There is only one electromagnetic spectrum, but by using different carrier frequencies, numerous RF devices can coexist.
The world of RF is a world of frequencies. This is true within a single system or even a single PCB, considering that one RF design may involve signals in multiple frequency ranges. But at this point we want to look at the broad context in which a particular RF system exists; the name we give to this concept is “the electromagnetic spectrum.”
More specifically, we will discuss the portion of the electromagnetic spectrum that is commonly used for RF communication. Light is included in the electromagnetic spectrum, and so are extremely-low-frequency radio waves that have limited use in engineered systems. Light is a useful means of transmitting information, but it behaves very differently from medium-frequency electromagnetic radiation (EMR), and consequently we place it in its own category—optical communication instead of wireless communication. Low-frequency EMR has specialized uses and is also generated constantly all over the world by the power grid, but it is not a part of mainstream wireless communication.
Before we discuss the various frequency categories, let’s review two fundamental issues: Why do we use so many different frequencies? And how does a designer decide which frequency is appropriate for a certain application?
Two or more transmitters operating at the same frequency create interference, i.e., they make it difficult for a receiver device to separate the relevant RF signal from irrelevant RF signals. This problem largely disappears when different frequencies are used. EMR at one frequency does not “corrupt” EMR at a different frequency, and the irrelevant signals are easily ignored via filtering.
Of course, interference doesn’t disappear just because two signals are separated by a fraction of a hertz—more frequency separation leads to less interference. Nevertheless, the use of different frequencies for different types of RF communication is amazingly effective: every day, all over the world, numerous wireless systems operate simultaneously with no significant loss of functionality.
The characteristics of EMR vary according to frequency. For example, extremely-low-frequency waves can effectively penetrate water and thus can be helpful when you need to communicate with a submarine. As another example, certain frequencies enable a radio signal to travel very long distances because these frequencies experience atmospheric refraction. The point is, the dominant objectives of a particular RF system heavily influence the process of choosing the operational frequency range.
The previous paragraph mentioned examples in which frequency affects propagation characteristics. Often, though, a more important consideration is bandwidth (in analog systems) or data rate (in digital systems).
If you want to wirelessly transmit an audio signal that has frequency components as high as 10 kHz, you cannot use a 5 kHz transmitter (i.e., carrier) frequency. Frequency corresponds to the rate at which a signal can transmit information, so you cannot “fit” 10 kHz of audio information into a 5 kHz carrier. Furthermore, practical considerations require the carrier frequency to be significantly higher than the information (i.e., baseband) frequency. Thus, wider-bandwidth and higher-data-rate systems must occupy higher-frequency portions of the electromagnetic spectrum.
The radio spectrum—i.e., the radio-communication portion of the electromagnetic spectrum—extends from the VLF (very-low-frequency) band to the EHF (extremely-high-frequency) band, i.e., from about 3 kHz to 300 GHz. The other bands that separate VLF from EHF are
These divisions are rather arbitrary and there is no dire need to know the exact frequency ranges. It would be better to simply give some examples of wireless-communication categories that are found in different portions of the spectrum, because this will help us gain an intuitive awareness of which frequency ranges are more appropriate for certain types of systems.
The satellite frequencies mentioned above mostly remain within the SHF section of the radio spectrum. The EHF band serves as the transition between radio waves and optical waves; EHF signals are more seriously obstructed by the gases and moisture in the atmosphere, and this reminds us of optical radiation and its inability to penetrate opaque objects. Signals with frequencies above those of the EHF band are classified as infrared radiation, not as radio waves:
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