Radio Frequency

Radio-frequency electrical currents are usually carried by specially-designed transmission line such as coaxial cable, as ordinary electrical cables would have high power loss.

Radio frequency (RF) refers to the oscillation rate of an alternating electric current, voltage, or a magnetic, electric, or electromagnetic field, or even a mechanical system, within a specific frequency range. This range typically spans from approximately 20 kHz up to about 300 GHz. To put that into perspective, it’s the spectrum nestled between the highest frequencies humans can audibly perceive (though those are not electromagnetic in nature) and the lower threshold of infrared light. It also conveniently encompasses the entire microwave band. The significance of this particular frequency range lies in its ability to efficiently radiate energy from an oscillating current in a conductor out into space, manifesting as radio waves. This fundamental property is the bedrock of all radio technology and a myriad of other applications. It’s worth noting that the precise boundaries of what constitutes “radio frequency” can vary slightly depending on the source and context, a minor quibble in the grand scheme of things.

Electric Current

Boy allowing the arc from a Tesla coil to strike his hand. Coming in contact with radio frequency arcs as shown here is extremely dangerous and can result in electrocution.

Electric currents that oscillate at radio frequencies, often termed RF currents, exhibit peculiar behaviors that set them apart from their direct current (DC) or lower-frequency alternating current (AC) counterparts, such as the ubiquitous 50 or 60 Hz currents powering our homes and cities via electrical power distribution.

Radiant Energy: A key characteristic is the capacity for energy to detach itself from the conductor and propagate through space as electromagnetic waves, specifically radio waves. This phenomenon is the very essence of radio communication.
Skin Effect: RF currents possess a pronounced tendency to congregate and flow along the surface of electrical conductors, rather than penetrating deeply within. This is the well-documented skin effect.
Physiological Impact: Unlike their lower-frequency brethren, RF currents often fail to elicit the familiar, painful sensation of electric shock or the involuntary muscular contractions it triggers. The reason for this is the sheer speed at which the current reverses direction, far too rapid to stimulate the depolarization necessary for nerve signal transmission. However, this absence of immediate shock does not equate to harmlessness. RF currents can inflict internal damage and cause severe superficial burns, often referred to as RF burns.
Ionization Potential: RF currents possess the ability to ionize air, effectively creating a conductive pathway through it. This property is ingeniously harnessed in high-frequency units employed in electric arc welding, which utilize currents oscillating at frequencies significantly higher than those found in standard power distribution.
Capacitive Behavior: Another intriguing characteristic is the apparent ability of RF currents to traverse paths that include insulating materials, such as the dielectric insulator within a capacitor. This is a direct consequence of capacitive reactance diminishing as frequency increases within a circuit.
Inductive Opposition: Conversely, a coil of wire, or even a simple bend or turn within a wire, can effectively impede or block RF current. This is due to the inductive reactance of a circuit escalating with rising frequency.
Wave Reflections: When RF current is confined within an ordinary electric cable, it tends to bounce back from any discontinuities, like connectors, and travel toward the source. This creates a phenomenon known as standing waves. To mitigate this, RF currents are more efficiently managed and transmitted over specialized transmission lines, such as coaxial cables.

Frequency Bands

The vast expanse of the radio spectrum is systematically partitioned into distinct bands, each bearing conventional names and designations established by the International Telecommunication Union (ITU). These designations provide a standardized framework for allocating and managing these crucial frequencies.

Frequency Range	Wavelength Range	ITU Designation	IEEE Bands	Full Name	Abbreviation
Below 3 Hz	>10⁵ km	—	—	—	—
3–30 Hz	10⁵–10⁴ km	Extremely low frequency	ELF	—	—
30–300 Hz	10⁴–10³ km	Super low frequency	SLF	—	—
300–3000 Hz	10³–100 km	Ultra low frequency	ULF	—	—
3–30 kHz	100–10 km	Very low frequency	VLF	—	—
30–300 kHz	10–1 km	Low frequency	LF	—	—
300 kHz – 3 MHz	1 km – 100 m	Medium frequency	MF	—	—
3–30 MHz	100–10 m	High frequency	HF	HF	HF
30–300 MHz	10–1 m	Very high frequency	VHF	VHF	VHF
300 MHz – 3 GHz	1 m – 100 mm	Ultra high frequency	UHF, L, S	Ultra high frequency	UHF
3–30 GHz	100–10 mm	Super high frequency	S, C, X, Ku, K, Ka	Super high frequency	SHF
30–300 GHz	10–1 mm	Extremely high frequency	Ka, V, W, mm	Extremely high frequency	EHF
300 GHz – 3 THz	1 mm – 0.1 mm	Tremendously high frequency	—	Tremendously high frequency	THF

The convention dictates that frequencies at or above 1 GHz are generally categorized as microwaves, while those starting at 30 GHz and extending upward are referred to as millimeter waves. Beyond these ITU designations, more granular classifications exist, such as the IEEE letter-band frequency designations and the EU/NATO designations, providing further detail for specific applications and engineering contexts.

Applications

The utility of radio frequencies is vast and touches nearly every facet of modern life. Its applications span from the ubiquitous broadcasting of entertainment and news, through essential voice and data communication systems, to sophisticated technologies like radar for detection and navigation, precise radiolocation, and even vital medical treatments and remote control systems.

Measurement

The apparatus required for measuring radio frequencies varies significantly with the frequency itself. At the lower end of the RF spectrum, standard laboratory instruments may suffice. However, as frequencies climb, the test equipment becomes increasingly specialized and sophisticated, demanding custom-designed solutions to accurately capture and analyze these high-frequency phenomena.

Mechanical Oscillations

While the term "RF" is most commonly associated with electrical oscillations, it's important to acknowledge that mechanical systems can also operate at these high frequencies. Examples include mechanical filters, which use resonant mechanical elements to select specific frequencies, and RF MEMS (Micro-Electro-Mechanical Systems) devices that leverage mechanical principles for radio frequency applications.