# Coaching Materials in Transmission Lines and Antennas Part 3 for ECE Board Exam

Compiled coaching materials in Transmission Lines and Antennas Part 3 as one topic in ECE Board Exam taken from different sources including but not limited to Communications books, past Board Exams Questions, Journals and the Internet.

This is the Coaching materials in Transmission Lines and Antennas Part 3 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 3

101. in which plane will the half-wave antenna be operating if it is mounted horizontally?

Vertical plane.

102. Since the radiation pattern of a dipole is similar to that of a doublet, what will happen to the pattern if the length of the doublet is increased?

The pattern would flatten.

103. What is the simplest method of feeding power to the half-wave antenna?

To connect one end through a capacitor to the final output stage of the transmitter.

104. What is the radiation pattern of a quarter-wave antenna?

A circular radiation pattern in the horizontal plane, or same as a half-wave.

105. Describe the physical arrangement of a ground screen?

It is composed of a series of conductors arranged in a radial pattern and buried 1 to 2 feet below the ground.

106. What is the difference in the amount of impedance between a three-wire dipole and a simple-center fed dipole?

Nine times the feed-point impedance.

107. Which has a wider frequency range, a simple dipole, or a folded dipole?

Folded dipole

108. What is the purpose of antenna stubs?

To produce desired phase relationship between connected elements

109. What is the primary difference between the major and minor lobes in a radiation pattern?

Major lobes have the greatest amount of radiation.

110. What is the maximum number of elements used in a collinear array?

Four.

111. Why is the number of elements in a collinear array limited?

As more elements are added, an unbalanced condition in the system occurs which impairs efficiency.

112. How can the frequency range of a collinear array be increased?

By increasing the lengths of the elements of the array.

113. How is directivity of a collinear array affected when the number of elements is increased?

Directivity increases

114. What is the primary cause of broadside arrays losing efficiency when not operating at their designed frequency?

115. When more that two elements are used in a broadside array, how are the elements arranged?

Parallel and in the same plane.

116. As the spacing between elements in a broadside array increases, what is the effect on the major lobes?

They sharpen

117. What are some disadvantages of the end-fire array?

Extremely low radiation resistance, confined to one frequency, and affected by atmospheric conditions

118. Where does the major lobe in the end-fire array occur?

Along the major axis

119. To maintain the required balance of phase relationships and critical feeding, how must the end-fire array be constructed?

Symmetrically

120. What two factors determine the directivity pattern of the parasitic array?

Length of the parasitic element (tuning) and spacing between the parasitic and driven elements.

121. What two main advantages of a parasitic array can be obtained by combining a reflector and a director with the driven element?

Increased gain and directivity.

122. The parasitic array can be rotated to receive or transmit in different directions. What is the name given to such an antenna?

Rotary array.

123. What are the disadvantages of the parasitic array?

Their adjustment is critical and they do not operate over a wide frequency range.

124. What is the advantage of adding parasitic elements to a Yagi array?

Increased gain.

125. The Yagi antenna is an example of what type of array?

Multi-element parasitic array

126. To radiate power efficiently, a long-wire antenna must have what minimum overall length?

One-half wavelength.

127. What is another name for the Beverage antenna?

Wave antenna

128. What is the polarity of the currents that feed the V antenna?

Opposite

129. What is the main disadvantage of the rhombic antenna?

It requires a large antenna site.

130. What is the primary reason for the development of the turnstile antenna?

For omnidirectional vhf communications

131. Microwave antennas and low-frequency antennas are similar in what ways?

Operating principles and electrical characteristics

132. What term is used to express the efficiency of an antenna?

Power gain or power ratio.

133. What term is used to express the measurement of the degree of mismatch between a line and its load?

Standing-wave ratio (SWR)

134. What type of antenna radiates in and receives energy from all directions at once?

Omnidirectional

135. What is the term that is used to describe narrowness in the radiated beam of an antenna?

Antenna directivity

136. What characteristic allows the same antenna to both transmit and receive?

Reciprocity.

137. What type of reflector is most often used in directive antennas?

Parabolic.

138. Microwaves can be reflected and focused in the same way as what other type of waves?

Light waves.

139. How many major lobes are radiated by a parabolic reflector?

One

140. A horizontally truncated parabolic antenna is used for what purpose?

Determine elevation

141. The beam from a horizontally positioned cylindrical paraboloid is narrow in what plane?

Vertical.

142. What is a purpose of a collimating lens?

Forces the radial segments of a wavefront into parallel paths

143. What type of lens decelerates a portion of a spherical wavefront?

Delay lens

144. What is a set of antenna elements called?

Antenna array

145. What type of antenna has all elements connected to the same energy source?

Driven array

146. What determines the beam elevation angle of an antenna that is electronically scanned in elevation?

Frequency or phase of radiated energy

147. What is the polarization of the energy radiated by a vertical slot?

Horizontal

148. Calculate the length of a half-wave dipole for an operating frequency of 20MHz.

149. A dipole antenna has a radiation resistance of 67 ohms and a loss resistance of 5 ohms measured at the feedpoint. Calculate the efficiency.

93%

150. Determine the efficiency with the following: Pin = 1000w, I = 10 A, R = 8 ohm.

80%

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