1.1 Radio Frequency

The abbreviation of radio frequency RF is used both as noun and as a qualifier as seen for example in RF devices. Actually, the distinction between RF and other objects is based on different historical considerations, such as the bandwidth-based, frequency-based, application-based, and size-based definition [95]:

Bandwidth-Based Definition: RF amplifiers are treated synonymously with tuned amplifiers, implicating that RF circuits are necessarily narrow-band ones with bandwidths of a small-fraction of the center frequency. This definition is not much in use any more today.

Frequency-Based Definition: RF is frequently defined as the range of electro-magnetic waves lying between the low-frequency and the microwave frequency bands, thus consisting of the three frequency bands:

• MF (Middle Frequency) refers to the frequency band between $300$kHz - $3\,$MHz. The waves are propagated in the troposphere and absorbed in the ionosphere. They are used for AM radio, maritime radio, radio direction finding, and emergency applications.
• HF (High Frequency): The band between $3\,$MHz - $30\,$MHz is propagated in the ionosphere and used for amateur radio, CB radio, international broadcasting, military communication, long-distance aircraft and ship communication, telecommunication.
• VHF (Very High Frequency): The frequencies between $30\,$MHz - $300\,$MHz are characterized by line-of-sight propagation and are used for VHF television, FM radio, aircraft AM radio, and aircraft navigation.

Application-Based Definition: Especially in communication system engineering, RF is distinguished by considering the role of the signals at these frequencies. RF signals were historically used as carriers rather than containing information. Whereas an amplifier extending signals from $600\,$kHz to $1600\,$kHz in the AM radio band would be an RF amplifier, a video amplifier having a pass-band from $50\,$Hz to $6\,$MHz is not, since the information itself is the frequency range.

Size-Based Definition: An RF device is characterized by taking the phase shift of a signal, occurring over the extent of the device, into account. Compared to the wave-length of the electro-magnetic wave, the size of RF devices is not negligible.

The qualifier RF is used to refer to the frequency range lying just below the microwave range. But this definition is inconsistent with the term RFIC (Radio Frequency Integrated Circuits) referring to integrated circuits operating at the millimeter and sub-millimeter wave frequencies. Furthermore, signals are referred as RF from the AM band to the sub-millimeter and even IR region. For that reason, microwave would be a sub-range of RF and the distinction between RF and microwave would be obsolete. Though problematic, the phrase RF and microwave is very popular [95].

As RF cannot be properly defined by etymological and historical considerations, the most rational basis for defining RF seems to be a feature-based definition. So the distinction between the RF and non-RF objects is based on required design considerations [95], such as phase shift, reactances, dissipation, noise, radiation, reflections, and nonlinearity. That set of issues combines any kind of device employed from LF up to IR ranges.