# Wireless & Mobile Communications Questions & Answers – Free Space Propagation Model

This set of Wireless & Mobile Communications Multiple Choice Questions & Answers (MCQs) focuses on “Free Space Propagation Model”.

1. The mechanism behind electromagnetic wave propagation cannot be attributed to ___________
a) Reflection
b) Diffraction
c) Scattering
d) Sectoring

Explanation: The mechanisms behind electromagnetic wave propagation are diverse. They can be greatly attributed to reflection, diffraction and scattering. Due to multiple reflections from various objects, the electromagnetic waves travel along different paths of varying lengths.

2. The propagation model that estimates radio coverage of a transmitter is called ___________
a) Large scale propagation model
b) Small scale propagation model
d) Okumura model

Explanation: Large scale propagation model are useful in estimating the radio coverage area of a transmitter. They can predict the mean signal strength for an arbitrary transmitter-receiver (T-R) separation distance. They characterize signal strength over large T-R separation distances.

3. Propagation model that characterize rapid fluctuation is called _________
a) Hata model
c) Large scale propagation model
d) Okumura model

Explanation: Fading models characterize the rapid fluctuations of the received signal strength over very short travel distance (a few wavelengths) or shot time durations (on the order of seconds).

4. Small scale propagation model is also known as _________
b) Micro scale propagation model
c) Okumura model
d) Hata model

Explanation: Small scale propagation model is also called fading model. Fading model characterize the rapid fluctuations of the received signal strength over very short distance of a few wavelengths or short time duration. The propagation models are used to estimate the performance of wireless channels.

5. Free space propagation model is to predict ______
b) Transmitted power
c) Gain of transmitter

Explanation: Free space propagation model predicts the received signal strength when there is an unobstructed line of sight path between transmitter and receiver. It assumes the ideal propagation condition that the environment is empty between the transmitter and receiver.
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6. Which of the following do not undergo free space propagation?
a) Satellite communication system
d) Wired telephone systems

Explanation: EM signals when traveling through wireless channels experience fading effects due to various effects. But in some cases the transmission is with no obstruction and direct line of sight such as in satellite communication, microwave and wireless line of sight radio links.

7. The free space model predicts that received signal decays as a function of _________
a) Gain of transmitter antenna
b) T-R separation
c) Power of transmitter antenna
d) Effective aperture of the antenna

Explanation: As with most large scale radio wave propagation models, the free space model predicts that received signal decays as a function of the T-R separation distance raised to some power. Often it is given as a function of negative square root of the distance.

8. Relation between gain and effective aperture is given by ______
a) G=(4πAe)/λ2
b) G=(4π λ2)/Ae
c) G=4πAe
d) G=Ae2

Explanation: The gain of the antenna is proportional to effective aperture area. Therefore, antennas with large effective apertures are high gain antennas and have small angular beam widths. Most of their power is radiated in a narrow beam in one direction, and little in other directions.

9. Relation between wavelength and carrier frequency is _________
a) λ=c/f
b) λ=c*f
c) λ=f/c
d) λ=1/f

Explanation: Wavelength is inversely proportional to carrier frequency. For electromagnetic radiation in free space, wavelength is a ratio of speed of light (c) and carrier frequency (f). Speed of light is 3*108 m/s. The unit for wavelength is meters.

10. Which of the following antenna radiates power with unit gain uniformly in all directions?
a) Directional antenna
b) Dipole antenna
c) Isotropic antenna
d) Loop antenna

Explanation: Isotropic antenna radiates the power with unit gain uniformly in all directions. It is an ideal antenna. From practical point of view, there is no actual isotropic antenna. But, an isotropic antenna is often used as a reference antenna for the antenna gain.

11. EIRP is abbreviated as __________
c) Effective and immediate radiated power
d) Effective and immediate ratio of power

Explanation: EIRP stands for Effective Isotropic Radiated Power. It is the amount of power that a theoretical isotropic antenna would emit to produce the peak power density observed in the direction of maximum antenna gain. EIRP also takes into account the losses in transmission line and connectors and includes the gain of the antenna.

12. Path loss in free space model is defined as difference of ________
a) Effective transmitted power and gain
b) Effective received power and distance between T-R
d) Effective transmitter power and receiver power

Explanation: I Path loss is defined as difference of effective transmitter power and receiver power. Free-space path loss is the loss in signal strength of an electromagnetic wave that would result from a line-of-sight path through free space, with no obstacles nearby to cause reflection or diffraction.

13. Far field region is also known as _________
a) Near field region
b) Fraunhofer region
c) Erlang region
d) Fresnel region

Explanation: The far field is the region far from the antenna. In this region, the radiation pattern does not change shape with distance. Also, this region is dominated by radiated fields, with the E- and H-fields orthogonal to each other and the direction of propagation as with plane waves.

14. Fraunhofer distance is given by _____
a) 2D2/λ
b) 2D/λ
c) D/λ
d) 2D/λ2

Explanation: Fraunhofer distance, also known as far field distance is inversely proportional to wavelength. It depends on the largest physical dimension of the antennal (D). This distance basically denotes the boundary between far field and near field region.

15. Which of the following is called an ideal antenna?
a) Dipole antenna
b) Directional antenna
c) Isotropic antenna
d) Loop antenna

Explanation: Isotropic antenna is an ideal antenna that directs the power uniformly in all directions. It is a theoretical point source of electromagnetic. It is practically not possible. It is mainly used as a hypothetical antenna to measure the gain.

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