Radio broadcast .. (1922-30)

RADIO BROADCAST ADVERTISER Dubilier condenser block TYPE: PL-381 as specified by RADIO BROADCAST Type PL-38i condenser unit has been selected by Radio Broadcast because of its long life and general adaptabil- ity. It is the ideal eliminator con- denser because of the improved dielec- tric material used. Each unit must withstand severe tests in order to pass our rigid inspection standards. Du- bilier nigh quality workmanship and reputation protect your investment and eliminate trouble. Note the Flexible, Rubber-Covered Leads and convenient mounting tabs. Metallic con- tainers protect condenser element from moisture and mechanical injury. Capacities 2,2,4,4,1, 1 mfd. operating voltage —and now DUBILIER BUFFER Condensers For satisfactory service using the Raytheon rectifi- er tube, the new Dubilier buffer condenser is absolutely essential. Made of two .1 mfd. condensers electrostatically shielded from each other and from external capacity effects. Flexible rubber-covered leads. Metallic con- tainers with mounting tabs. Two types: PL-91 for 400 volts D.C.; PL-340 for 600 D.C. volts. At all Dealers Write For Booklet (^4 new booklet "17 Ways to Improve your Set" will be mailed on receipt of ten cents. Tells how to use fixed condensers and gives circuit details. Be sure to get your copy.) Dubilier CONDENSER CORPORATION 4377 BRONX BLV'D NEW YORK, N.Y. The Radio Broadcast SHEETS TNQUIRIES sent to the Questions and Answers department of RADIO BROADCAST were at one time •*• answered either by letter or in " The Grid." The latter department has been discontinued, and all questions addressed to our technical service department are now answered by mail. In place of " The Grid," appears this series of Laboratory Information Sheets. These sheets contain much the same type of information ax formerly appeared in " The Grid," but we believe that the change in the method of pre- sentation and the wider scope of the information in the sheets, will make this section of RADIO BROADCAST of much greater interest to our readers. The Laboratory Information Sheets cover a wide range of information of valu-e to the experimenter, and they are so arranged that they may be cut from the magazine and preserved for constant reference. We suggest that the series of Sheets appearing in each issue be cut out with a rajor blade and pasUd on 4" by 6" filing cards, or in a notebook. The cards should be arranged in numerical order. Several times during the year an index to all sheets previously printed will appear in this department. The first index appeared in November. Those who wish to avail themselves of the service formerly supplied by " The Grid," are requested to send their questions to the Technical Information Service of the Laboratory, using the coupon which appears on page 60 of this issue. Some of the former issues of RADIO BROADCAST, in which appeared the first sets of Laboratory Sheets, may still be obtained from the Subscription Department of Doubleday, Page &• Company at Garden City, New York. No. 89 RADIO BROADCAST Laboratory Information Sheet May, 1927 Short-Wave Coils SOME DATA ON THEIR RESISTANCE 'THIERE are, at present, a great many excellent •*• coils on the market for use in short-wave re- ceivers. They are generally of the "plug-in" type so that different coils are used to obtain the various ranges required. These coils should have as low a radio-frequency resistance as is possible, consistent with a construc- tion sufficiently rugged to prevent their being dam- aged if they are handled somewhat roughly. It would be preferable if the coils could be wound on some solid form but the question then arises whether or not a form can be used without increasing the resistance of the coil to a considerable extent. The General Radio Company has conducted some experiments along this line to determine just how much the form used affects the coil's resis- tance and also to determine what size wire is best to use. Tests were made using a standard bakelite form having a diameter of 2J". The curve given on this Sheet indicates how the radio-frequency resis- tance of the coil varies with the size of the wire used. Evidently, from the curve, the wire size is not especially critical but best results are obtained with a wire size of about No. 12 or 14 gauge. It was found that the use of good binders to hold the turns in place has no appreciable effect upon the resistance. A coil was wound in such a manner that a form could be slipped in and out of it without disturbing the wire. Measurements on the coil with and without the form indicated that the difference in efficiency was negligible. Tests were also made with regard to shielding and it was found that the shielding could be placed very near the coil and have no appreciable effect. The result of the tests may be summed up as fol- lows: When designing a coil for use on the 40-meter 2.0 1.5 J.fl 14 16 18 20 22 B&S WIRE GAUGE (7500-kc.) short-wave band (all these tests were made at this frequency), it is well to (1.) use about No. 12 to 14 wire; (2.) use a coil form if desired; (3.) use any good dope as a binder; (4.) use any reasonable amount of shielding where advanta- geous; (5.) keep the form factor (diameter divided by length) around 1 to 2.5. These data are taken from the February, 1927, issue of the General Radio Experimenter. No. 90 RADIO BROADCAST Laboratory Information Sheet Loop Antennas May, 1927 SOME OF THEIR ADVANTAGES operation of a transformer is usually ex- •*• plained by saying that the current flowing in the primary sets up an alternating magnetic field which in turn causes a current to flow in the secon- dary. This is also the simplest way to explain the operation of a loop antenna, the only difference being that the alternating magnetic field that causes the current to flow in the loop is in the form of radio waves. The number of volts induced in a loop by the pas- sage of radio waves is: 2 TC f n A H x 10-s where H is the amplitude of the wave, f the fre- quency, n the number of turns in the loop, and A the area of the loop. The voltage calculated from this formula is only correct when the plane of the loop is vertical and perpendicular to the direction of the magnetic field. That is, the loop must be pointing toward the transmitting station. If rotated about a vertical axis only a quarter of a turn, no voltage will be induced. This feature is the most important advantage of a loop, for two stations using exactly the same wave- length may often satisfactorily be separated (pro- vided they do not lie in the same or exactly opposite directions) by simply turning the loop at right angles to the interfering station. Loops are com- ing into greater use as transmitting stations be- come more powerful, and they will probably ulti- mately be used almost exclusively on account of the small space required, ease of installation, por- tability, lack of necessity to safeguard against light- ning, and the improvement of the ratio of signal strength to interfering noises, due to their direc- tional properties. If a loop is compared in size to an antenna of the ordinary type it would appear that the amount of energy intercepted by the loop would be exceedingly small indeed. The fact is, however, that a good loop antenna, tuned with a condenser having low insulation losses, will pick up signals much better than might be expected from a comparison of its size to that of an outdoor antenna. This is due to the fact that the loop has a very much lower resis- tance than an elevated antenna. The loop type antenna has been used most fre- quently in conjunction with super-heterodynes be- cause, with this type of receiver, it is easy to obtain a large amount of radio-frequency amplification. During the last year, however, several receivers of the neutrodyne type have been placed on the mar- ket designed for use with a loop. These receivers are generally completely shielded so as to prevent interaction between the loop and the coils in the receiver.