How to Rate the Power of Radio Stations

Radio Record, Volume I, Issue 40, 20 April 1928, Page 3

How to Rate the Power of Radio Stations

(By

Carl

Dreher

itt "Radio Broad-

cast ")

BEFORE broadcasting appeared to amuse and vex the world, radio engineers were concemed with the power of wireless ivlegraph transmitters. These, in 1916 wud thereabouts, were mainly of the wark type, consisting essentially of a rivtor generator feeding 400-cycle alternating current to a high tension transformer, which charged a condenser in connection with a suitable spark gap and oscillating circuit, and so ultimately produced high frequency oscillations in a radiating anteuna. Such sets, in America, were rated in terms ef energy input to the primary of the step-up transformer. The most popular size of set for ship-toshore work was rated at 2 kilowatts on this basis. Possibly 15 per cent. of this energy got to. the antenna, which thereforereceived something in the neighbour--hood of 800 watts of radio frequency power, The set designers of that. day preferred to talk about input to the transformer, for one reason which vas obvious-the greater ease of measuring power at a commercial fre-_ quency and low voltage, and another reason which their self-esteem probably did not allow into the upper. stories of their cunsciousness very of-ten-the fact that 2060 watts sounded — more impressive than 300, and did not iiluminate the miserable over-all ef--ficiency of the transmitter too unspar--ingly. Tle Germans, however, with that taciless thoroughness which has given rise to various emotions in other peoples, were already rating their radio transmitters in terms of radio-fre-quency power in the antenna-"Turm-Kraft," us they called it, which, literally translated, means ‘‘Tower-Power." When » German said he had a 250watt radio set, he meant that, it could put thet much energy into a suitable antenna. In the United States, when radio telephony came into its own, we adopted this basis of grading transmutters. Practice in this regard is not uniform all over the world, howéver, This is pointed ont by Captain P.

P. Eckersley, chief engineer of the British Broadcasting Company, in an article on ‘Power: a Vexed Question," in the "Radio Supplement" (London) for March 5. Eckersley gives the following table for a "Standard 1} Kilowatt Set? :-

When Eckersley speaks of a "standard" 1f-kw. set, he means the British standard, of course. ‘The British rate their sets on the basis of power delivered to the plates of the oscillators. As tubes now go, their efficiency in converting high tension direct current into radio frequency oscillating energy is about 60 per cent. ‘Thus the power delivered by the cscillators to the antenna is about two-thirds of that received by the plates, or working in the other direction, if we multiply the output of the oscillators by 1; we get the high tension input. It follows

that to convert the power of an American station to the British standard, we must. multiply by a factor of 1.5, while if we wish to rate British stations according to American practice we must multiply the nominal power by 0.66. To me the American-German method of expressing power seems more logical. ‘he output is what counts, ‘The ultimate energy (radiated energy) is very difficult to ascertain, thus we are unable to follow the really scientific procedure of expressing the power of the station in terms of that quantity. The next best thing is to work in terms of radio-frequency power in the antenna. The antenna current may be measured with reasonable accuracy, and tlie resistance of the antenna at a given frequency is likewise determinable. ‘The povter equals the square of the current multiplied by tlle resistance-a law not confined to antennas, If telephone stations are

to be rated by power at all, the an‘tenna power seems the most rational figure to be chosen. | If we are to stup at any point ahead of the antenua, the total input to the rectifiers is just as pertinent as the high tension input to the oscillators, and will please the press agent of the station much more. In the above table the G kw. rating is just as defensible as the 1.5 kw. rating, and perhaps more so, for there is some warrant for expressing the power of a machine in terms of the total input, when the output is in a form not readily measur-

ed. For example, we tall about a 0.5-kw, electric heater or flat-iron, this being the power absorbed by the lieating element. In such cases the power consumption, which determines the operating expense, as well as the effect on the supply circuits, is the chief quantity of interest to the user. To show how the power yaries, we may analyse the actual conditions in one modern — station at an arbitrary power level chosen for illustration only, and considerably helow the full power and ratiug of the station. Fig. 1 will aid the reader in following the energy transformations involved. We start with the rectifier, which consists of 12 tubes fed from an ac. source, suitable alternating voltages being applied to plates and filaments. The first wattmeter, W1, reads 185 kw. The > output of the rectifier is about 11 amperes de. at 9300 volts, corresponding to 102 kw. on the d.c. side. This high tension power is split equally between the oscillator and modulater frames of the set. Thus, W2 being 102 kw., W8 and W4 will each equal 51 kw, As W8 is the plate input of the oscillators, the British rating of the set at this level would be 61 kw. Assuming 66 per cent. efficiency, the oscillators deliver 53.6 kw. of radiofrequency energy (W5). ‘This would be the American power rating of the transmitter for the illustrative output power we have chosen, Another factor, lhowever, must be taken into account. The oscillators and modulators require filament current, which in this case is in the form of dic. Each tube takes 45 amperes at 15 volts, or 0.675 kw. If eight oscillators and 12 modulators are used, the former ahsorb 5.4 kw. and the latter 8.1 kw. for filament heating alone. The total power consumption of the set is 185 kw., a.c., and 18.5 kw., d.c., adding up to a total of 113.5 kw. The mean r.f. power delivered to the antenna is 33 kw., an approximate power efficiency, overall and for this particular adjustment, of 20 per cent. Of course we must take into account, in comparing this figure with efficiencies of other machines, that we require at least as much energy to modulate | the carrier as to produce it in the '

first piace. Another element which prevents the over-all efficiency of eveu the best modern radio transmitters from attaining a more respectable level is the fact that several units must function in tandem, owing to the peculiar nature of the machine as a whole. ‘The rectifier has an efficieac of about 75 per cent., and the oscil--lator of 65 per cent., which are middling good performances in the field of energy transformation. Taken together, however, they drop gomewhat below 50 per cent. ‘Then when we add the burden of filament heating, and the large energy consumpiton of the modulator, we ruin the efficiency of the transmitter considered as simply an energy transformer. But toa consider it in this way alone is meaningless. . It is as much as to say that a beautiful and healthy woman is useless because she is a poor piano movyer. We rate such a woman by her physical beauty; we rate a modern radio telephone transmitter by its acoustic beauty, i.e., fidelity of reproduction. Efficiency in the wmnarrow sense is allowed to go hang, very properly. However, as energy is sacrificed apparently so recklessly in the best broadcasting stations, that is the more reason for not rating them at any intermediate stage, when the question of range aud power is being considered. If power ratings in radio are to be made uniform -internationally, therefore, the radio-frequency power delivered to the antenna wonld seem to be the most yalid basis of comparison. The metre-ampere product as an expression of the effective range of stations takes the radiating qualities of the antenna into account, and is hence a step in advance of power ratings in terms of mere watts. Tiven then, however. we must consider percentage modulation, as Captain Eckersley points out, and as yet no one lias worked out a rigid formula including this variable. Practicaliy, as probaby most stations run their modulation around 80 per cent. peaks, this factor caucels out in many instances, leaving metre-amperes or watts in the antenna as a yalid basis of comparison between transmitters the frequency of which does not differ too widely.

Point of measurement, Total high tension input to set from transformers, Power to anodes of oscillaiing vales. Power to aerial, Meter-amperes. Power, Used by G6kw. Some Continental organisations. British and all members ot the Geneva Bureau for comparison purposes. American and some Continental. $00 Governments and = scientific bodies, Lo 1.0