Radiated signal spectrum

The emission spectrum of a radio signal is the relative intensity of electromagnetic radiation along the frequency scale.

The radio frequency spectrum is the total set of radio frequencies, within the limits established by the International Telecommunication Union, that can be used for the operation of radio-electronic devices or high-frequency equipment.

The set of harmonic electromagnetic oscillations into which a complex signal can be decomposed is called the spectrum of that signal. There are amplitude-frequency (AF) and phase-frequency (PF) spectra. To construct an AF spectrum, the frequencies of the harmonic oscillations forming the spectrum are plotted on the horizontal axis, while perpendicular segments are drawn from these points with lengths corresponding to the amplitudes of the respective harmonic components.

The physical meaning of the spectrum lies in defining the set of harmonic components (with given amplitudes and frequencies) that form the specific signal shape in the time domain. In general, the spectrum of time-limited signals is infinite, meaning that an infinite number of harmonics is required to achieve a desired waveform. However, harmonic amplitudes decrease with increasing frequency, allowing the practical spectrum to be limited to a certain frequency band sufficient to reproduce the signal with the required accuracy.

For example, without impairing speech intelligibility, the frequency range of a speech signal in telephone networks is limited to 300–3400 Hz.

Radio Signal Spectrum Width

The spectrum of a harmonic oscillation with a constant frequency F is represented by a single line. The spectrum of a complex modulated signal is more complex and occupies a frequency band. The width of this band — the spectrum width — allows comparing different types of radio signals, which are classified as wideband or narrowband.

For different signals, the spectrum width is determined differently. If the signal spectrum is limited by frequencies fmin and fmax, its width is calculated as fmax x fmin. If the spectrum has unlimited width, the concept of active spectrum width is used, meaning the frequency range covering the most intense harmonics that contain 95% of the total signal energy.

Spectrum width is an important characteristic of a radio signal because it defines the bandwidth of the circuits through which the signal is transmitted. A multi-tone sound signal perceived by human hearing has a frequency range from 16 Hz to 20 kHz and is considered narrowband. A television signal has a frequency range from 10 Hz to 4–5 MHz and is considered wideband. Land mobile radios and radio modems typically have a narrowband spectrum, while digital radio systems (WiFi) are wideband.

Pulse signals are used in radio communications for controlling HF signals, encoding, and processing information. Pulse shapes include rectangular, trapezoidal, and sawtooth. Key parameters of pulses and their sequences are amplitude, duration, rise and fall times, repetition period, repetition frequency, and duty cycle. Pulse signals are wideband, containing numerous harmonics for which it is difficult to define an exact cutoff frequency.

Radio Frequency Spectrum Allocation

Radio waves used in radio engineering occupy the frequency range from 10,000 m (30 kHz) to 0.1 mm (3000 GHz). This is only part of the electromagnetic wave spectrum. Beyond radio waves (toward shorter wavelengths) are infrared rays, followed by visible light, then ultraviolet, X-rays, and gamma rays — all of the same electromagnetic nature, differing only in wavelength and therefore frequency. Although the spectrum is divided into regions, their boundaries are conventional. The regions follow one another continuously, sometimes overlapping. By international agreement, the radio spectrum used in communications is divided into ranges:

These conventional spectrum ranges are large and are further divided into frequency subranges, which include broadcasting and television bands, bands for terrestrial, aviation, space, and maritime communications, for data transmission and medicine, for radar and radionavigation, and more.