Radio Broadcast (May 1928-Apr 1929)

RADIO BROADCAST _5T 16 80 12„ 60 8 h 40 20 an impedance Zl, is N squared 20 100 times Zl, or the effect of the transformer is to multiply the load impedance by the square of the turns ratio which may be a fraction or an integer depending upon whether the high or the low side of the transformer is loaded. In the foregoing discussion, the turns ratio of the transformer is assumed to be the only transformer characteristic entering the relation. This ideal condition does not actually exist and, in designing or selecting an impedance adjusting transformer, other factors must be considered. There are, of course, the power losses which invariably accompany the passage of power 0 fl through a conversation. These, however, are small in a welldesigned transformer. Most important, however, is the impedance of the transformer itself. In discussions of impedanceadjusting transformers, the statementisoften made that the primary impedance of the transformer should equal the impedance of the source, and its secondary impedance equal that of the load. At best this is a careless statement. The effective impedance of the loaded transformer is determined by the t 1 \ Power \ V — ^ Voltage > s — X / / / / / — / — * Current / / / / It 4 6 8 10 LOAD RESISTANCE IN KILO-OHMS 12 Fig. 3 — Variation of voltage, current, and power as the load is varied. Values were computed on the basis of a 5000-ohm tube impedance and a resistance load impedance of the load over thefrequency range for which good efficiency is maintained. The complete equivalent circuit of the loaded transformer is shown in Fig. 2. The series impedance, Rt and XQ, repre sents the loss in voltage due to transfer losses and leakage reactance. The shunt impedance, Zt, is equivalent to the primary open circuit impedance of the transformer, to which one would expect the term transformer impedance to apply. This impedance has no definite value unless a frequency is specified, i. e., it varies with frequency. The fact that this impedance was ignored in the original discussion gives the clue to its proper value — it must be so high as to take no appreciable current from the source. The useful frequency range of the transformer is the range of frequencies over which this impedance is high enough to prevent appreciable current flow through it. At any frequency at which the transformer is useful, this impedance must be, not equal to, but several times the impedance of the source. An impedance-adjusting transformer should have a turns ratio equal to the square root of the ratio of the impedances to be coupled. The input impedance of the transformer with the secondary open circuited should be several times that of the source at all frequencies in the range to be covered. Table of Wavelength Allocations The following table gives the wavelength allocations which were adapted by the International Radiotelegraph Conference at Washington, D. C. The data show the type of service permitted in wavelength bands between 5 and 30,000 meters (60,000 and 10 kilocycles per second). This radio allocation plan, of course, is used in all civilized countries of the world, as it was adopted at an international conference. Frequencies inkilocycles per second (kc/s) 10100110125 100 110 125 1501 150 160 160 194 Approximate wavelengths in meters 30,000-3,000 3,000-2,725 2,725-2,400 2,400-2,0001 2,000-1,875 1,875-1,550 194 285 1,550-1,050 285 315 1,050 950 315 3502 950 8502 350 360 850 830 360 390 830 770 390 460 770 650 460 485 650 620 435 5153 620 5803 515 550 580 545 Services Fixed services. Fixed services and mobile services. Mobile services. Maritime mobile services open to public correspondence exclusively. Mobile services. (a) Broadcasting. (6) Fixed services. (c) Mobile services. The conditions for use of this band are subject to the following regional arrangements: All regions where broadcast i ing stations now exist work ( broadcasting. ing on frequencies below j fa 300 kc/s (above 1000m). J r,.. ( Fixed services. Other regions j Mobi,e services Regional arrangements will respect the rights I of other regions in this band, (a) Mobile services. (6) Fixed services. (c) Broadcasting. The conditions for use of this band are subject to the following regional arrangements: ( (a) Air mobile service exclusively. \ (6) A ir fixed services exclusively. (c) Within the band 250-285" kc/s (1200-1050m). Fixed service not ) open to public correspondence. I (d) Broadcasting within the band L 194-224 kc/2 (1550-1340m). / (a) Mobile services except l commercial ship sta j tions. Other regions < (6) Fixed air services excluJ sively. I (c) Fixed services noi open to V. public correspondence. Radio beacons. Air mobile services exclusively. Mobile services not open fo publ ie correspondence, (a) Radio compass service. (6) Mobile services, on condition that they do not interfere with radio compass service. Mobile services. Mobile services (except damped waves and radiotelephony) . Mobile services (distress, call, etc.). Mobile services not open to public correspondence (except damped waves and radiolelephony) . Europe Frequencies in kilocycles per second (kc/s) 550 1.3001 1,300 1,500 1,500 1,715 I, 715 2,000 2,000 2,250 2,250 2,750 2,750 2,850 2,850 3,500 3,500 4,000 4,000 5.500 5,500 5.700 5,700 6.000 6,000 6,150 6,150 6,675 6,675 7,000 7,000 7,300 7,300 8,200 8,200 8,550 8,550 8,900 8,900 9,500 9,500 9,600 9,600-11,000 II, 000-11,400 11,400-11,700 11,700-11,900 11,900-12,300 12,300-12,825 12,825-13,350 13,350-14.000 14,000-14,400 14,400-15,100 15,100-15,350 15,350-16,400 16,400-17,100 17,100-17,750 17,750-17,800 17,800-21,450 21,450-21,550 21,550-22,300 22,300-23,000 23,000-28,000 28,000-30,000 30,000-56,000 56,000-60,000 Above 60,000 Approximate wavelengths in meters 545 230* 230 200 175 150 150 133 133 109 109 105 105 85 85 75 54 52.7 50 48,8 45 75 5452.7 5048 8 45-42 . 8 42 . 8-41 41-36.6 36 6-35 . 1 35.1 33.7 33.7 31.6 31.6-31.2 31 .2 27.3 27.3 26.3 26.3 25.6 25.6-25.2 25.2-24.4 24 . 4 23 . 4 23 . 4-22 . 4 22 . 4-21 . 4 21 . 4-20 . 8 20 . 8 19 . 85 19 . 85-19 . 55 19 55-18.3 18.3 -17.5 17.5 -16.9 16.9 -16.85 16.85-14 14 -13.9 13.9 -13.45 13.45-13.1 13.1 -10.7 10.7 -10 10 -5.35 5 . 35-5 Below 5 Services Broadcasting. (a) Broadcasting. (6) Maritime mobile services, waves of 136! kc/s (220m) exclusively. Mobile services. {Mobile services. Fixed services. Amateurs. Mobile services and fixed services. Mobile services. Fixed services. Mobile services and fixed services. Mobile services. Fixed services. Amateurs. • Mobile services and fixed services. Mobile services. Fixed services. Broadcasting. Mobile services. Fixed services. Amateurs. Fixed services. Mobile services. Mobile services and fixed services. Fixed services. Broadcasting. Fixed services. Mobile services. Fixed services. Broadcasting. Fixed services. Mobile services. Mobile services and fixed services. Fixed services. Amateurs. Fixed services. Broadcasting. Fixed services. Mobile services. Mobile services and fixed services. Broadcasting. Fixed services. Broadcasting. Mobile services. Mobile services and fixed services. Not reserved. Amateurs and experimental. Not reserved. Amateurs and experimental. Not reserved. iThe wave of 143 kc/s (2,100m) is the calling wave for mobile stations using long continuous waves. 2The wave of 333 kc/s (900m) is the international calling wave for air services. ^he wave of 500 kc/s (600m) is the international calling and distress wave. It may be used for other purposes on condition that it will not interfere with call signals and distress signals. 4MobiIe services may use the band 550 to 1,300 kc/s (545-230m) on condition that this will not cause interference with the services of a country which uses this band exclusively for broadcasting. Note. — It is recognized that short waves (frequencies from 6.000 to 23,000 kc/s approximately — wavelengths from 50 to 13m approximately) are very efficient for long distance communications. It is recommended that as a general rule this band of waves be reserved for this purpose, in services between fixed points. • march, 1929 page 323