Ⅰ. What are electromagnetic waves?

People nowadays are familiar with electromagnetic waves, however, the term "electromagnetic wave" was not coined until 150 years ago, despite the fact that electromagnetic waves actually exist. Maxwell (link) develops Maxwell's equations by combining the results of earlier experiments and theoretically predicting the existence of electromagnetic waves, of which light is one. Hertz's experiment demonstrated the existence of electromagnetic waves more than 120 years ago. Only then did humanity realize that this phenomenon exists and that its propagation speed is equal to that of light. Marconi created wireless telegraph communication across the Atlantic by loading the telegraph signal onto electromagnetic waves. Who'd have guessed that electromagnetic waves had found their way into every area of our lives? Wifi, 5G, drones, unmanned vehicles, satellite communications, and other forms of electromagnetic wave transmission are all dependent on the carrier of electromagnetic waves.

So, what are electromagnetic waves and how do they work? Electromagnetic waves are a type of electromagnetic energy. As indicated in the diagram below, the good brothers of electric and magnetic field energy are not separated from one other, are perpendicular to each other, and travel at the speed of light. What's more intriguing is the fact that vacuum is the finest carrier for electromagnetic wave transmission. Even the air will have an impact. When it comes into contact with metal, it reflects, and when it comes into contact with a medium, it attenuates. It can be considered the most perfect signal carrier because it can be carried without restriction in open space, as well as through metal transmission lines and controlled freely. The wireless world arose as a result of this.

Electromagnetic waves have three distinct characteristics: wavelength, frequency, and wave speed. The wavelength is equal to the wave speed divided by the frequency, and these three characteristics are interrelated.

To put it another way, when the wavelength is known, the frequency is known; when the frequency is known, the wavelength is likewise known. However, keep in mind that this represents the wavelength and wave speed of open space. The medium has changed yet again.

Electromagnetic wave speed: Electromagnetic waves travel at the speed of light in open space. The speed of light is commonly assumed to be 300,000,000 meters per second in general calculations, however, a more exact number is 299,792,500 meters per second. It is fast enough, and it is the fastest speed that humans can achieve, yet it still takes time to go a certain distance. With modern radio technology, the time it takes for a signal to travel a specific distance must be taken into account. To measure the distance to a target, radar, for example, employs the fact that a signal takes a certain amount of time to travel. Other applications (such as mobile phones) must account for the time it takes for a signal to propagate in order to ensure that the system's key timing is not disrupted and that the signals do not overlap.

Electromagnetic wave wavelength:

As indicated in the diagram, this is the distance between a specific position in one cycle and the same point in the next cycle. Peaks are the easiest locations to choose because they are the easiest to find. The wavelength was used to determine the placement of a signal on a device's dial in the early days of radio or wireless. It is still a vital characteristic of any radio signal or electromagnetic wave, even though it is no longer employed for this function. Because frequency gives a more precise and convenient means to determine the properties of a signal, its position on the dial of a radio equipment or its position within the radio spectrum is now determined by its frequency.

Electromagnetic wave frequency

This is the number of times a specific spot on the wave rises and falls in a given amount of time (usually one second). The frequency unit is Hz, which stands for one cycle per second. Hertz, the German physicist who discovered radio waves, is honored with this unit. Radio frequencies are typically extremely high. As a result, the prefixes kilo, mega, and giga are frequently used.

1 kHz is 1000 Hz

1 MHz is one million hertz

1 GHz is one hundred million hertz or 1000 MHz.

The frequency unit was not designated at first, and the number of cycles per second (c/s) was used instead. Some older texts may list these units and their prefixes, such as kc/s for lower frequencies and Mc/s for higher frequencies.

Ⅱ. Radio spectrum

The frequency range of electromagnetic waves is extraordinarily broad, ranging from ultra-long waves of a few Hz to visible light to incredibly short rays (link). In fact, it's surprising that individuals can only distinguish visible light out of such a large electromagnetic wave spectrum. Can anyone perceive electromagnetic waves outside of the visible light spectrum since the human visual organ was created for them? Is it possible that other animals have a broader visual range than humans?

Different frequencies of electromagnetic waves correlate to different wavelengths, and the wavelength of this electromagnetic wave is the source of human awareness, but who determines that the frequency and wavelength of electromagnetic waves are equal? As a result, we can only discuss wavelength or frequency. The best radio frequency spectrum currently used by humans is the part below, which runs from very low frequency (VLF) to extremely high frequency (EHF), with wavelengths ranging from ultra-long waves to centimeter waves to millimeter waves. Will terahertz, on the other hand, be used in radio?

Ⅲ. Polarization of electromagnetic waves

Polarization is another significant characteristic of electromagnetic waves. So, what does it mean to be polarized? In electromagnetic waves, it is the change route of the greatest electric field. The propagation of electromagnetic waves is greatly influenced by the polarization of electromagnetic waves. Different polarization modes of electromagnetic waves can be used in various wireless applications, such as satellite communications. Circularly polarized waves are employed more frequently. 45. ° linearly polarized waves are the most widely employed in mobile communications. The polarization of electromagnetic waves is critical not just for sending and receiving antennas, but also for signal transmission.

Electromagnetic waves with polarization

Circular and linear polarization are the most prevalent types of electromagnetic wave polarization, while elliptical polarization is the most common.

Circular polarization denotes that the maximum value of the electromagnetic wave electric field changes in the shape of a circle, i.e., the electric field spins in circles and sings a song while transmitting the electromagnetic wave, as illustrated in the diagram below. Of course, because it rotates, there is a distinction between forward and reverse: right-handed circular polarization is right-handed circular polarization, and left-handed circular polarization is left-handed circular polarization.

As seen in the diagram below, a circularly polarized wave can be split into two orthogonal linearly polarized waves. Feeding the antenna with two orthogonal feeders is also a common approach to achieving a circularly polarized antenna. There must be a direction when it comes to linearly polarized waves. The horizontal plane is commonly used as a reference. Vertical linear polarization is defined as an electric field change that is perpendicular to the horizontal plane, whereas horizontal linear polarization is defined as a change that is parallel to the horizontal plane. The horizontal plane is also used in the most popular 45° antenna design.

As previously stated, the most frequent method of polarizing electromagnetic waves is elliptical polarization. Why? Refer to the circularly polarized wave composite diagram in the preceding image. The composite wave is circularly polarized when two orthogonal linearly polarized waves have the same amplitude. The composite wave is an elliptical pole whenever the amplitudes are uneven. Equality is difficult to achieve, and discrepancy is the norm.