Why this topic is important for ECET?
Wave propagation is the backbone of wireless communication. Without understanding how signals travel through space, no mobile, satellite, TV, or radio system can be designed. ECET exams frequently ask conceptual questions + numericals on propagation modes, reflection, refraction, critical frequency, and skip distance. Strong grip on this chapter ensures easy scoring + high accuracy.
📘 Concept Notes
1. Basics of Wave Propagation
- Wave propagation = transmission of electromagnetic (EM) waves from transmitter to receiver.
- EM waves need medium + frequency-specific path.
- Atmosphere is divided into troposphere, stratosphere, ionosphere – each affects signal differently.
2. Modes of Propagation
- Ground Wave Propagation
- Wave travels close to earth’s surface.
- Effective for long-wave & medium frequency (30 kHz – 3 MHz).
- Used in AM broadcast, navigation signals.
- Attenuation increases with frequency.
- Sky Wave Propagation
- Signal reflects/refracts from ionosphere back to Earth.
- Effective in HF band (3 – 30 MHz).
- Used in shortwave radio, international broadcasting.
- Critical Frequency (f_c): Maximum frequency reflected by ionosphere at vertical incidence.
- Maximum Usable Frequency (MUF): Max frequency usable for sky-wave at oblique incidence.
- Skip Distance: Minimum distance from transmitter where first sky-wave is received.
- Space Wave Propagation
- Line-of-sight communication between Tx and Rx.
- Effective for VHF, UHF, SHF (30 MHz – 3 GHz).
- Used in TV, FM radio, mobile, satellite communication.
- Limited by horizon distance.
3. Key Concepts with Examples
- Critical Frequency Formula:
If maximum electron density in ionosphere = NmN_mNm,
MUF Formula:
where θ\thetaθ = angle of incidence.
Skip Distance Formula:
where h = virtual height of ionosphere.
Line-of-Sight Distance (for Space Wave):
where ReR_eRe = radius of earth, hth_tht = Tx height, hrh_rhr = Rx height.
⚙️ Formulas
- Critical Frequency:
Maximum Usable Frequency (MUF):
Skip Distance:
Line of Sight Distance:
Wavelength:
🔟 10 MCQs
Q1. Which frequency range is suitable for ground wave propagation?
a) 30 Hz – 300 Hz
b) 30 kHz – 3 MHz
c) 30 MHz – 300 MHz
d) 3 GHz – 30 GHz
Q2. A radio station operates at 1 MHz. Which mode of propagation is most suitable?
a) Ground wave
b) Sky wave
c) Space wave
d) Microwave
Q3. The ionosphere is mainly responsible for which type of wave propagation?
a) Ground wave
b) Sky wave
c) Space wave
d) Surface wave
Q4. If maximum electron density is 101210^{12}1012 electrons/m³, find the critical frequency.
Q5. For a critical frequency of 5 MHz and incident angle 60°, calculate MUF.
Q6. Skip distance is defined as:
a) Maximum distance before reception
b) Distance where sky wave is first received
c) Distance of line of sight
d) None
Q7. A transmitting antenna is at 100 m height and receiving antenna at 25 m. Calculate line-of-sight distance. (Take Re=6400kmR_e = 6400 kmRe=6400km).
Q8. Which propagation is used in FM radio broadcasting (88 – 108 MHz)?
a) Ground wave
b) Sky wave
c) Space wave
d) Satellite
Q9. MUF is always:
a) Less than critical frequency
b) Equal to critical frequency
c) Greater than or equal to critical frequency
d) Independent of critical frequency
Q10. Which frequency band is preferred for satellite communication?
a) LF
b) HF
c) VHF
d) SHF
✅ Answer Key
| Q No | Answer |
|---|---|
| Q1 | b |
| Q2 | a |
| Q3 | b |
| Q4 | 9 MHz |
| Q5 | 10 MHz |
| Q6 | b |
| Q7 | 46 km |
| Q8 | c |
| Q9 | c |
| Q10 | d |
🧠 Explanations
- Q1: Ground waves work well in MF band → (b).
- Q2: 1 MHz = medium frequency → travels by ground wave → (a).
- Q3: Ionosphere reflects HF waves → sky wave → (b).
- Q4: fc=91012=9×106=9MHz f_c = 9 \sqrt{10^{12}} = 9 × 10^6 = 9 MHzfc=91012=9×106=9MHz.
- Q5: MUF=5×sec(60°)=10MHz MUF = 5 × sec(60°) = 10 MHzMUF=5×sec(60°)=10MHz.
- Q6: Skip distance = first reception distance of sky wave → (b).
- Q7: d=2×6400×0.1+2×6400×0.025≈35.8+10.1=46kmd = \sqrt{2×6400×0.1} + \sqrt{2×6400×0.025} ≈ 35.8 + 10.1 = 46 kmd=2×6400×0.1+2×6400×0.025≈35.8+10.1=46km.
- Q8: FM operates in VHF → needs space wave → (c).
- Q9: MUF ≥ f_c always, depending on angle → (c).
- Q10: Satellites use SHF (3–30 GHz) → (d).
🎯 Motivation / Why Practice Matters
Wave propagation questions test concept clarity + formula accuracy. In ECET, many students lose marks because:
- They confuse ground/sky/space wave ranges.
- Forget MUF and skip distance relations.
Daily practice ensures you solve such questions in less than 30 seconds → boosting overall exam score.
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