An Introduction to RF Theory, Practices, and Components

The Ins and Outs of RF

Image Source: spyrakot/stock.adobe.com; generated with AI

By JJ DeLisle for Mouser Electronics

Published October 31, 2025

A wide range of technologies used for wireless communication, power transfer, navigation, and sensing have become vital to virtually all human activities, from daily life to space exploration. Many of these devices and systems operate within the radio frequency (RF) spectrum, which ranges from approximately 3kHz up to the infrared light spectrum, near 30THz. RF technology is responsible for the vast number of communication services we use daily (and, in some cases, nearly constantly). It is responsible for the sensing and navigation technologies that keep planes in the air and ships on course; it is even behind the microwave energy generators that cook, sanitize, and seal much of the world’s food and personal products. Due to the diversity of applications and markets, RF technology knowledge is also very diverse. While this diversity has led to various naming conventions across the different applications, which can sometimes be confusing to those new to the field, this article will help decipher some of these colloquialisms.

This article explains high-level concepts associated with RF and provides a basic overview of the key principles of RF phenomena and technology. In that light, the article is meant to provide a primer on RF technology concepts to help initiate readers into the field or offer a shallow review for those familiar with the topic.

What Is RF?

Within the RF frequency range, there exists an extremely diverse range of natural and artificial electromagnetic radiation sources and systems. RF generators include celestial objects like the sun, which generates broad-spectrum radiation (RF and beyond), terrestrial sources such as lightning and atmospheric interactions with space weather, and the myriad human-made RF sources and systems.

Because RF technology originated in wireless communications, some schools of thought limit RF to relatively low-frequency wireless communications. In some cases, the term RF is used to describe all radiation that could be encompassed within the RF spectrum, whether made by humans or of natural origin. RF is generally considered to encompass a frequency range from roughly 3kHz to 300GHz, with some now considering it to extend into the terahertz (THz) range. The submillimeter-wave spectrum (above 300GHz to 30THz) is being actively studied for sensing and short-range, extremely high-data-rate communications.^[1]

Nomenclature for RF technologies and phenomena is often specific to the application, industry, or field of science that uses the concepts. For instance, one engineer may refer to the microwave spectrum as a subset of the RF spectrum, while another may use it to describe the shorter-wavelength and higher-frequency electromagnetic radiation beyond legacy RF communications frequencies.

The millimeter-wave (mmWave) spectrum—typically assigned the range of 30GHz to 300GHz but may be extended to 3THz in some contexts—has a higher frequency range than the microwave spectrum. In this case, "millimeter" is used to describe the range of wavelengths that are on the order of a millimeter to hundreds of millimeters. Terahertz, like mmWave, is a physically accurate descriptor that describes the submillimeter-wave frequencies beyond mmWave frequencies and below optical frequencies (which are in the tens of terahertz).

Many applications and industries have specific terms for regions of the RF spectrum and frequency bands, such as the radar frequency bands designated using letters, the International Telecommunication Union (ITU) spectrum designations, NATO radar bands, and cellular communications frequency ranges.

This article uses the term RF to describe the portions of the electromagnetic spectrum (Figure 1) used for wireless communication, data signals, power transfer, and RF-based sensing up to the lower regions of the infrared spectrum.

Figure 1: Electromagnetic frequency ranges, where radio and microwaves are designated as separate portions of the spectrum. (Source: VectorMine/stock.adobe.com)

RF Communications

RF communications encompasses the theory, techniques, and technologies that enable the transmission of information encoded as electromagnetic signals between RF communication devices or systems. The most common mediums for RF signal transfer are open-air channels, conductive paths in circuits, and transmission lines and waveguides.

RF communication relies on standardized frequency allocations, often subdivided into channels, with signals that may be modulated using analog or digital techniques. One of the main benefits of RF communications technology is that radio signals can propagate through free space across long distances to multiple receivers while conveying substantial amounts of information.

RF communications can be either point-to-point/peer-to-peer (P2P) or point-to-multipoint (P2MP or PMP), depending on the applications. They can even extend to vast mesh networks or interlinked radio communication networks, such as cellular communications and Wi-Fi. The common use of Bluetooth^® connectivity is an example of P2P RF communications, while Wi-Fi is generally used as a P2MP, with a wireless router broadcasting to multiple devices.

RF communications can support wide-area information broadcasts, such as AM/FM radio stations, or precise and narrow P2P links designed to provide an optimized communication channel between two points.

To convey information with RF signals, they must be modified so RF transmitter circuits can perform the signal manipulation designed to meet operation criteria for specific frequency ranges and channel dynamics (modulation) while also being able to be successfully demodulated by the intended receiving circuits. Because many of the information types transferred using RF communications do not operate at the same frequencies as a desirable RF communications medium, these electrical signals are often upconverted to the desired RF spectrum during transmission and downconverted on the receiving side. This concept is called superheterodyne, where the signal that contains original information (i.e., the low-frequency [LF] baseband signal) is modulated to an intermediate frequency (IF) that is then frequency translated (using a mixer and local oscillator) to a higher-frequency RF signal, which serves as the carrier for further processing. If one or more upconversion and downconversion stages are used, the frequency stages between the baseband and RF are known as IF stages, and the system is referred to as a multiple (i.e., dual, triple) conversion superheterodyne system. Common examples of superheterodyne RF communications are AM/FM, two-way, and citizens band (CB) radios, as well as many communication systems in the microwave spectrum and beyond.

For RF communications within the frequency and performance range of modern analog-to-digital converters (ADCs) and digital-to-analog converters (DACs), it is possible to digitally synthesize and sample RF signals directly. This approach is known as full-digital, fully digital, all-digital, or direct-digital RF communications, where upconverters and downconverters are not required to shift RF to IF for synthesis or sampling, and discrete analog modulators and demodulators are unnecessary. Instead, digital signal processing generates and conditions the communication signals with ADCs and DACs, directly converting between digital and RF signals.

The quality of the components and RF circuit design greatly impact the performance of RF communication systems. With both analog and RF systems, the frequency accuracy, noise, phase noise, and amplitude accuracy must meet certain thresholds for successful communication. In the case of digital communications, the digital data contained within the RF signals can be corrupted by the nonidealities of the RF circuit components, interconnect, and transmission medium. This is why many digital wireless communication systems use sophisticated signal processing techniques and error correction to ensure an optimized communication link.

RF Sensing and Radar

RF sensing is the use of RF spectrum radiation and the distinct interactions this electromagnetic radiation has with materials and interfaces. RF sensing can be either passive or active. With passive RF sensing, specialized RF circuits are designed to capture ambient or focused RF energy generated or reflected throughout an environment. These signals then stimulate transducers that are strategically located to capture the desired RF radiation. In some cases, arrays of transducers and signal-processing technology are used to create high-resolution images of incoming radiation or otherwise modify the transducer dynamics to enhance sensor directivity. Examples include radio telescopes that capture RF radiation from distant stars and celestial objects for astronomical observation and astrophysics research.

Active RF sensing uses generated RF radiation with specific signatures that are directed using probes, antennas, waveguides, or other RF transducers toward target objects. The reflected or penetrating signals from the RF radiation circuitry are then captured by either the same transducers sending the generated signals or dedicated receiving transducers. These captured signals are then analyzed to determine the characteristics of the materials, medium, or interface through which the signals passed or were reflected. With appropriate circuit design targeting specific RF signal parameters, a variety of material and interface properties can be extracted from analysis of the signals. Examples of active RF sensing technologies include radar, weather radar, RF atmospheric measurements, non-invasive medical analysis, and grain silo food quality analysis. These also encompass material characterization techniques such as microwave free-space characterization and split-post dielectric resonator measurements.

Radar is a very prevalent use of RF sensing techniques to determine the distance, velocity, shape, and structure of stationary and mobile objects, including environmental obstacles and terrain. With radar, a continuous, ramped, or pulsed RF signal is produced by the radar transmitter and directed to an RF transducer (antenna, aperture, or antenna array) that sends directed wavefronts (Figure 2). The resulting reflections from environmental features and targets are then captured and analyzed to extract radar target characteristics. Based on the radar signature of a target, it is even possible to identify characteristics, such as Doppler shifts and cross-sectional and reflection patterns. This use allows the differentiation of aircraft. Radar can also be used for topographical analysis and terrain mapping by aerial or orbiting platforms. Technologies such as synthetic aperture radar (SAR) can be used by satellites and high-altitude aircraft to pass over terrain and generate highly accurate topographical data. Other examples of common radar use are automobile safety systems that detect objects to the front, sides, and rear of a vehicle for automotive safety system use or even fully autonomous driving.

Figure 2: A legacy radar system with separate transmit and receive antennas. (Source: Koapan/stock.adobe.com)

RF Circuit Components and Interconnects

RF signal synthesis and generation, conditioning, modulation, and conversion are all performed by RF circuit components connected by RF interconnects. The following list provides the primary functions of RF components:

• Frequency translation
• Frequency and phase adjustments
• Amplitude adjustments
• Frequency selection
• Switching
• Signal directivity isolation
• Multiplexing (time, frequency, phase, directivity, etc.)
• Signal splitting or combining
• Generation and synthesis
• Capture and translation/conversion to other electrical signal types (e.g., analog, digital) or as energy

Active vs. Passive RF Components

There is an extensive diversity of RF circuit components and fabricating methods, which can lead to confusion regarding terms (Table 1). In some applications, RF circuit elements that require external power sources to perform their functions are referred to as active devices, but are commonly called devices. Circuit elements that perform their functions with only the signal energy that passes through them are known as passive components and are commonly referred to simply as components. In this case, RF circuit elements refer to the physical items that perform specific RF functions. Circuit elements can also refer to basic electronic components like resistors, capacitors, and inductors.

However, this is not always the case when discussing RF, as individual basic circuit elements are not frequently used on their own and are commonly referred to by their function in an RF circuit instead of their element type. These designations are not a convention and appear inconsistently throughout the literature. This is further confounded as several RF circuit elements can be realized as active or passive variants.

Moreover, there are also more complex arrangements of RF circuit elements and components that make up devices, modules, subsystems, and systems that have their own distinct designations or fit multiple designations. RF nomenclature for components is also inconsistent between markets, applications, and, in some cases, among manufacturers. Standards bodies and legacy designation methods are used for some types of RF components, such as RF coaxial connectors and cabling types still being labeled by legacy US Department of Defense standards.

Table 1. Basic RF Device & Component Types

RF Components: Fabrication and Nomenclature

Further confounding RF component nomenclature is that many passive RF components can be fabricated using purely physical structures in tandem using common RF circuit fabrication technologies, such as printed circuit boards (PCBs), low-temperature co-fired ceramic (LTCC) and high-temperature co-fired ceramic (HTCC), semiconductor processes, 3D printing (e.g., ceramics, dielectrics, conductive materials, composite materials), custom laminates, and even proprietary microfabrication methods. For example, an RF circuit may include filters, delay lines (phase delays), power dividers and combiners, transformers, transmission lines, waveguides, and couplers—all fabricated on single or multiple layers of PCB laminates using stacked conductor and dielectric layers.

RF Interconnects

RF interconnects, such as transmission lines and waveguides, are too often considered separately from RF components, as they are the medium that carries the RF energy between RF components and are not generally used to perform RF circuit functions. However, in physical RF systems, the RF interconnect exhibits behavior that significantly affects the RF signals and the energy that passes through them, potentially allowing external interference to enter the signal paths. The impact of RF interconnects is also a function of frequency, and signal integrity concerns at higher frequencies necessitate deep consideration of circuit interconnect dynamics.

RF technologies require substantial consideration of the error and variability in all components and interconnections through which the information and carrier signals pass. This is often referred to as a signal chain. Each component of a signal chain introduces some non-linearities, noise (amplitude inaccuracy and noise content), and phase noise (frequency inaccuracy and frequency content). Each component and interconnection point in a signal chain also allows noise and interference to be injected at that ingress point.

Where noise, interference, and distortion are injected within an RF signal chain dictates the significance of the nonidealities in the performance of the RF circuit. For instance, unwanted signals injected after a low-noise amplifier (LNA) do not get amplified as much as those injected before the LNA. They may have only a minor effect on the signal quality and the receiver circuitry processing the signal if the desired signal gain remains significantly stronger than the unwanted signals. However, if unwanted signals are injected before an LNA or power amplifier (PA), they will be amplified by the same gain factor as the desired signal. This can degrade RF signal processing and, in some cases, damage upstream or downstream components.

This potential degradation can be especially challenging with nonlinear components such as amplifiers and mixers, as unwanted signal content can be further unintentionally distorted or even mixed into sensitive frequency ranges that conflict with the desired performance of the RF circuit or nearby RF circuits. In communication systems with many closely spaced channels, intermodulation distortion is a common concern. With intermodulation distortion, two or more signals that enter a nonlinear circuit feature could mix, producing the sum and difference frequencies and a series of harmonics (Figure 3). If the signals are of high enough amplitude, the resulting distortions could be enough to desensitize a receiver or even overwhelm a frequency channel and prevent communications.

Figure 3: RF channel measurement showing the frequency spectrum of intermodulation distortion. (Source: RF Intermodulation at 280 MHz, © Nader Moussa, used under Creative Commons Attribution-ShareAlike 3.0 Unported)

RF Systems

There are many different aspects to RF systems; depending on the complexity, an RF system may include electronic power, analog processing circuits, digital control, thermal management, and many other electronic components. Many modern RF systems are now software-defined. This means that the level of digital signal generation, control, and synthesis is significant enough that radio and sensing technologies’ transmission and reception functions can be configured using software. Modern smartphone radios and advanced radar function in this way. For many RF systems, the distinct RF parts are contained within the front end, often referred to as the RF front end (RFFE). Of course, digital synthesis and conversion electronics are nominally RF components, but they are traditionally recognized in RF design as distinct and separate domains like baseband signal generation and processing (which was historically analog and now more commonly digital). However, there are many radio, radar, and other sensing technologies for high-performance applications where digital electronics are not yet capable enough to meet high-performance standards. Examples include some test and measurement applications, extreme frequency (i.e., upper microwave and mmWave), high-precision sensing, high power, latency-sensitive applications, and other applications where the flexibility enabled by digital electronics in RF systems does not outweigh the cost and increased complexity.

The frequency and power regimes that an RF system operates over largely dictate many aspects of the system. For instance, higher-frequency RF systems are often smaller in critical dimensions than lower-frequency RF systems due to the proportional relationship that transmission lines or waveguides and many RF circuit elements have with the wavelength of the frequency limits. For instance, both waveguides and transmission lines have upper-frequency cutoffs for the dominant or desired transmission or waveguide modes, usually based on the widest bandwidth or lowest loss modes. High-power RF systems are often larger to accommodate higher voltages, which could cause voltage breakdown if conductive or dielectric components are too close. These competing concepts create a challenging trade-off for reaching high power levels at higher frequency regimes, as high-frequency RF structures generally have smaller dimensions with less conductive material available for removing thermal energy. Moreover, most losses increase with frequency, meaning that more signal energy is converted to thermal energy at higher frequencies than at lower frequencies.

Another common RF system trade-off is between system bandwidth and noise. Wider-bandwidth RF systems also allow for greater noise levels and may be more prone to interference. Wider-bandwidth communications can allow for much greater data throughput, and wider-bandwidth radar can provide greater positional and velocity accuracy for a target, but it also makes the systems more vulnerable to intentional and unintentional interference. This is why high-reliability narrow-band RF systems are still in use when much of the non-critical RF technology is embracing wideband operation.

Conclusion

This article provides a high-level view of the main topics of RF technology and science. Beyond this material, a vast array of knowledge, both in breadth and depth, is used to design RF circuits that enable technologies from broadcast radio to astrophysics research.

Sources

[1]https://microdevices.jpl.nasa.gov/capabilities/submillimeter-devices/