A GOOD AERIAL

Radio Record, Volume I, Issue 21, 9 December 1927, Page 2

A GOOD AERIAL

_ AND CORRECT EARTH | | | If the receiving set is to be located | in a country district at a considerable cistance from a local station (says the Melbourne ‘‘Listener-In’’) the degree ot selectivity is not a paramount consideration, as interference is not to be expected, but the sensitivity of the set to signals from distant stations is a feature which probably will be greatly desired. Under these conditions an aerial -106 feet or more in length can be em- } ployed to advantage. This aerial should consist of a single wire supported at least thirty feet above the ground and located in such a position that it is not near any large objects. ‘Lhe lead-in ‘may be a single wire anywhere from ten to thirty-five feet in length. If, on the other hand,_the receiver is to be installed in a city where interferUSEFUL ADVICE FOR BEGINNERS. ence may ~-be expected from several ‘powerful local broadcasting stations, telephone, electric light and power wires, tramways, generator plant, and ene hundred. and one other causes, it will usually -be found necessary to employ a very short aerial. In some cases a single wire about sixty feet in length will best answer the requirements, and in extreme cases it may be found necessary to use one not more than forty feet long. Fer Suburban Area. Under average conditions in a suburban district an outside aerial consisting of a single wire approximately eighty feet in length will -be found most satisfactory, and it is recommended that this size aerial be tried by all persons installing a receiver for the first time. Jf it is found that interference ‘cannot be overcome by manipulating the tuning controls of the receiver, it may be necessary to reduce the length. However, it should not be shortened any more than necessary to eliminate the interference, On the other hand, if no trouble is experienced from interference, the sen. Sitivity of the set may be improved by adding to the length. When erecting this aerial every effort should be made to see that it does not pass near to any object which would be apt to absorb electrical energy. The lead-in wire to be used in connection with the antenna may be from 15 to 20 feet in length, thus making the over-all length of the aerial and lead-in from 95 to 100 feet. Reliable Support. Another point to consider is the method of support. Care should be taken to see that the wire is tightly stretched, and also that the screw eyes or other terminals are attached to a. ‘non-moving object. With regard to insulation, it is most. important that the ‘‘free’’ end of the } aerial should be well insulated, for at this end the voltages set up in the aerial by the incoming signals are at the highest value. This does not mean, of course, that the insulation of the leadin end of the aerial should be neglected. The Earth Must be Good. The earth is perhaps even more neglected than the aerial, a poor earth often being the unsuspected csuse of insensitivity anda unselectivity. ‘The most usial symptoms of a poor earth, assuming all other things to be above suspicion, are poor signal strength and lack of sensitivity, together with flat tuning. A poor earth is tantamouut to introducing a resistance in series with jthe aerial, thereby bringing about an }appreciable loss of sensitivity, Instability is also introduced for the reason that the earth terminal of the receiver is not actually at earth potential as it should be. Types of Earth. If from a variety of circumstances outside the control of the reader it is impossible to erect a good aerial, then the inefficiency of the latter can be offset to a large extent by the use of a good earth. The question now arises: Which of the various types of earths are the best? There are the commonly used water-pipe earth, the cutside vuried earth, the counterpoise earth, and others too numerous to mention. Gen|crally speaking, the outside buried earth is better than the wate1-pipe earth, if the former is obtained by a short earth lead. Otherwise, the waterpipe is better, but a good joint must be {1 made. If the water-pipe is used, it should be made certain that the right pipe is chosen, and for this purpose the destinations of the various pipes should be traced out with the object of finding the "ascending main’-that is, the pipe connected directly to the underground) main passing up to the taps and the cistern, This pipe is always filled with water, and thus makes a good conductor. The other pipes traverse long distances, and have many joints before entering the earth. Don't Use the Gas Main. The gas main should not be used as an earth, for, apart from the danger of an explosion following a lightning dis(charge down the aerial, the joints of gas pipes are notoriously bal as conductors of electric currents, Where neither an outside buried earth nor a water-pipe earth are easily obtainable, the counterpoise may be substituted. This type of earth consists of a wire or system of wires running directly underneath the aerial, and insulated from earth, being suspended by insulators at a heirht of three to six feet above the earth. Local interference from motor generators, etc,, can be very much subdued by an "earth" of this tune. ,