Free Astronomy Magazine November-December 2020

42 NOVEMBER-DECEMBER 2020 ASTROBIOLOGY phase are known. If we listened to a radio station that transmits only the carrier, we could at most hear a sin- gle note or a con- tinuous noise. Instead, we hear voices and music, and this is because the carrier has been modulated with the electrical signals generated by the transducer. The addition of in- formation causes the regular sinu- soidal shape of the carrier to un- dergo a change in amplitude (AM = amplitude modu- lation) or in fre- quency (the afore- mentioned FM = frequency modulation). The signal thus ob- tained is amplified and radiated through an antenna in the form of electromagnetic ra- diation. AM was the first method used to transmit radio programs, but achieving quality recep- tion was often difficult due to adverse weather conditions or sources of electric fields of various kinds that produced alter- ited centers, interference may nevertheless occur that researchers must remove with ap- propriate tricks. However, interference as a whole limits the potential of the instrument by more than 30%. Therefore, one wonders the reason for looking for a hypothetical alien transmission precisely in the FM band, occupied by thousands of radio stations (mostly commercial), with all of the compli- cations that this entails. To understand that choice, we must start from the very nature of radio waves. They are a subset of the electromagnetic spectrum and, by conven- tion, are divided into bands. Depending on the bands used, radio waves can be em- ployed to transmit data, sounds, images and any other element that can be converted with a transducer (encoder, microphone, camera, etc.) into electrical signals of vari- able amplitude. These electrical signals are, de facto , the information you want to trans- mit. To transmit it, however, it is necessary to generate the so-called “carrier wave” (or simply “carrier”) with an oscillating circuit, of which the frequency, amplitude and B y modulating a carrier radio wave with a suit- ably produced signal, we can have two types of resulting sig- nals: one modu- lated in frequency (FM) and one modulated in am- plitude (AM). Tremblay and Tingay searched for FM signals from a large re- gion of the sky centered on the constellation Vela, but with negative results. Below, an anima- tion showing the two types of radio signals.