POWER OF STATIONS

Evening Post, Volume CXI, Issue 106, 7 May 1931, Page 21

POWER OF STATIONS

Curiosity often arises over the moaning of the statement that a broadcasting station has such and such a power. What does it mean? It certainly does not mean that the power named is the amount necessary to run the transmitter —that power is always very much higher. Until fairly recently the "rating" of broadcast transmitters was largely a matter of taste or opinion—in somo cases, one fears,' of sheer imagination. The effective power of a broadcasting station is, of course, the amount which is available for the actual work of* broadcasting, regardless of the amount whiehhas to be used up to pro-duce-it. The International Union of Badiotelepkony has given a definition whicli is now generally observed. A "transmitter is rated according to the unmodulated radio-frequency output when this is adjusted to tho greatest value that can be fully modulated; it is the full load power input to the antenna system when the microphone is idle It does not follow that two stations of the same nominal rating on this basis have the sanio actual useful output;-that will;- depend upon the degree of 'modulation.;'lf the output valves are nearly fully loaded when delivering, the .rated'power, tho available 'degree'; of : ./; will.', be 'small; There will.'bo a: fultpowered carrier wave arid a weak musical Output. If>, however, the valves are of ample power and the modulation is high—up to 100 per cent.—the musical output will be large. When the carrier is fully modulated the. actual output is 1% times the "nominal'Vratirig.: But this is possible only when /the 'output valves are able without overloading'to generate four times' the rated power.. This is because the instantaneous peak value of the power of a fully modulated carrier wave is four times its: averago.unmodulated ■value. ~.'.. .- ..... ■' ■■•'■• '•/ How much power has to be used in the station "in front" of the output valvos to produce the required output is a matter of. radio-engineering design. There are; various arrangements, isome more efficient than others. Modern transmitters of high "efficiency have the following total power consumptions: For 1 kilowatt output, 11 k.w.; 2 lew., 17 k.w.; 5 k.w., 32 k.w.; 10 lew., 60 k.w.; 15 k.w., 90 k.w.; 50 k.w.,; 250 k.w. The average overall power efficiency of the station—-output to input—from five kilowatts. upwards, is thus about 16 per cent.-.