Understanding Radio Frequency (RF) Theory and the possible applications as a result of a basic knowledge is truly an enjoyable and useful skill. Almost every electronic device in the average 21st century American household uses RF. Television remotes, AM/FM radios, Bluetooth, Wireless-Fidelity (Wi-Fi), and of course, Push To Talk Radios (PTT) all use principles studied by the genius minds of days long gone. In order to understand RF Theory, all that is required is a brief history lesson and a few long-winded explanations of terms.
RF Theory is rooted to Heinrich Rudolf Hertz’s “Theory of Electromagnetism”. Hertz’s work proved the idea that electromagnetic energy could be manipulated (referred to as ‘modulation’) and transmitted through open space. Hertz was such a pioneer in the field of wireless communications that the very way in which to measure the ‘Frequency’ of open-air electromagnetic signals is with “Hertz” or Hz. Radio Frequencies on the Electromagnetic Spectrum are measured from 30 kilohertz (kHz) to 300 Gigahertz (gHz).
Hz is the measurement used to describe how many times a Radio Wave changes direction in 1 second. The moment the wave changes directions, until the next change of direction, is one cycle. So, to say a frequency of 162 kHz is saying that the Wave changed directions 162,000 times in 1 second. Radio Waves, as the name implies, make a ‘wave’ pattern. Naturally, people decided to regulate and allocate certain frequencies of r certain purposes, these are called Frequency bands. Typically, numerically linear and can also be arbitrarily selected for allocation purposes such as Cell phone networks using the 1700mHz band for the phone to the tower, and the 2100 mHz band for tower to phone, named uplink and downlink, respectively.
Frequency band are allocated into what is called ‘bandwidth’. An simple way to understand this is to put the frequencies on a number line. The bandwidth is literally the numeric ‘width’ of the frequency band. So to say the bandwidth of the FRS band is 125kHz, is saying that the step to the next frequency channel is plus or minus 125 kHz. Channel is just another name for a designated frequency. Instead of saying, frequency 462.5625 mHz, simply say, FRS Channel 1. Another reason for allocation of frequencies is wavelength.
Wavelength which is the physical distance between and two matching points between waves of the same signal. To demonstrate, make a wave pattern across a piece of paper. Mark a vertical line at the highest point of a wave. This point is known as the ‘crest’, the bottom most point is known as the ‘trough’. The distance between the two ‘crests’ is the wavelength. Wavelength is measured in meters(m). When an amateur radio operator or ‘HAM’ (named for their dashing good looks) says “I’ll be on the 2m band”, this means that the operator will be transmitting on a frequency that has a wavelength of 2m.
To review, Frequency is the number cycles within 1 second, Wavelength is the physical length between crests of the Radio Wave. It should be noted that the term ‘equilibrium’ is the imaginary straight line perfectly between the crest and the trough. Wavelength and Frequency are connected. The higher the frequency, the shorter the wavelength. The Lower the frequency, the longer the wavelength. But how is all of this done intentionally?
Modulation, in the technical sense can be very complex and quite confusing to the novice or even for many considered to be intermediately skilled radio operators. Modulated signals, regarding wireless communications is a Radio Wave that contains information. To perform modulation, a transceiver must create what is known as carrier wave. Think of this as an ‘outer shell’, protecting the inner signal that contains the information from interference and maintaining direction. “Please keep hands and feet, inside the ride at all times”. Two types of analog modulation are Amplitude Modulation (AM) and Frequency modulation (FM). A simple, yet comprehensive look at the two is needed. Digital methods of modulation are used, however, do assist in explaining basic RF Theory.
AM and FM are the same AM/FM known to anyone familiar with a car radio. Amplitude, referring to the height from the equilibrium to the crest or trough of the radio wave is the key feature of AM. AM is could be described as, the carrier wave itself is changing amplitude as the signal travels. Frequency modulation is much more easily visualized. It is more of beam with a constant amplitude and slight change in wavelength and frequency during travel. AM channels are more affected by various environmental conditions to include Radio Wave Propagation than FM channels. Perhaps this why the music channels are broadcast on FM and politics are discussed on AM.
Radio Wave Propagation could be broadly categorized into either reflection, refraction, or diffraction. Reflection is easily visualized by taking a flashlight and pointing it into a mirror, notice that the light now changes directions and continues onto the wall. This can be repeated with multiple mirrors and the correct angle, though… not recommended unless loss of sight is a goal. Refraction occurs when waves pass through condensed matter such as glass and changes direction. A low watt laser pointer can be used through glass and will have a slight deviation from the original path. Refraction leads to ‘multi-path’ as can be observed with the laser that somewhere close to 180 degrees of the original path through the window, a faint yet visible “reflection” can be observed. This means the light both reflected back and continued through the window. Diffraction is when the wave simply goes around matter, such as corners of structures to give an easy example. An incandescent flashlight is best suited for this experiment, notice the ambient light around the corner that is not in line with the equilibrium of the bulb. Voila! Diffraction in real life! The bulbs from the lights and lasers in radio terms, are called antennas.
Antennas are made of conductive material, used to transmit and receive signals. Several types of antennas are commonly used around the world. Directional and Omni-directional are easily described types. Directional only transmit in almost one focused direction, as where omni-directional transmit almost equal in all directions. Almost because there is what is described as a ‘bleed off’ of electromagnetic radiation. Signal meters can put this into visuals and signal bleed off can be seen coming from the “sides” of the antenna and even small amounts to the rear of say, a Yagi-Uda directional dipole antenna.
Considering the knowledge that has been transmitted thus far, let’s apply the fundamentals. Radios can be either receivers or transceivers. Meaning they either only Receive (referred to as ‘Rx’) signals and amplify the signal into a human audible format or can both Transmit (referred to as ‘Tx’) and receive radio frequencies. Tx modulates the signal and Rx de-modulates the signal. Code to decode, cypher to decipher are other ways of thinking about the process. Next time the car is on, change from FM to AM and hear the difference in clarity, sound, and loudness. The difference is a result of the modulation method. Next time the music turns to static, it is better understood what is happening to that signal due to environmental propagation.