Understanding RF - Radio frequency technologies

The electrical outlets in a home provide a voltage or current that rises and falls over time. The rate at which the current rises and falls is called the frequency. Frequency describes how many times per second the current goes through one complete cycle of rising and falling before returning to its original level. The number of cycles per second is measured in units of Hertz. For example, if in one second the current goes through four complete cycles, the frequency is four hertz. E.g. in the United States the frequency of the AC power in wall outlets is normally 60 Hz, meaning the current completes 60 cycles in 1 second.

AC generating electromagnetic fields

Any alternating current that moves through a conductor – such as a wire or an antenna – also generates an electromagnetic field that travels outwards into space. At very low frequencies, such as the 60 Hz AC in wall outlets, this electromagnetic field is not particularly strong and will not travel very far.

Three general uses for RF

There are three general uses for RF. The first category of RF applications involves heating objects. This 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.

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.

Two things happen 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. The opposite is true for higher frequencies. Broadcast AM radio uses frequencies in the 100s of kHz, and broadcast FM radio uses frequencies around 100 MHz because these, relatively low frequency signals, can travel many kilometers and can be received inside of houses or businesses.

Wi-Fi is at either 2.4 or 5 gigahertz, and these frequencies are 25 to 50 times higher than AM or FM. One of the reasons for using these much higher frequencies is that Wi-Fi signals do not need to be traveling too far: one access point is essentially noise or interference to everyone else’s access points. Wi-Fi signals don’t extend much beyond homes or businesses.

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, of 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.

RF safety

Radiated energy, such as X-rays, gamma rays, and UV or ultraviolet rays, are so-called ionizing radiation. Ionizing means that this energy can break apart atoms or molecules, including DNA, and can be a direct cause of certain forms of cancer and other health issues.

RF is non-ionizing radiation, which doesn’t break apart atoms or molecules. That doesn’t mean that RF is completely harmless. RF can create heating and at very high levels this can lead to tissue damage. However, these is no firm or conclusive evidence that RF poses a significant danger to living creatures at common power levels or under “normal” circumstances. High power transmitters do however require caution, and the regulatory and industry guidelines need to be followed when it comes to RF exposure.