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Question 1

What are the characteristics of electro-magnetic waves (cycle, amplitude, phase,…)

Answer 1

Wavelength: is the length corresponding to a cycle, usually expressed in meter or centimeters
Frequency: is the number of cycles per second;
velocity: propagation speed v=landa x f = c/n
amplitude: the strength of the signal
cycle: refers to one complete oscillation of the wave
period: T=1/f (time of 1 cycle)
phase: difference between 2 waves in degrees
em waves are sinusoidal

Question 2

What are the propagation characteristics of electro-magnetic waves ?

Answer 2

Laws of Maxwell :
* Constant velocity: speed remains constant in a medium with constant density.
* Frequency remains constant during propagation.
* Wavelength stays constant only in a medium with constant density.
* Wave damping causes the amplitude to decrease over distance.

Question 3

What are the specific propagation characteristics and propagation modes of direct electro-
magnetic radiowaves ?

Answer 3

• Very small wave bending (only refraction bending)
• Usual mode for very high frequencies
• Maximal range (radio-horizon) depending on :
  o Antenna height
  o Emitting power
  o ‘Line Of Sight

Question 4

What are the specific propagation characteristics and propagation modes of radio electro-
magnetic groundwaves ?

Answer 4

Groundwave Propagation:

Ground Reflected Wave:
    The wave reflects off the ground, helping it travel along the Earth's surface.
Surface Wave:
    Follows the curvature of the Earth.
    Energy travels more easily because the Earth's surface acts as a medium.
    Propagation depends on the electric conductivity of the ground.
    Can travel long distances, with a range greater than 1000 km.

Question 5

What are the specific propagation characteristics and propagation modes of radio
electromagnetic skywaves

Answer 5

Skywave Propagation:

Reflection by the Ionosphere: Skywaves are reflected by the ionosphere’s layers (D, E, F1, F2), enabling long-distance communication.
Factors Affecting Propagation:
    Ion Density: Varies with altitude, time of day (highest at noon), season, latitude, and the 11-year solar cycle.
    Frequency:
        Lower frequencies are refracted (bent) by the ionosphere.
        Higher frequencies tend to pass through the ionosphere.
Nighttime Propagation: Reflection is more effective at night due to changes in ionospheric density.

Question 6

What is the difference between ducted waves and hopped skywaves ?

Answer 6

ducted waves follow the Earth’s surface due to atmospheric conditions, while hopped skywaves bounce between the ionosphere and Earth, covering greater distances by “hopping.”

Ducted Waves:
    Follow the Earth’s surface due to atmospheric conditions (temperature, pressure, humidity).
    Ducting occurs when there’s a sudden change in the refraction index, allowing waves to travel farther than usual, sometimes for minutes to days.

Hopped Skywaves:
    Bounce between the ionosphere and Earth (multiple reflections or "hopping").
    Can cover greater distances but may cause signal distortion, delay, or interference due to path length variations.

Question 7

What is the difference between amplitude modulation, frequency modulation and pulse
modulation ? Why is pulse modulation more and more used ?

Answer 7

• Amplitude Modulation (AM):
  Amplitude changes, while frequency and phase stay the same.
  Requires small bandwidth.
• Frequency Modulation (FM):
  Frequency changes, while amplitude and phase stay the same.
  Requires medium bandwidth.
• Pulse Modulation (PM):
  The signal is divided into small parts (pulses).
  Requires large bandwidth and high power, leading to more interference.
  Allows precise signal encoding.

Pulse modulation is more used because it allows efficient encoding with a unique signature, offering better signal processing

Question 8

What disturbing factors limit the range of radiosystems ? How can the range be improved
(antenna requirements, …) ?

Answer 8

1. Absorption: Atmospheric molecules absorb EM wave energy, converting it into heat, especially at higher frequencies or with more humidity. Solution: Use lower frequencies
2. Obstacles: Physical barriers (e.g., buildings or terrain) can block or weaken signals, creating shadow zones. Solution: Increase antenna height to overcome physical barriers, use directional antennas to focus the signal
3. Interference/Multipath: Simultaneous arrival of multiple waves can cause phase shifts or cancellation, leading to signal loss. Solution: Use multiple antennas at different locations or using different signal polarizations
4. Scattering: Reflections and diffraction of waves can cause signal distortion and energy loss. Solution: Use higher frequencies that are less affected by scattering,

Question 9

different frequency signals

Answer 9

UHF/SHF signals are mostly limited to direct Line-of-Sight (LOS) transmission, offering excellent accuracy but limited range due to strong absorption of groundwaves. directional antenna
HF/VHF signals can experience reception holes at skip distances, with weaker accuracy and limited groundwave transmission. short antenna
MF signals benefit from both good skywave and groundwave transmission, resulting in a good range but potential interference between the two. very high antenna + ground net antenna
VLF/LF signals offer very large ranges, but with low accuracy, primarily due to the dominance of groundwave and skywave reflections. very high antenna + ground net antenna