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Electricity can be static, like the energy that can make your hair stand on end. Magnetism can also be static, as it is in a refrigerator magnet. A changing magnetic field will induce a changing electric field and vice-versa—the two are linked. These changing fields form electromagnetic waves.

Electromagnetic waves differ from mechanical waves in that they do not require a medium to propagate. This means that electromagnetic waves can travel not only through air and solid materials, but also through the vacuum of space. In the 's and 's, a Scottish scientist named James Clerk Maxwell developed a scientific theory to explain electromagnetic waves.

He noticed that electrical fields and magnetic fields can couple together to form electromagnetic waves. He summarized this relationship between electricity and magnetism into what are now referred to as "Maxwell's Equations. Heinrich Hertz, a German physicist, applied Maxwell's theories to the production and reception of radio waves. The unit of frequency of a radio wave -- one cycle per second -- is named the hertz, in honor of Heinrich Hertz.

His experiment with radio waves solved two problems. First, he had demonstrated in the concrete, what Maxwell had only theorized — that the velocity of radio waves was equal to the velocity of light!

Radio Waves

This proved that radio waves were a form of light! Second, Hertz found out how to make the electric and magnetic fields detach themselves from wires and go free as Maxwell's waves — electromagnetic waves. Light is made of discrete packets of energy called photons.

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Photons carry momentum, have no mass, and travel at the speed of light. All light has both particle-like and wave-like properties. How an instrument is designed to sense the light influences which of these properties are observed. An instrument that diffracts light into a spectrum for analysis is an example of observing the wave-like property of light. The particle-like nature of light is observed by detectors used in digital cameras—individual photons liberate electrons that are used for the detection and storage of the image data.

One of the physical properties of light is that it can be polarized. Polarization is a measurement of the electromagnetic field's alignment. In the figure above, the electric field in red is vertically polarized. Think of a throwing a Frisbee at a picket fence. In one orientation it will pass through, in another it will be rejected. This is similar to how sunglasses are able to eliminate glare by absorbing the polarized portion of the light.

The terms light, electromagnetic waves, and radiation all refer to the same physical phenomenon: electromagnetic energy.

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This energy can be described by frequency, wavelength, or energy. All three are related mathematically such that if you know one, you can calculate the other two.


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Radio and microwaves are usually described in terms of frequency Hertz , infrared and visible light in terms of wavelength meters , and x-rays and gamma rays in terms of energy electron volts. This is a scientific convention that allows the convenient use of units that have numbers that are neither too large nor too small. The number of crests that pass a given point within one second is described as the frequency of the wave.


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  • One wave—or cycle—per second is called a Hertz Hz , after Heinrich Hertz who established the existence of radio waves. A wave with two cycles that pass a point in one second has a frequency of 2 Hz.

    How does radio path loss affect systems

    Electromagnetic waves have crests and troughs similar to those of ocean waves. The distance between crests is the wavelength. The shortest wavelengths are just fractions of the size of an atom, while the longest wavelengths scientists currently study can be larger than the diameter of our planet!

    An electromagnetic wave can also be described in terms of its energy—in units of measure called electron volts eV. A changing electric field generates a magnetic field, and a changing magnetic field generates an electric field. This mutual regeneration is manifested as a distinct entity, namely, an electromagnetic wave. Once generated, this wave will travel outward from its source, careening day after day, at the speed of light, toward the depths of the unknown.

    Designing an entire RF communication system is not easy. Any time-varying signal in any circuit will generate EMR, and this includes digital signals. In most cases this EMR is simply noise.

    Tour of the Electromagnetic Spectrum

    In some cases it can actually interfere with other circuitry, in which case it becomes EMI electromagnetic interference. We see, then, that RF design is not about merely generating EMR; rather, RF design is the art and science of generating and manipulating and interpreting EMR in a way that allows you to reliably transfer meaningful information between two circuits that have no direct electrical connection.

    There are a few reasons:. EMR is a natural extension of the electrical signals used in wired circuits. Time-varying voltages and currents generate EMR whether you want them to or not, and furthermore, that EMR is a precise representation of the AC components of the original signal. Imagine that a room contains two separate devices.

    The transmitter device heats up the room to a certain temperature based on the message it wants to send, and the receiver device measures and interprets the ambient temperature. This is a sluggish, awkward system because the temperature of the room cannot precisely follow the variations of an intricate electrical signal. EMR, on the other hand, is highly responsive. Transmitted RF signals can faithfully reproduce even the complex, high-frequency waveforms used in state-of-the-art wireless systems.

    In AC-coupled systems, the rate at which data can be transferred depends on how quickly a signal can experience variations. In other words, a signal must be doing something —such as increasing and decreasing in amplitude—in order to convey information. It turns out that EMR is a practical communication medium even at very high frequencies, which means that RF systems can achieve extremely high rates of data transfer.

    The pursuit of wireless communication is closely linked to the pursuit of long-distance communication; if the transmitter and receiver are in close proximity, it is often simpler and more cost-effective to use wires. Though the strength of an RF signal decreases according to the inverse-square law, EMR—in conjunction with modulation techniques and sophisticated receiver circuitry—still has a remarkable ability to transfer usable signals over long distances. The only wireless communication medium that can compete with EMR is light; this is perhaps not too surprising, since light is actually very-high-frequency EMR.

    But the nature of optical transmission highlights what is perhaps the definitive advantage offered by RF communication: a clear line of sight is not required. Our world is filled with solid objects that block light—even very powerful light. We have all experienced the intense brightness of the summer sun, yet that intensity is greatly reduced by nothing more than a thin piece of fabric.

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    In contrast, the lower-frequency EMR used in RF systems passes through walls, plastic enclosures, clouds, and—though it may seem a bit strange—every cell in the human body. But compared to light, lower-frequency EMR goes just about anywhere. Load More Articles.