Radio Broadcast (May 1929-Apr 1930)

Raytheon\ This lamp is made in numerous types and styles, which provide suitable light sources and lightsensitive relays for all systems. List Price, $7-50 Ray the on \ V P>to(ell JF Sending This is an extra-sensitive broadcasting tube, supplied in either hard vacuum or gas-filled types, and in two sizes of each. Information and prices on application Over a hundred different makes of "B" Eliminators require this tube, and take no other. There are millions of them in daily, satisfaction-giving use. List Price, *4-50 Write for further information on any of this equipment RAYTHEON MFG. CO. CAMBRIDGE, MASS. The Radio Broadcast LABORATORY INFORMATION SHEETS By HOWARD E. RHODES T^HE aim of the Radio Broadcast Laboratory Information Sheets is to present, in a convenient form, concise and accurate information in the field of radio and closely allied sciences. It is not the purpose of the Sheets to include only new information, but to present practical data, whether new or old, that may be of value to the experimenter, engineer, or serviceman. In order to make the Sheets easier to refer to, they are arranged so that they may be cut from the magazine and preserved, either in a blank book or on 4" x 6" filing cards. The cards should be arranged in numerical order. Since they began, in June, 1926, the popularity of the Information Sheets has increased so greatly that it has been decided to reprint the first one hundred and ninety of them (June, 1926-May, 1928) in a single substantially bound volume. This volume, " Radio Broadcast's Data Sheets," may now be bought on the newsstands, or from the Circulation Department, Doubleday, Doran & Company, Inc., Garden City, New York, for $1.00. Inside each volume is a credit coupon which is worth $1.00 toward the subscription price of this magazine. In other words, a year's subscription to Radio Broadcast, accompanied by this $1.00 credit coupon, gives you Radio Broadcast for one year for $3.00, instead of the usual subscription price of $4.00. — The Editor. No. 278 Radio Broadcast Laboratory Information Sheet ]VIay, 1929 Inductance-Capacity Products 'T'HE formula for determining the frequency to which a circuit will tune is 159,000 f = / VLC where f equals the frequency in cycles per second L equals the inductance of the coil in microhenries C equals the capacity of the circuit in microfarads It is evident from this formula that the frequency to which a circuit tunes is not determined by the inductance or the capacity alone but by their product. Tables of LC products are to be found in many textbooks, and in "Laboratory Sheet" No. 279 is given a table of LC products covering the broadcast band. The usefulness of this table will become evident from the following examples. Example 1: Suppose we have a radio receiver which uses 0.0005-mfd. tuning condensers and which tunes-in|a station broadcasting on 525 meters at 100° on the dial, i.e., with the condenser plates all in. What is the inductance of the tuning coils used in the set? Answer: From the table the LC product for 525 meters is 0.0776. Therefore, L times C equals 0.0776. We know that C is 0.0005. Therefore, 0.0776 divided by 0.0005 gives 155 microhenries as the inductance of the coil. i Example 2: Suppose we wanted to rebuild this set to use 0.00025-mfd. condensers? What would the inductance of the coil have to be? The LC product must remain the same, [0.0776. Therefore, 0.0776 divided by 0.00025 gives 331 microhenries for the coil inductance. Example 3: The receiver described in example No. 1 will tune down to only 230 meters. Therefore, what is the minimum capacity of the circuit and what must it be reduced to to permit the set to tune down to 200 meters? Answer: The LC product for 230 meters is 0.01489. From example No. 1 the inductance of the coil is 155 microhenries. Therefore, 0.01489 divided by 155 gives 0.000096 mfd. as the minimum capacity of the circuit. To tune down to 200 meters the capacity must be reduced to 0.01126 (the LC product for 200 meters) divided by 155 microhenries. The quotient is 0.000073 which is the minimum capacity (in mfd.) the circuit must have if the set is to tune down to 200 meters. No. 279 Radio Broadcast Laboratory Information Sheet May, 1929 Inductance-Capacity Products THIS table gives the inductance-capacity products to tune to various frequencies throughout the broadcast-frequency band. L is in microhenries and C is in microfarads. The use of the table is explained in "Laboratory Sheet" No. 278. Meters f L X C Meters f LX C 200 1,500,000 0.01126 410 732,000 0.0473 210 1,429,000 0.01241 420 715.000 0.0496 220 1,364,000 0.01362 430 698,000 0 . 0520 230 1,304,000 0.01489 440 682,000 0.0545 240 1,250,000 0.01621 450 667,000 0.0570 250 1,200,000 0.01759 460 652,000 0 . 0596 260 1,154,000 0.01903 470 639,000 0.0622 270 1,111.000 0.0205 480 625,000 0.0649 280 1,071,000 0.0221 490 612,000 0.0676 290 1,034,000 0.0237 500 600,000 0.0704 300 1,000,000 0 . 0253 505 594,000 0.0718 310 968,000 0.0270 510 588,000 0 . 0732 320 938,000 0.0288 515 583,000 0 . 0747 330 909,000 0.0306 520 577,000 0.0761 340 883,000 0 . 0325 525 572,000 0 . 0776 350 857,000 0.0345 530 566,000 0.0791 360 834,000 0.0365 535 561,000 0 0800 370 811,000 0 . 0385 540 "556,000 0.0821 380 790,000 0.0406 545 551,000 0 . M36 390 769,000 0.0428 550 546,000 0.0852 100 750,000 0 0450 • may, 1929 . . . |>uro 52 •