|What is The Need to Modulate a Radio Signal?
Not many would ask this question. We all know the answer, don’t we? Nevertheless, for the benefit of those of us who might just be getting initiated to the world of amateur radio, let us do a quick recap.
We all agree that radio communication is used to transfer some form of information (voice, picture, text, data, etc) from one place to another distant location through an open and physically unconnected medium, without the use of wire conductors. Hence, radio communication is also referred to as wireless communication.
Somehow the information that we need to send across has to be first converted to its electrical equivalent. For instance, the human voice that propagates for short distances through the air by virtue of the variation of air pressure has to be converted to an equivalent electric voltage variation. A microphone is typically used to do so. The electrical output from a microphone can thereafter be processed and amplified as desired to produce a strong electrical signal that continuously varies in frequency and amplitude in accordance with the original sound of the voice. We will from hereon refer to this information signal as the Baseband signal. Thereafter, in a radio communication environment, we perform the task of radio signal modulation to transfer the amplitude and frequency variant baseband signal to a radio frequency electrical waveform called the Carrier. The carrier is typically at a much higher frequency than the baseband signal. However, they are both forms of electrical energy.
An inquisitive reader may ask at this point since both the baseband and the carrier are electrical energies, why do we have to complicate matters by going through the process of radio signal modulation? Why can’t we simply amplify the baseband electrical signal sufficiently and then transmit it directly as electromagnetic wave using an antenna? After all, both the baseband signal and the modulated radio signal are electrical signals and hence both can be easily amplified to produce enough power. Isn’t it?
Bingo! you got it right. The above notion is absolutely correct. However, even though prima-facie it could be done, there are a few major nasty practical hurdles in doing so. That is why we opt to go via the radio signal modulation route rather than directly transmitting the baseband frequency signal… Let us see what these hurdles are.
Firstly, if all of us were to transmit our baseband audio signals, then the frequency span of all our transmissions are going to be more-or-less the same. Since the spectral frequency range would be identical, our transmissions will be overlapping and interfering with one another. That will create a mess.
Secondly, the frequency of our baseband signals will be low and fall in the audio (AF) frequency spectrum, the wavelength of our transmission will be very large. If we take approximately 1 kHz as the mean frequency of the baseband signal, it would translate to a wavelength of 300 Km. Even a typical quarter-wave (λ/4) length vertical antenna will turn out to be 300/4 = 75 Km tall with perhaps equally long radials that run in all directions around the antenna. Similarly, a half-wave (λ/2) dipole will be 150 Km long. An antenna of that size is clearly unimaginable. Even if we chose a smaller length ELF antenna that is meant for extremely narrowband signal, it will not be able to tune and operate over the wide baseband signal span from approximately required 250-2500 Hz band span.
Thirdly, even if we were to hypothetically surmount the above problems, we would be restricted only to the radio signal propagation behavior of the ELF band. We will not be able to exploit the various different EM wave propagation characteristics that are available at the higher portions of the frequency spectrum. Consequently, most of the modern-day radio communication capabilities would not be available.
All said and done, we need to use and exploit the wave propagation characteristics and other features offered by radio signals at higher frequencies like the MF, HF, VHF, UHF, and beyond. Therefore we need to have effective and efficient ways and means of transferring the baseband information on to the radio frequency signals through the process of radio signal modulation.
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