Observations/Radio wave propagation

Radio wave propagation

Radio Wave Propagation deals with the behavior of radio waves when the waves travel. It is sometimes known as simply wave propagation or propagation of electromagnetic waves. Radio wave transmission is a quite prominent method by which information can be transferred from one end to another.

Radio wave propagation is used in radio communication in order to transmit signals to short or long ranges. Along with this, it also finds applications in radar, direction finding, remote machine controlling, etc.

RF Propagation Properties

The behavior of RF signals during propagation is influenced by several key properties, each of which contributes to the complexity of wireless communication systems:

• Attenuation: As RF signals travel through space, they experience attenuation, which is the reduction in signal strength due to absorption, scattering, and divergence. Higher frequencies generally suffer more attenuation than lower ones, making them suitable for short-range communication.
• Fading occurs when there are variations in signal strength at the receiver due to constructive and destructive interference of multiple signal paths. This phenomenon can be caused by changes in the environment, such as moving obstacles or changes in atmospheric conditions.
• Refraction: When RF signals encounter a change in the density of the medium they are traveling through, such as transitioning from air to water or from one layer of the atmosphere to another, refraction can occur. This bending of the signal path can lead to signal coverage beyond the line of sight.
• Diffraction: As RF signals encounter obstacles like buildings, hills, or other obstructions, they can diffract or bend around these obstacles. This phenomenon enables communication in non-line-of-sight scenarios.
• Scattering refers to the redirection of RF energy in various directions when it interacts with irregular surfaces or small objects. Scattering can contribute to multipath propagation and may cause signal degradation in urban environments.

Methods of Radio wave propagation

Radio wave propagation is associated with the phenomena that occur when a wave travels between transmitter and receiver. However, the wave can travel between transmitter and receiver in two ways:

• By propagating in free space
• By guided within a medium such as coaxial cable or waveguide.

When the wave is allowed to propagate through free space, then during propagation, the available spectrum must be shared amongst the existing users in a proper manner. This includes providing services to the devices which are in close geographical proximity to each other. In this case, there are high chances of interference between the devices operating at nearly the same frequency. The environment through which the radio waves propagate includes discontinuities like hurdles or variations in medium parameters. For practical radio wave propagation, the earth and the objects present in its surroundings are highly considerable.

When propagation takes place through a guided media then the signal is subjected to less attenuation with other users. Also, in this case, the user gets access to the full range of frequency band. When a wave propagates through the earth environment then the manner of wave propagation does not depend only on the properties of waves but also on the environmental conditions. The effect of the medium in which the waves propagate is of great importance as it accounts for the intermediary objects in the path.

Types of Propagation

Free Space Propagation

In an ideal, open space environment, electromagnetic waves propagate in a straight line from the transmitter to the receiver. This type of propagation is characterized by minimal obstacles or interference and is commonly used for long-distance communication, such as satellite communication. In free space, these waves adhere to the inverse-square law, which dictates that the power density exhibited by an electromagnetic wave correlates directly with the inverse of the square of the distance from the originating point source. In simpler terms, when the distance between a transmitter and a receiver is doubled, the resulting effect is a reduction in the power density of the radiated wave at the new location to just one-quarter of its former value.

Ground Wave Propagation Diagram

Ground Wave Propagation

Definition: It is also known as surface wave propagation in which the radio waves transmit by passing through the semi-conductive surface of the earth. Here the transmission of waves takes place at a region close to the surface of the earth traveling beyond the horizon. This mode of propagation requires vertically polarized waves as the horizontally polarized waves in this case will get absorbed by the earth.

A major disadvantage associated with ground wave propagation is that it is suitable only for short-range operation. This is so because the induced wave in ground wave propagation causes attenuation of the propagated signal. Therefore, in order to transmit the signal with the least attenuation, it is preferred that the signal is transmitted only to short ranges, in the case of ground wave propagation.

Sky Wave Propagation Diagram

Sky Wave Propagation

Definition: A type of radio wave communication in which the electromagnetic wave propagates due to the reflection mechanism of the ionospheric layer of the atmosphere is known as sky wave propagation. Due to propagation through the ionosphere, it is also known as ionospheric wave propagation.

The permissible frequency range in the case of sky wave propagation lies between 3 MHz to 30 MHz.

Basically the electromagnetic waves in the range of 3 to 30 MHz get reflected by the ionosphere. However, the signals with frequency beyond 30 MHz despite undergoing reflection get penetrated. So, due to this reason, sky wave propagation is suitable only for this particular range of frequency.

Tropospheric Scatter Propagation Diagram

Tropospheric Scatter Propagation

It is sometimes called forward scatter propagation or scatter propagation and is suitable for VHF, UHF and microwaves. In this, the waves propagate through forward scattering due to the irregularities of the troposphere. This propagation technique uses the properties of the troposphere. This mode offers reliable communication between 160 km to 1600 km (99 to 994 miles).

It is called so because in this case, the propagation occurs beyond the horizon i.e., the fine layers of the troposphere. This type of propagation sometimes leads to the production of unwanted noise or fading.

It is to be noted here that other than these four modes of propagation, propagation occurs through the inversion layer of the troposphere and this case is named duct propagation or super refraction.

Line-of-Sight (LOS) Propagation

LOS propagation occurs when the transmitter and receiver have a direct line of sight with minimal obstacles between them. This type of propagation is crucial for microwave and millimeter-wave communication, often used in point-to-point communication and cellular networks. This method of transmission finds application in medium-distance radio communication, such as cell phones, cordless phones, walkie-talkies, wireless networks, FM radio, television broadcasting, radar, and satellite communication (e.g., satellite television). The distance of line-of-sight transmission at ground level is limited by the visual horizon, which relies on the heights of both transmitting and receiving antennas. Notably, this propagation technique is exclusively viable for microwave frequencies and higher frequencies.

Multipath Propagation

In urban environments or areas with obstacles, RF signals can encounter reflections, diffractions, and scattering, leading to multipath propagation. This phenomenon can cause signal interference and fading, but it's also harnessed in technologies like MIMO (Multiple-Input Multiple-Output) to improve data rates and reliability.

Radio Frequency Bands

Understanding the intricacies of RF propagation is essential for designing robust and reliable wireless communication systems. Different types of propagation and their associated properties have profound effects on signal coverage, interference, and overall system performance. As technology continues to advance, engineers and researchers must continue to explore and refine our understanding of RF propagation to meet the ever-increasing demands of wireless communication.

Frequency, Wavelength, and Band Name of RF Signal and Associated Propagation Methods

Band	Frequency	Wavelength	Propagation Method
ELF	3–30 Hz	100,000-10,000 km 62,137-6,213 miles	Guided between the Earth and the ionosphere.
SLF	30–300 Hz	10,000-1,000 km 6,213-621 miles	Guided between the Earth and the ionosphere.
ULF	0.3–3 kHz	1,000-100 km 621-62 miles	Guided between the Earth and the ionosphere.
VLF	3–30 kHz	100-10 km 62-6.2 miles	Guided between the Earth and the ionosphere and ground wave propagation.
LF	30–300 kHz	10-1 km 6.2-0.6 miles	Guided between the Earth and the ionosphere and ground waves propagation.
MF	300–3000 kHz	1000-100 m 3,280-328 feet	Ground waves propagation and slight ionospheric refraction.
HF	3–30 MHz	100-10 m 328-32 feet	Ionospheric refraction
VHF	30–300 MHz	10-1 m 32-3.2 feet	Line-of-sight propagation.
UHF	300–3000 MHz	100-10 cm 39-3.9 inches	Line-of-sight propagation.
SHF	3–30 GHz	10-1 cm 3.9-0.39 inches	Line-of-sight propagation.
EHF	30–300 GHz	10-1 mm 0.39-0.039 inches	* Line-of-sight propagation, limited by atmospheric absorption to a few kilometers (miles).
THF	0.3–3 THz	1-0.1 mm 0.039-0.0039 inches	* Line-of-sight propagation, limited by atmospheric absorption to a few meters.

* limited by atmospheric absorption means the radio wave will not bounce off the atmos back down to the surface.

References

https://www.arrl.org/distance-records

ARRL RF Distance Records May 19, 2023