This is the Coaching materials in Transmission Lines and Antennas Part 4 as one topic in ECE Board Exam taken from various sources including but not limited to past Board Examination Questions in Electronic System and Technologies (EST), Communications Books, Journals and other Communications References. This particular Coaching Notes in Communications Engineering has random Questions and Answers in random topics. Make sure to familiarize this review notes to increase the chance of passing the ECE Board Exam.
Transmission Lines and Antennas Coaching Materials Part 4
151. A dipole antenna has an efficiency of 85%. Calculate the gain in decibels.
152. The ERP of a transmitting station is 17W in a given direction. Express this as an ERP in dBm so that it can be used with the path loss equation. Also find EIRP.
ERP = 42.3 dBm; EIRP = 44.44 dBm
153. A helical antenna with eight turns is to be constructed for a frequency of 1.2GHz (a) calculate the optimum diameter and spacing for the antenna and find the total length of the antenna. (b) calculate the antenna gain in dBi (c) calculate the bandwidth.
(a) 62.5 mm; (b) 14.8 dBi; (c) 36.6 degrees
154. A parabolic antenna has a diameter of 3 m, an efficiency of 60%, and operates at a frequency of 4 GHz. Calculate the gain and beamwidth.
G = 39.8 dBi; beamwidth = 1.78 degrees.
155. A power of 100 W is supplied to an isotropic radiator. What is the power density at a point 10 km away?
79.6 nW/square meter
156. Find the electrical field strength for a signal power of 100W at a distance of 10 km away.
157. A 5 kW power, produces a field intensity of 50 uV/m at the receiver, what field intensity will be received if the power is raised to 20 kW?
158. A transmitter has a power output of 150 W at a carrier frequency of 325 MHz. It is connected to an antenna with a gain of 12 dBi. The receiving antenna is 10km away and has a gain of 5 dBi. Calculate the power delivered to the receiver, assuming free-space propagation. Assume also that there are no losses or mismatches in the system.
159. A taxi company uses a central dispatcher, with an antenna at the top of a 15 m tower, to communicate with taxi cabs. The taxi antennas are on the roofs of the cars, approximately 1.5 m above the ground. Calculate the maximum communication distance: (a) between the dispatcher and a taxi (b) between taxis
(a) 21 km (b) 10.1 km
160. In the right-hand rule for propagation, the thumb points in the direction of the E field and the forefinger points in the direction of the H field. In what direction does the middle finger point?
Direction of wave propagation.
161. Which two composite fields (composed of E and H fields) are associated with every antenna?
Induction field and radiation field
162. What composite field (composed of E and H fields) is found stored in the antenna?
163. What composite field (composed of E and H fields) is propagated into free space?
164. If a transmitting antenna is placed close to the ground, how should the antenna be polarized to give the greatest signal strength?
165. What is one of the major reasons for the fading of radio waves which have been reflected from a surface?
Shifting in the phase relationships of the wave.
166. What are the three layers of the atmosphere?
Troposphere, stratosphere and ionosphere
167. Which layer of the atmosphere has relatively little effect on radio waves?
168. What is the determining factor in classifying whether a radio wave is a ground wave or a space wave?
Whether the component of the wave is travelling along the surface or over the surface of the earth.
169. What is the best type of surface or terrain to use for radio wave transmission?
Radio horizon is about 1/3 farther.
170. What is the primary difference between the radio horizon and the natural horizon?
171. What three factors must be considered in the transmission of a surface wave to reduce attenuation?
(a) electrical properties of the terrain (b) frequency (c) polarization of the antenna
172. What causes ionization to occur in the ionosphere?
High energy ultraviolet waves from the sun.
173. How are the four layers of the ionosphere designated?
174. What is the height of the individual layers of the ionosphere?
D layer is 30-55 miles, E layer is 55 to 90 miles, F layers are 90 to 240 miles.
175. What factor determines whether a radio wave is reflected or refracted by the ionosphere?
Thickness of ionized layer
176. There is a maximum frequency at which vertically transmitted radio waves can be refracted back to earth. What is this maximum frequency called?
177. What three main factors determine the amount of refraction in the ionosphere?
(a) density of ionization of the layer (b) frequency (c) angle at which it enters the layer
178. What is the skip zone of a radio wave?
A zone of silence between the ground wave and sky wave where there is no reception.
179. Where does the greatest amount of ionospheric absorption occur in the ionosphere?
Where ionization density is greatest
180. What is meant by the term multipath?
A term used to describe the multiple pattern a radio wave may follow.
181. When a wide band of frequencies is transmitted simultaneously, each frequency will vary in the amount of fading. What is this variable fading called?
182. What are two main sources of EMI with which radio waves must compete?
Natural and man-made interference
183. Thunderstorms, snow storms, cosmic sources, the sun, etc., are a few examples of emi sources. What type of EMI comes from these sources?
184. Motors, switches, voltage-regulators, generators, etc., are a few examples of EMI sources, what type of EMI comes from these sources?
185. What are the two general types of variations in the ionosphere?
Regular and irregular variations.
186. What is the main difference between these two types of variations?
Regular variations can be predicted but irregular variations are
187. What are the four main classes of regular variation which affect the extent of ionization in the ionosphere?
Daily, seasonal, 11-year, and 27-days variation.
188. What are the three more common types of irregular variations in the ionosphere?
Sporadic E, sudden disturbances, and ionospheric storms.
189. How do raindrops affect radio waves?
They can cause attenuation by scattering.
190. How does fog affect radio waves at frequencies above 2 Gigahertz?
It can cause attenuation by absorption
191. How is the term “temperature inversion” used when referring to radio waves?
It is a condition where layers of warm air are formed above layers of cool air.
192. How does temperature inversion affect radio transmission?
It can cause vhf and uhf transmission to be propagated far beyond normal line-of-sight distances.
193. In what layer of the atmosphere does virtually all weather phenomena occur?
194. Which radio frequency bands use the tropospheric scattering principle for propagation of radio waves?
VHF and above.