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Show ANTENNAE OF OTHER TYPES ARE DESCRIBED Cag-3, Loop and Umbrella Forms Have Each Certain Advantages Advan-tages and Special Uses. Some of the newer generation of radio amateurs may have been led to think that the cage antenna Is more efficient than other forms. One of the arguments advanced for their use Is that they have been adopted by the United States navy on all ships. There Is a good reason for the adoption of this type of antenna for the navy, but It Is not because of the great efficiency of the cage type. In fact, the only rea-i-a the cage type Is used at all In The ry Is because of the fact that It would be difficult for an enemy shell to carry away the antenna with a single shot. There seems to be a good deal of misinformation on the merits of the cHge-type antenna. They make a partly by it lnnerem cliarat-ionsu1.-:-One of the most important applications appli-cations of the loop ani-T.na is as a radio compass, lor ship, airplane and submarine. Ty mean-; of the Bcllini-Tosi Bcllini-Tosi system, in which (wo single wire antennae, eroded at right angles to each other, rre used, the exact bearing bear-ing of a distant transmitter may be determined. This system is the one used by the numerous naval radio compass stations along the sea coast. The umbrella-type antenna, as illustrated illus-trated in Figure 10, receives 16s name from its general shape and similarity to an umbrella. Referring to the figure fig-ure it will be seen that this type of antenna consists of a number of wires connected at the upper end of a vertical ver-tical mast and radiating from this point In several directions. The lead-in lead-in is taken from the central point of the antenna, system. In general, length of the radiating wires or "ribs" of the umbrella Is about two-thirds the total height of the mast, although this dimension is not fixed, and a different ratio may be employed with success. As can be seen in the illustration, insulators are placed about one-third of the way from the top of the "rib" wires, so that the actual antenna will be some distance above the ground. The over-all length of the "rib" wires, including the guy wire, should be several times as long as the mast is high. This guy wire Coye "Typs Antenna Oil 3ZZZ7" Loop Antcnrto M' Loop LfJ "2 fjp Drtcfiono Chorocfialt I r f l-WI'H'lr1 Urnbrefa fin fenna. s IrrSLtltrfor Ground. S k set pretty appearance, it is true, but unfortunately un-fortunately in radio, pretty is as pretty does, and it takes more than any artistic appearance to do the work. One of the so-called advantages of the cage Is that high tension corona losses are reduced to a minimum. If the amateur Is using undamped oscillations, oscil-lations, that Is, using a vacuum tube transmitter, he need not be concerned about corona losses. However, if a damped or spark transmitter is used, the cage antenna will be advisable. Also uniform current distribution is secured by the use of the cage antenna. an-tenna. This Is not the case with the flat-top antenna, where the currents are not equal, the outer carrying more current than the Inner wires. From the foregoing It may be said that the cage antenna is to be preferred to the flat-top only because of the reduction of the high-voltage corona losses and because of the uniform current distribution. distri-bution. It has been shown that antennae have certain well defined directional characteristics. char-acteristics. The one having the most clearly defined characteristics Is the loop antenna shown in Figure 9. In actual practice a very satisfactory satisfac-tory type of loop antenna for recep-tlonal recep-tlonal purposes consists of eight turns of No. 10 wire, spaced one-eighth, and wound on a frame 30 inches square. The fundamental wave length of this antenna Is approximately ISO meters. Obviously loop antennae need not be square. They may he triangular, circular, or any closed shape, and their linear dimensions are not necessarily nec-essarily fixed. Due to their small dimensions, the amount ' of energy picked up by them is also small, and therefore they should be used only with several stages of radio frequency amplification before the detector. See Figure S, in which the loop Is shown with only a detector. In the hook-up, L is the loop antenna, anten-na, and forms the Inductance In the oscillating circuit. C Is the variable condenser connected across the loop terminals and Is used to tune ths loop to the proper wave length. A loop antenna In this manner has extremely sharp directional characteristics character-istics and receives signals only from stations In the plane of the loop. That Is, the loop must be parallel to the direction of travel of the wave sent out by the transmitter. Many good circuits have recently been devised whereby loop antennae are used with success In eliminating static Interference. This elimination is accomplished partly by the small linear dimensions at the loop and can be fastened either to a small post or to a peg driven into the ground. Due to the fact that the wires of the umbrella-type antenna cover a large area of ground, the capacity of the antenna is very large. The total Inductance, however, Is only a little greater than that of the vertical portion por-tion of the antenna. The umbrella-type antenna has not found a very wide field of application in commercial or amateur telegraphy. During the war It was used to some extent in portable army seta. Due to Its directional characteristics, which are nil or in other words, energy Is radiated equally in all directions it has found some favor with some high-powered high-powered commercial stations for transoceanic trans-oceanic communication. |