A radio frequency (RF) signal refers to a wireless electromagnetic signal used as a form of communication, if one is discussing wireless electronics. Radio waves are a form of electromagnetic radiation with identified radio frequencies that range from 3Hz to 300 GHz. Frequency refers to the rate of oscillation (of the radio waves.) RF propagation occurs at the speed of light and does not need a medium like air in order to travel. RF waves occur naturally from sun flares, lightning, and from stars in space that radiate RF waves as they age. Humankind communicates with artificially created radio waves that oscillate at various chosen frequencies. RF communication is used in many industries including television broadcasting, radar systems, computer and mobile platform networks, remote control, remote metering/monitoring, and many more.

While individual radio components such as mixers, filters, and power amplifiers can be classified according to operating frequency range, they cannot be strictly categorized by wireless standard (e.g. Wi-Fi, Bluetooth, etc.) because these devices only provide physical layer (PHY) support. In contrast, RF modules, transceivers, and SoCs often include data link layer support for one or more wireless communication protocols.

< 1 GHz
Although wireless communication is often associated with the 2.4 GHz frequency range, many devices and technologies use radio frequencies below 1 GHz (1000 MHz). The 900 MHz band, or 33-centimeter band, is a well-known ISM (industrial, scientific and medical) frequency range used for cordless phones, walkie-talkies, amateur radio and even amateur television. ZigBee, a specification for low-power communication in wireless personal area networks (WPANs), as well as the IEEE 802.15.4 standard it is based on, can use the 900 MHz ISM band in the Americas.

Also in the sub- GHz range is Citizens’ Band (CB) radio, a popular two-way amateur radio service occupying the 26.79 MHz to 27.4 MHz range in the United States. Both AM broadcast radio (535 KHz – 1.7 MHz) and FM broadcast radio (87 MHz – 108 MHz) are in this lower range of frequencies as well, and over-the-air television in the U.S. spans a wide (54 MHz to 806 MHz) range of carrier frequencies. Recent near field communication technology (NFC), found today in many of the newest smartphones, operates at 13.56 MHz.

1 GHz - 5 GHz

The 2.4 GHz ISM (industrial, scientific and medical) band is perhaps the most common in this frequency range. Its unlicensed nature has made the ISM bands a popular choice for many wireless technologies, such as ZigBee (IEEE 802.15.4), Bluetooth (IEEE 802.15.1), and Wi-Fi (802.11).

Many other 2.4GHz technologies exist as well; WiMAX, GPS, cordless phones, car alarms, and even microwave ovens operate in this frequency range.

> 5 GHz
Wireless signals in the upper end of the super high frequency (SHF) band and higher (5 GHz+) are often referred to as microwaves. At these frequencies water is more or less “opaque”, so a microwave signal will experience significant attenuation and scattering issues due to moisture in the air. For this reason, extremely high frequency communications are either severely limited in range or always require line of sight.

Still, these types of signals can be made highly directional, and are seeing increasing use in modern technology. The ISM (industrial, scientific and medical) bands include several frequencies in the high GHz range such as Wi-Fi based on IEEE 802.11a and 802.11n wireless standards, which both 5 GHz capable. IEEE 802.11ac is the latest standard standard for emerging WiFi technology, operating exclusively in the 5 GHz band and offering significant improvements in speed, range, power consumption, and reliability. Other new technologies are designed to opearate on frequency bands much higher still; WirelessHD is a wireless specification based and frequencies as high as 60 GHz and features an impressive maximum nominal data rate of 25 gigabits per second – rivaling that of HDMI.

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