[1] Electromagnetic radio waves (formerly called Hertzian waves) are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in the electromagnetic spectrum, often frequencies below 300 GHz and wavelengths above 1 millimeter, about a rice grain's diameter. Those of ~1 GHz and wavelengths shorter than 30 cm are called microwaves.
[1] Fig. 1 Animation of a half-wave dipole antenna radiating radio waves, showing the electric field lines. The antenna in the center is two vertical metal rods connected to a radio transmitter (not shown). The transmitter applies an alternating electric current to the rods, which charges them alternately positive and negative. Loops of electric field leave the antenna and travel away at the speed of light; these are the radio waves. In this animation the action is shown slowed down tremendously.
Radio waves are electromagnetic for being combined of electric and magnetic fields oscillating together through space.
The name "radio" waves refers for its usage in radio communication and broadcasting, having the lowest frequencies and longest wavelengths in the electromagnetic spectrum.
[1] Like all electromagnetic waves, radio waves in a vacuum travel at light speed and in the Earth's atmosphere at slightly lower speeds. Radio waves are generated by charged particles undergoing acceleration, such as time-varying electric currents. Naturally occurring radio waves are emitted by lightning and astronomical objects, and are part of the blackbody radiation emitted by all warm objects.
Notice how in the GIF above, at a certain position of the north and south pole, the center vectors point in a certain direction, in this case it's inward, as they propagate.
[1] Fig. 2
...Before they point outward when the poles switch positions as they propagate.
[1] Fig. 3
Radio waves are used for travelling at light speed, move over long distances, and go through solid walls and air without being blocked, making them great to send invisible messages for phones, Wi-Fi, radio, and television.
Radio waves are thus essential in telecommunication, where high frequency (HF) radio wave
Radio waves consist of 2 intertwined, coupled, oscillating energy fields traveling together through space at light speed, the oscillating electric E and magnetic B fields.
[6] The electric and magnetic fields are present even if with constant magnitude voltage/current. But the fields won't propagate. For a wave to propagate out into the universe, we must alter the voltage and current.
An electric field (E-field) is created by differences in voltage or electric charge. E-field vectors oscillate back and forth in a specific plane perpendicular (in space quadrature) to the direction the wave is moving. The E-field indicates the antenna's polarity
An magnetic Field (B) is created by moving electric charges (electric currents). B-field vectors oscillate perpendicular to both the electric field and the propagation direction.
[6] Fig. 5 Electromagnetic waves' electric and magnetic components are represented as perpendicular sinusoids.
[2] Fig. 7 The E and B-field vector lines in space quadrature to one another, representing the peak of their respective waves.
[8] Fig. 8 An electromagnetic wave oscillating, with its E and B vectors.
Thought they're perpendicular to one another, they're in time-phase, i.e., they occur simultaneously.
[2] So radio waves' E and B vectors are in space quadrature and in time-phase.
[3] The oscillation depends on the received radio wave's frequency and corresponds to a dipole antenna's resonant frequency (the radi owave at which an antenna naturally vibrates via the max physical efficiency)
[2] To obtain a max signal, both the transmitting and receiving signals must be in the E-field plane. The direction the fields propagate is determined by the cork screw rule.
This is applied by choosing any pair of the vectors where a cork screw is imagined at the center
[2] Fig. 9 The E and B-field vector lines in space quadrature to one another, representing the peak of their respective waves.
[1] A radio band is a small frequency band (a contiguous section of the range of the radio spectrum) where channels are often used or set aside for same purposes. To prevent interference and allow for efficient use of the radio spectrum, similar services are allocated in bands. E.g., broadcasting, mobile radio, or navigation devices, will be allocated in non-overlapping ranges of frequencies.
Lower-frequency radio bands span from 3 Hz to 3 MHz, covering Extremely Low Frequency (ELF) up to Medium Frequency (MF), which feature very long wavelengths (100,000 km down to 100 m), low signal attenuation, and a high ability to bend around obstacles or penetrate water and earth.
Medium frequency (MF) and HF (high frequency) are frequencies sitting in the middle of the radio frequency spectrum. MF spans 300 kHz to 3 MHz and HF spans 3-30 MHz.
VHF (very high frequency), UHF (ultra high frequency), and SHF (super high frequency) are higher categories in the radio frequency spectrum. Unlike lower frequencies that bounce off the ionosphere, these waves can travel in straight lines without being reflected by the ionosphere, through space where are they received and retransmitted elsewhere to the globe, why they are essential for satellite and modern line-of-sight communications.
[1.1] High frequency (HF) is the ITU designation for radio waves band of 3-30 MHz, i.e., they occupy the electromagnetic spectrum at 3-30 MHz. They are known for long-distance, intercontinental communication, due to the signals bouncing off the Earth's ionosphere layer in the skywave propagation process.
The ionosphere is a region of Earth's upper atmosphere, sitting at 48-965 km above sea level, where it's ionized by the sunrise, i.e., high solar radiation knocks electrons free from atoms here, creating a sea of charged particles reflecting radio waves and creates auroras.
The ionosphere acts as a giant mirror for radio waves. It bends and reflects signals back to Earth, allowing radio messages to travel past horizons and around the globe.
[5] Fig. 3: Radio-wave transmission reaching beyond line of sight by means of the sky wave reflect
An antenna uses radio waves as a bridge between electricity and space. Pushing an electric current through a metal wire makes electrons wiggle back and forth, a movement creating invisible energy ripples that are the radio waves. When they hit another metal antenna, they push its electrons back and forth to recreate the original electrical signal.
[3] Antennas are designed according to the purpose for which they are meant. Radio telescopes are huge arrays of antennas for receiving radio signals from extremely high distances in the cosmos but ordinary radio communication like smart phones don't need such huge arrays, which use tiny antennas.
[3] Fig. 4 A parabolic antenna.
[7] Fig. 7 Two older mobile phones with antennas.
Older mobile phones used small antennas which could even be extended for better reception. But now with wider availability of cellular coverage with numerous relay towers small antennas within the phone usually printed on the circuit board are enough. Still those who have traveled to remote areas would have faced the problems of signal loss in cellphones, that is because of scarcity of relay towers in those less populated regions."
[3] A dipole antenna is one of the simplest antennas that can convert electric signals/currents into into radio waves (Fig. 4), which are just 2 metal rods ¼ of the resonant frequency's wavelength (½ wavelength total) attached to a radio through transmission line.
The rods apply an AC current to oscillate the magnetic field to continuously switch the north and south poles back and forth thus charging its ends, while receiving radio signals.
[3] Fig. 4 A dipole antenna applying AC current to oscillates its magnetic field back and forth.
[3] A device like a radio that receives the radio wave extracts the audio signals from it and presents through the data through its speaker.
[1.2] Fig. 6 A parabolic antenna
[3] Satellite communication need better antennas like the parabolic antenna (Fig. 6. Simpler antennas can be used if the satellites are located in the low earth orbits like the International Space Station but the communication is possible only for a short period like 10 min during which the satellite is above the horizon.
[1] Wikipedia
[1.1] High frequency
[1.2] Parabolic antenna
[1.3]
[3.1]
[4] Why do electromagnetic fields (like radio waves) travel so much further than magnetic or electric fields alone? (+ are there two types of "photons"?) - T12J7M6 (AskPhysics; Reddit) - Caution: This page contains inaccurate infos
[5] Britannica