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Sky Wave Propagation is a term used for the radio signals which pass through the space or sky. It is also known as radio wave propagation because it is neither refracted nor reflected back to the Earth from the ionosphere. Among all kinds of electromagnetic waves, radio waves are extensively used in day-to-day life. Radio wave propagation is the transmission of waves through different layers of the atmosphere. Sky wave propagation is a subcategory of radio wave propagation.
| Table of Contents |
Key Terms: Sky wave propagation, Microwave propagation, critical frequency, skip distance, Sky, Space, Radio wave, Earth, Electromagnetic wave, atmosphere
Sky Wave Propagation
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Sky Wave Propagation is the radio wave propagation that occurs within the troposphere of our Earth's atmosphere. These waves are directed and reflected from the troposphere to the Earth at an altitude of 20 kilometers.
Sky Wave Propagation
| Examples Related to Sky Wave Propagation In our everyday life, we experience various examples of sky wave propagation. Such examples include mobile communications and listening to music. Here are the examples related to Sky Wave Propagation:
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Components of Sky Wave Propagation
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The sky wave propagation depends on three significant components, which are
- Direct Waves - It is defined as the direct receiving of radio waves when transmitted from the transmission antenna.
- Ground Reflected Waves - This term is defined as transmitting radio waves to the receiving antenna after reflection from the ground.
- Tropospheric Waves - The successful reaching of radio waves to the receiving antenna. This process takes place after the reflection of radio waves from the troposphere.
| Frequently Asked Questions Question: What is the difference between microwave and sky wave propagation? Answer: The difference between microwave and sky wave propagation is that Sky wave has a higher wavelength and lower frequency. In contrast, microwaves have a lower wavelength and higher frequency. Question: What are the characteristics of sky wave propagation? Answer: Sky wave propagation is a type of radio wave propagation. It is either the reflected or refracted back waves to the earth from the ionosphere that is an electrically charged layer of the upper atmosphere. |
Difference between Space Wave and Sky Wave Propagation
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Space Wave and Sky Wave Propagations are the types of radio wave propagation. In sky wave propagation, electromagnetic waves are reflected from ionosphere in the atmosphere. Space wave propagation is transmitted from one antenna to another in the troposphere.
Here are other differences between Spacewave and Sky Wave Propagation:
| Space Wave Propagation | Sky Wave Propagation |
|---|---|
| Waves transmit from one antenna to another in the troposphere | Waves reflected by the ionosphere |
| A higher range of frequency, generally from 30 MHz | Frequency ranges from 3 MHz to 30 MHz |
| Examples: T.V and satellite communication | Examples: Amateur radio and S.W radio |
What is the Critical Frequency of a Sky Wave Propagation?
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The critical frequency is nothing but the maximum frequency where the total internal reflection occurs in the ionosphere. The equation can be expressed as:
| fc = 9√Nmax |
Where,
- fc is the critical frequency in Hz
- Nmax is the maximum electron density per meter-cube
Further, in relation to MUF, the critical frequency equation can be rewritten as:
| fc = MUF/ secϴ |
Where,
- MUF is the maximum usable frequency
- Fc is the critical frequency in Hz
- Ө is angle of incidence
Here, the critical frequency differs depending on the condition of the atmosphere, time of the day, and the angle of propagation of radio waves transmitted by the antenna.
Read more:
| Important Concepts Related to Sky Wave Propagation | ||
|---|---|---|
| Difference Between Analog and Digital Signal | What is internet: History of Internet | Pulse Amplitude Modulation |
| Space Travel Equipment | Difference Between AM and FM | Electronic Communication Systems: Important Terms |
What is Skip Distance?
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Skip distance is the minimal distance from the earth and the point of radio signal transmission. For flat earth surface, the skip distance can be defined as:
| \(D_{SKIP} = 2h \sqrt {(\frac{f_{MUF}}{f_C})^2 - 1}\) |
Where,
- Skip is the Skip distance
- h is the altitude at which the reflection occurs
- fMUH is the maximum usable frequency
- Fc is the critical frequency in Hz
How to prevent loss of signals or signal strength during line-of-sight propagation?
The line-of-sight propagation is not smooth and faces a series of obstacles resulting in the loss of signal strength. Therefore, we need to adjust the heights and distances of transmitting and receiving antennas from each other. In order to do that, the formula will help us tune the adjustments.
Where, DM is the distance between transmitting and receiving antennas.
- R is the radius of the Earth
- hT is the transmitting antenna height
- hR is the receiving antenna height
Applications of Sky Wave Propagation
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The sky wave propagation has broader applications in various sectors, which are as follows
- Line-of-Sight Communication
- Communication between Satellites and the Ground.
- Radar communication in defense and space sectors.
- Radio and Television broadcasting services.
- Linking of Microwaves in various communication sectors.
- Mobile Communication Services
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Things to Remember
- Sky wave is used for very high-frequency band transmission (within a frequency range of 30-40 MHz)
- At 30-40 MHz, the ground wave propagation fails to propagate.
- There are two types of sky wave propagation which are electromagnetic and microwave propagation.
- Sky wave has a higher wavelength compared to the microwave, which has a smaller wavelength.
- Sky wave has a lower transmission frequency than the microwave, which has a higher transmission frequency.
- Sky wave propagation has broader applications in radar and satellite communication, television broadcasting, line-of-sight communication, and mobile communications.
Sample Questions
Ques: Calculate the maximum distance up to which the signal transmitted from a tower with a height of 80m can be received? (2 Marks)
Ans: Given that, (altitude) h = 80 m, R = Radius of the earth = 6400 km
We know that, the maximum distance formula is
Therefore, the required distance will be 32km.
Ques: Name the type of communication system in which the signal is a discrete and binary coded version of a message or information? (1 Marks)
Ans: Digital Communication
Explanation: A digital communication system is one in which the signal is a discrete and binary-coded rendition of a message or information.
Ques: What is the important purpose behind modulating a signal during transmission? (1 Marks)
Ans : The lower frequency signals cannot be transmitted to longer distances. Modulation is done to increase the transmission frequency range.
Ques: What is the requirement for transmitting microwaves from one position to another position on the Earth? (1 Marks)
Ans : The transmission and receiving antenna must be in the same line-of-sight.
Ques: Why is ground wave propagation not suitable for high frequencies? (2 Marks)
Ans : Signals which have a frequency higher than 1500 KHs are absorbed mainly by the surface of the Earth. Therefore, it cannot be transmitted from the surface. This is the reason behind its non-usage for ground wave propagation.
Ques: A signal jumps from one level to the another level instantaneously. What will be its frequency? (2 Marks)
Ans : If the signal jumps from one level to another instantly, it denotes that the frequency of the particular signal is infinite.
Ques: Sky has no limit, but the sky wave propagation has a limit. Explain why? (2 Marks)
Ans : Sky wave propagation has a limit because it occurs due to the reflection of radio waves through the ionosphere. But, the ionosphere absorbs the high frequency and does not reflect them back.
Ques: What is the role of the atmospheric layer in communication? (2 Marks)
Ans : The top layer of our atmosphere is called the ionosphere. It lies at an altitude of 400 kilometers above the surface. It remains as a reflecting layer for high-frequency radio waves.
Ques: Which of the following frequencies will be suitable for beyond-the-horizon communication using sky waves? (2 Marks)
(A) 10 KHz,
(B) 10 MHz,
(C) 1 GHz, and
(D) 1000 GHz
Ans : Option (B) 10 MHz
Ques: Frequencies in the UHF range normally propagate by means of: (2 Marks)
(A) Ground Waves
(B) Sky waves
(C) Surface Waves
(D) Space Waves
Ans : (D) Space Waves
Explanation: An ultra-high frequency (UHF) wave cannot travel down the ground’s trajectory or be reflected by the ionosphere because of its high frequency. Line-of-sight communication is nothing but space wave propagation. It is used to transmit UHF Signals.
Previous Year Questions
- Sky wave propagation is used in… [VITEEE 2011]
- In amplitude modulation, the amplitude of carrier wave is… [VITEEE 2019]
- Amplitude modulation has… [KCET 2015]
- The attenuation in optical fibre is mainly due to… [VITEEE 2010]
- Sinusoidal carrier voltage of frequency 1.5MHz and amplitude… [VITEEE 2010]
- The frequency of an FM transmitter without signal input is called… [VITEEE 2011]
- The process of superimposing message signal on high frequency carrier wave… [KCET 2016]
- The radius of the Earth is 6400 km. If the height of an antenna… [KCET 2018]
- A basic communication system consists of… [KCET 2017]
- An antenna uses electromagnetic waves of frequency… [KCET 2019]
- In frequency modulation… [KEAM]
- A ground receiver receives a signal at… [KEAM]
- A ground receiver in line-of-sight communication cannot receive… [KEAM]
- In an amplitude modulation with modulation index… [KEAM]
- Audio signal cannot be transmitted because… [KEAM]
- For signal transmission, modulation is necessary… [KEAM]
- Identify the mismatched pair… [KEAM]
- If Ec=20sin105πt and Em=10sin 400πt it are… [KEAM]
- If the critical frequency for sky wave propagation is… [KEAM]
- In amplitude modulation, the bandwidth is… [KEAM]
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