The Application of RF

Three general uses for RF

There are three general uses for RF. The first category of RF applications involves heating objects. This function  includes both microwave ovens as well as industrial and medical applications.

Another area of use for RF is sensing or detecting objects. Here, RF is transmitted, and the characteristics of the received RF provides information about the objects it encountered.

Most known is transferring information. Radio and television broadcasts were among the first uses of RF for transferring information in the form of sounds and images. The properties of RF enable modern data-transmission technologies such as Wi-Fi, cellular voice and data and Bluetooth. Also, the ability to transmit information through space is extremely important for satellite applications, including GPS.

                  

Using RF to heat objects

Microwave ovens use something called a “magnetron” to create RF at a frequency of about two and a half gigahertz. This is in the same frequency range commonly used by Wi-Fi and Bluetooth. The RF then penetrates food or liquids and cause the molecules, especially water, to vibrate which creates heat. When using RF to heat food in a microwave having metal objects in the oven should be avoided because the metal can turn the radiated RF produced by the magnetron back into conducted RF. The resulting currents in the metal can cause sparks or fires.

In addition to warming up leftovers, RF is also used for heating in industrial applications, such as pasteurizing milk, and is also found in some medical applications, ranging from destroying cancerous cells to various cosmetic treatments.

Sensing objects using RF

Radar is an example of how objects can be sensed using RF. There are various radar applications, such as detecting planes or ships, or measuring the speed of a vehicle or a baseball. Another example of using RF for sensing are the body scanners that have largely replaced metal detectors in airports. Some types of motion sensors in alarm or other systems also use RF. A less well-known use of RF for sensing is something called material measurements. RF allows to non-destructively determine certain properties of materials, such as checking tissue for the presence of breast cancer, or trees for the presence of rot and termites.

Transfer information using RF

The most common use of RF in the modern world is to transfer information without wires or “over the air”. In order to transfer information using RF, one or more properties of the generated electromagnetic field must be changed, and this process is called modulation.

The simplest way of changing something about the radiated field is just turning it on and off, and this is essentially how Morse Code works. Turning the RF on a short time for a dot and a longer time for a dash. The next step up from this “on-off” approach is amplitude modulation (AM), where the strength of the RF is changed to convey information. In frequency modulation (FM), the frequency of the radiated RF is changed depending on the information to be sent.

Both AM and FM are used primarily for analog modulation such as radio broadcasts. For sending digital information more complicated modulation schemes are needed, often changing both the amplitude and the phase, or frequency shift, of the RF at the same time.

                                                                            Amplitude modulation (AM)

                                                                            Frequency modulation (FM)

RF frequencies explained

The definition of RF covers a very wide range of frequencies, but the specific frequency used is largely based on the application—this selection also directly influences the design of supporting components like directional couplers and common mode choke inductors, which are critical for signal integrity and interference control.

Two key phenomena occur when frequency is lowered: First, the radiated fields propagate, or travel, longer distances; second, lower-frequency signals also tend to penetrate, or pass through, objects more easily. For such low-frequency scenarios (e.g., broadcast communications), common mode choke inductors are often integrated into transmission circuits to suppress unwanted common-mode noise—this is especially important because low-frequency signals are more susceptible to picking up interference from power lines or nearby electrical devices. The opposite is true for higher frequencies: they have shorter propagation distances and weaker penetration, but they support higher data rates, which demands more precise signal monitoring. Here, directional couplersplay a vital role—they tap off a small portion of the RF signal without disrupting the main transmission, allowing real-time measurement of signal strength, power, or reflection coefficients, which is essential for optimizing high-frequency system performance.

Broadcast AM radio uses frequencies in the hundreds of kHz, and broadcast FM radio uses frequencies around 100 MHz. These relatively low-frequency signals can travel many kilometers and be received inside houses or businesses, and the inclusion of common mode choke inductors in radio receivers helps filter out hum or static caused by common-mode interference, ensuring clearer audio output.

Wi-Fi operates at either 2.4 or 5 gigahertz—frequencies 25 to 50 times higher than AM or FM. One reason for using these higher frequencies is that Wi-Fi signals do not need to travel far (they rarely extend beyond homes or businesses), and their short range reduces mutual interference between adjacent access points. For Wi-Fi routers and adapters, directional couplers are commonly used in the RF front-end to monitor the output power of the transmitted signal; this allows the system to adjust power levels dynamically, avoiding signal distortion while complying with regulatory power limits. Additionally, common mode choke inductors are installed in the power supply or antenna feed lines of Wi-Fi devices to mitigate common-mode noise that could degrade data transmission speeds or cause connection drops.

In most parts of the world, the right to use a given frequency or range of frequencies is set by a government or regulatory agency. In the United States, this is the Federal Communications Commission (FCC). Acquiring the “license” to use certain frequencies often requires a fee, and the cost can be quite substantial—cellular network operators, for example, pay billions of dollars for the exclusive right to use certain frequencies. Even in these licensed high-frequency cellular systems, directional couplers are indispensable for network maintenance (e.g., troubleshooting signal loss in cell towers) and common mode choke inductors help reduce interference between the cellular network and other nearby RF systems, ensuring reliable communication for millions of users.