The signal medium for communication between radios s is radio waves, which belong to the electromagnetic spectrum. 

Radio waves are a type of electromagnetic wave, specifically a form of electromagnetic radiation with wavelengths longer than infrared light within the electromagnetic spectrum.

Radio wave frequencies range from 30 Hz to 300 GHz, and wavelengths range from 1 mm to 10,000 km.

Radio waves propagate at the speed of light. 

C (speed of light) = λ (wavelength)*f (frequency)

3 * 10^8 m/s = 0.85 m * 350 MHz (350 * 10^6)

Electromagnetic Spectrum

2.1. Direct transmission

The receiving antenna can directly see the transmitting antenna, and the electromagnetic waves from the transmitting end propagate directly to the receiving end.

When an electromagnetic wave is emitted and shines on a flat object with a wavelength longer than the carrier wave, the reflected electromagnetic wave is then received by a receiving antenna.

2.3. Refraction

Refraction is the phenomenon where an electromagnetic wave changes its direction of propagation when it enters another medium (forming a certain angle with the original direction, but not returning to the original medium).

2.4. Transmission

The emitted electromagnetic waves penetrate directly through the object and propagate in the air behind the object.

2.5. Diffraction (or diffraction)

 Diffraction is a fundamental characteristic of wave propagation. It occurs when a wave encounters an obstacle or passes through an opening, causing a change in the direction of its propagation. The principle of radio wave diffraction is similar to that of light diffraction, both based on wave theory. During propagation, radio waves are interfered with by obstacles such as terrain, buildings, and vegetation, causing the wavefront to bend and thus bypass the obstacle.

Diffraction is ubiquitous: Radio wave diffraction is prevalent in nature; almost all radio waves undergo diffraction during propagation. This is because radio waves possess wave properties and exhibit the basic characteristics of diffraction.

The degree of diffraction is related to wavelength: The degree of diffraction of radio waves is related to their wavelength. The longer the wavelength, the more pronounced the diffraction. This is why low-frequency radio waves penetrate obstacles more easily.

The degree of diffraction is also related to the size of the obstacle: When the size of the obstacle is much larger than the wavelength, the diffraction is very significant. When the size of the obstacle is smaller than the wavelength, the diffraction is relatively weak.

The degree of diffraction is related to the shape of the wavefront: the shape of the wavefront also affects the degree of diffraction. When the wavefront is a plane wave, the diffraction phenomenon is strongest. When the wavefront is a spherical wave, the diffraction phenomenon is relatively weak.

2.6. Scattering

When emitted electromagnetic waves strike an object with a wavelength smaller than the carrier wave, they are reflected into multiple, weaker electromagnetic waves that then propagate to the receiving antenna.

When inhomogeneous clumps appear in the atmosphere or ionosphere, radio waves may be reflected in all directions by these inhomogeneous media, allowing some of the energy to reach the receiving point; this is called scattered waves.