Radio Broadcast (May 1927-Apr 1928)

240 RADIO BROADCAST ADVERTISER It Must Be CustonvBuilt ! The new improved HammarlundRoberts Hi-Q SIX Receiver has been designed by ten of America's foremost radio engineers entirely with an eye to finest possible recep' tion. From the outset price was a secondary consideration. Every modern constructional feature is included — the finest parts in America are used — perfect synchro' nidation at last is realised and through such advanced features as four completely shielded stages of tuned frequency, Automatic Variable Coupling and Symphonic transformers— a truly deluxe quality of performance is achieved! This wonderful instrument cannot be purchased ready-made. It must be CUSTOM-BUILT by yourself at home. Complete parts cost only $95.80, whereas to market it completely built would mean a price of nearly $300. Instruction book shows you how to build. Drilled foundation panels make construction a pleasure rather than a job. The finished instrument is accepted as the best in radio regardless of price. Ask your dealer for the "How to Build" Book or write direct. Price 25 cents. Ji&mmarlunci ROB El RTF S Hi Q SIX HAMMARLUND-ROBERTS, Inc. 1 182 Broadway, Dept. A, New York No. 155 Radio Broadcast Laboratory Information Sheet January, 1928 Wave Traps THREE CIRCUITS THE trend of broadcasting, for sometime, has ■*• been toward the use of high power, and this has made the problem of selectivity a serious one for many listeners located within a few miles of a highpower broadcasting transmitter. When difficulty is experienced in satisfactorily tuning-out such a station, it will be advisable to incorporate a wave trap in the antenna circuit. Wave traps are very easily constructed and cost little. They consist of any ordinary coil and a condenser, connected in the antenna circuit, and adjusted to absorb a large amount of the energy being received from the interfering station. The traps may be connected in several ways, as indicated on the diagram. The arrangement shown at A will give most complete elimination of the undesired signal but may also cause a considerable decrease in volume of stations operating on adjacent channels. The arrangement shown at B is probably the most flexible manner in which to connect a wave trap. If the coil is arranged with several taps an adjustment can be arrived at which gives most satisfactory results. Arrangement C is only useful in case of mild interference. The circuit tunes very sharply and will effectively eliminate interference provided it is not too great. In constructing a wave trap, coil L may consist of 47 turns of No. 22 wire on a 3-inch diameter form if the tuning condenser Ci has a capacity of 0.0005 mfd.; with a 0.00035 condenser coil L should consist of 60 turns. With either size, coil Li may consist of about 15 turns wound at the b end of the secondary coil. With arrangement B taps should be made at about every 10 turns. No. 156 Radio Broadcast Laboratory Information Sheet January, 1928 Wavelength-Frequency Conversion A TABLE FOR THE BROADCASTING BAND ON LABORATORY Sheet No. 157 is given a wavelength-frequency conversion table covering the broadcasting band. Broadcasting is assigned to channels 10 kc. apart on frequencies that are divisible by 10. It is simple to use the table. If we knew that some station was transmitting on 1000 kc. we can determine from the table the corresponding wavelength, which in this case is approximately 300 meters. The wavelength corresponding to any given frequency can be determined by dividing the frequency in kc. into 300,000. A 10-kc. separation between broadcasting stations is necessary to prevent bad interference between two stations on adjacent channels. When a broadcasting station is transmitting it actually uses a band of frequencies (side bands) 10,000 cycles wide — 5000 cycles either side of the "carrier" frequency. The carrier frequency is the frequency assigned a station by the Federal Radio Commission, but as mentioned above, in the ordinary process of modulation a frequency band 10,000 cycles wide is used. When a station is transmitting it also radiates a frequency exactly double its carrier frequency. The additional wave is called the second harmonic, being equal in frequency to the carrier frequency multiplied by two. Careful design and operation of the transmitter will keep these harmonics small in amplitude and this is essential if interference is to be prevented. If a station transmits on, say, 600 kc. and also radiates a strong second harmonic with a frequency of 1200 kc, it will interfere with another station transmitting on a carrier frequency of 1200 kc. Any radio station might be considered to have two ranges; first the broadcasting range, being the distance area over which the program on the station may be received satisfactorily and, secondly, the interference range, being the area over which a station causes interference due to the production of a heterodyne whistle between its carrier and the carrier of another station. The first range is much smaller than the second and a station having a service area of 100 miles will have an interference range of probably about 1000 miles. 157 Radio Broadcast Laboratory Information Sheet January, 1921 Table for Wavelength-Frequency Conversion Kc. Meters Kc. Meters Kc. Meters " Kc. Meters 550 545 1 800 374 8 1.050 285.5 1,300 230.6 560 535 4 810 370 2 1,060 282.8 1,310 228.9 570 526 0 820 365 6 1,070 280.2 1,320 227.1 580 516 9 830 361 2 1,080 277.6 1,330 225.4 590 508 2 840 356 9 1,090 275.1 1,340 223.7 600 499 7 850 352 7 1,100 272.6 1,350 222. 1 610 491 5 860 348 6 1,110 270.1 1,360 220.4 620 483 6 870 344 6 1,120 267.7 1,370 218.8 630 475 9 880 340 7 1,130 265.3 1,380 217.3 640 468 5 890 336 9 1,140 263.0 1,390 215.7 650 461 3 900 333 1 1,150 260.7 1,400 214.2 660 454 3 910 329 5 1,160 258.5 1,410 212.6 670 447 5 920 325 9 1,170 256.3 1,420 211. 1 680 440 9 930 322 4 1,180 254. 1 1,430 209.7 690 434 5 940 319 0 1,190 252.0 1,440 208.2 700 428 3 950 315 6 1,200 249.9 1,450 206.8 710 422 3 960 312 3 1,210 247.8 1,460 205.4 720 416 4 970 309 1 1,220 245.8 1,470 204.0 730 410 7 980 303 9 1,230 243.8 1,480 202.6 740 405 2 990 ■302 8 1,240 241.8 1,490 201.2 750 399 8 1,000 299 8 1,250 239.9 1,500 199.9 760 394 5 1,010 296 9 1,260 238.0 770 389 4 1,020 293 9 1,270 236. 1 780 384 4 1,030 291 1 1,280 234.2 790 379. 5 1,040 288 3 1,290 232.4