distributed radially from the foot of the an-tenna. The area covered by the counterpoise should preferablybe several times as large as the area of the antenna itself
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Keywords: ., bookcentury1900, bookdecade1920, bookpublisherwashi, bookyear1922
