SELECTIVITY AND HOW TO OBTAIN IT

Radio Record, Volume I, Issue 7, 2 September 1927, Page 14

SELECTIVITY AND HOW TO OBTAIN IT

ADDRESS BY MR. PRESTON BILLING

Now that we have a high powered broadcast station in our midst, the remarks which are to follow will be found of assistance generally to listeners. Commencing with the aerial itself, it is impossible for any receiver to be selective when it is connected to a long aerial. Aerials of this type, especially now the stations to which we listen are so powetful, are to be avoided. I would personally recommend those experiencing trouble in this direction to limit the aerial length, inclusive of | earth wire aud lead in wire, to 80 feet in all. ‘The choice of this length will, of course, result in a shorter distance between the poles, allowing for, say, 80ft. masts, the length of wire between them wonld be, say, up to 50 feet. The acrial should make entrance to the house through a porcelain tube, and once inside the house, it should be kept, as short as possible. If the set can be placed near the point of etitry of the aerial, so much the better,

The regulations require that a lightning arrester he installed. These may be fitted outdoors or indoors as preferredpersonally, I prefer the latter type. The Earth Lead. | This is a point which is often neglect-_ ed. Use an insulated wire of reasonably heavy gauge, bearing 1 mind also that this lead must be kept as short as possible. Run one end of this wire to the nearest cold water pipe and clean the pipe with a piece of carborundum cioth or a file. Next place the clamp in position and screw up tight after | attaching the earth wire. Just make sure you rally have a good earth conaection for, although you may have ttached the wire to the water pipe, have a look and make certain that the pipe docs not completely encircle the house services ere returning to earth. | } | The Receiver.

The receiver is by far the most important item of the whole installation. There are many makes of excellent receivers on the market, but here again both the selectiveness and the power of them are dependent on thie number of yalyes used and the type of circuit in usc. I am not going to enter into the respective merits and demerits of the various types of reccivers, for if one has already purchased a ready-made receiver there will be little or no chance of putting any alterations into it to make it more selective, and resort will have to be made to one or more forms of what. are known commercially as wave traps. If the receiver is to be in use close to a broadcast station then it would be advisaile to see that it is one of the shielded varicty This procedure will be found advisable, as there is the risk that the tuning jnductances themselves may act as miniature antennas, in any case shielding will have tie effect of cutting down wmterference from domestic electrical devices. The design of the radio-frequency |! transformers a‘id the material upon which the turns are wound have a good deal to do with the selectiveness and

| the strength of the reception. If your receiver is a factory-built one by a reputable manufacturer, you may rest assured the coils therein are of satisfactory design, in fact, many of the modern coils are as selective as they can be made without destroying their tonal properfies. If you wish to construct a set of these coils for use, in a neutrodyne circuit the following particulars are given. If these instructions are carried ont a very fine receiver should result-see circuit diagrain for wiring. How to Make the Coils. Procure some sheet celluloid about 1-32nd of an inch thick, cut a strip off the sheet equal te Ysin. wide, procure a wooden former such as that when one thickness of the celluloid is wound around it the overall diameter will be three inches, allow a lap over a » . of abottt an the ends fast by painting the joint with a mixture of celluloid and Amyl-Acetate mixed. Next proceed with the winding, Whieh should be done with 24 gauge double silk covered wire. Commencing from one ‘end about half

fan inch from the edge wind, in place fifteen turns and bring out a tap. Proceed with the winding until there are wound on a_ total of sixty-four turns, inclusive of the 15 turns previously wound. ‘The two ends can be secured in place by drilling two small holes and passing the wire cnds through each at their respective ends, — Prior to removing the coil from the former, coat sparingly with the mixture mentioned; allow to stand a while before removing from the former, when the coil will be found to come away readily and keep its shape on removal. ‘his completes the secondary coil, The primary coil is made in a similar manner, except that the overall diameter will be 23in. and the width of the strip will be ljin. It is secured in the same manner, and the winding is commenced at a similar distance from one edge and wound in the same direction. Five turns only ere wound in place, and these are spaced equally so that the five turns occupy the space required by the 15 turns of the secondary winding, Tventually the primary coil is placed directly beneath these

15 turns. senerally speaking, the Leginner is advised to buy reputable coils. The method of mounting these coils will be left to the ingenuity of the constructor. ‘They are usually fastened to the variable condensers and set at an angle of slightly under 55 degrees. This angle must be set accurately if the receiver is to be neutralised without difliculty, as if one coil is of 1 lesser diameter than the other it will be necessary to space the primary with thin strips of shect celluloid ef 1-8 of an inch thickness. . These are secured in place with the amyl-acetate solution. Three coils are wound = in this manner to complete the set. coils when built into the — circuit giyen with this article will be found very selective and very powerful.

Increasing Sclectivity by Means of Wave Traps, Much has been written svith regard to this subject, aud many of the socalled wave traps have been tried mostly with disarpointing results, for, whilst they partially eliminate’ the unwanted signal, vhev considerably diminish the volume of the station it is required to listen to. There have been only one or two devices which have come under nry notice which will really increase the selectiveness of any set, and, at the same time, actually increase the signal strength from tle station you desire to listen to. Radiating Receivers, This form of nuisance seems to he on the increase, and I would like to give a little advice to those who may be in doubt as to when their set is reradiating, The three-coil Armstrong receiver, whether of the two-circuit or single cirenit variety, is a positive offender, and a good deal of trouble comes from amateur sets constructed on this principle, and operated by own-

UEREEEDECAEQUAUERARSSESUEDUUMMSLENGOATEOAUREDEOESUERUSOD UNDE ATOEASSHSEAN ers who know nothing about them. In addition to the above mentioned ‘set there ate various forms of Neutrodynes and Browning-Drake receivers which are just as likely to cause annoyance to your neighbours if they are nof correctly nemtralised. If users of these home-made sets would make certain of their neutralising adjustments there would be fewer squealings on the air. Any receiver which is believed to be radiating can be cured by adding one staye of neutralised high-frequency amplification. ahead of the existing set. If one must go on using the type of circnit first mentioned, then bear in mind the following hints: (1) Do not use too high a B battery pressure. (2) Keep the det B positive voltage as low as is consistent with good results. (3) Use a small size tickler coil, and keep it as far as possible from the secondary coil. Violation of these instructions simply bring in distorted signals, which are mushy and useless to the listener, and, in addition, the owner of a good receiver is debarred from haying a good evening’s entertainment. Owners of small valve sets are frequent offenders through straining their re- ceivers to work loudspeakers with two and three valves, In conclusion, Mr. Billing stated that he would be pleased to answer any inquiries which may be addressed to me, care of this station. Queries to be brief and to the point, written in ink, on one side of paper only, and clearly endorsed with the name and address of the inquirer.