Finals Part 1 Flashcards
(206 cards)
1
Q
Simplest type of antenna; Very short version of a dipole antenna
A
Elementary doublet
2
Q
Antenna is much smaller than half of the signal’s wavelength—-usually less than one-tenth of it
A
Meaning of Electrically short in elementary doublet
3
Q
Also called a short dipole or Hertzian dipole
A
Elementary doublet
4
Q
An elementary doublet has ____________ throughout its length
A
Uniform current
5
Q
The equation i(t) = Isin(2pift + theta), means that the current in the elementary doublet varies _________. It follows the shape of a sinewave, just like how AC behaves
A
Sinusoidally in a wave like pattern over time
6
Q
The key idea of a elementary doublet is that its radiation is ___________ perpendicular to the ___________ and ________ along its ___________
A
Strongest; antenna (to the sides); weakest; axis
7
Q
Strong radiation in the elementary doublet occurs perpendicular to the antenna at _____ and ______
A
90 degrees and -90 degrees
8
Q
Weak or no radiation in the elementary doublet occurs along the antenna’s axis at ________ and _________
A
0 degrees and 180 degrees
9
Q
The two-lobed pattern/figure eight pattern means that an elementary doublet does not radiate well along its ___________ but is effective in directions _____________
A
Along its length; perpendicular to it
10
Q
One of the most common antennas especially for frequencies above 2 MHz.
A
Half-wave dipole antenna
11
Q
It is called half wave because its total length is ___________ of the signal it transmits or receives
A
Half of the wavelength
12
Q
The half wave dipole is also called a
A
HErtz Antenna
13
Q
Hertz Antenna is named after who?
A
Heinrich Hertz
14
Q
HAlf wave dipole is a _________, meaning it efficiently radiates at a frequency where its length is a multiple of quarter-wavelengths (lambda/4)
A
Resonant antenna
15
Q
Half wave dipole creates _________ along its length
A
Standing waves of voltage and current
16
Q
Voltage & current distribution in the Half-wave dipole is that the voltage is ________ at the ends and _________ in the middle, while the current is ________ in the middle, and _________ at the ends
A
Highest ; lowest ; highest; lowest
17
Q
The center of the dipole has maximum current flow which is __________, while the ends have maximum voltage which is similar to an __________
A
Good for feeding power; like an open circuit
18
Q
The impedance of a half-wave dipole varies around _________ at the ends to about _______ at the center
A
2500 ohms; 73 ohms
19
Q
The center fed dipole has an input impedance of about ________, which matches well with standard 75 ohms coaxial cable
A
73 ohms
20
Q
A half wave dipole acts like _______ placed end to end
A
Many tiny dipoles
21
Q
Two key configurations of a half wave dipole
A
- Vertical
- Horizontal
22
Q
A key configuration of a half wave dipole where:
1. main radiation is in two lobes perpendicular to the antenna
2. No radiation directly above or below
3. Forms a figure eight pattern when viewed from the side
A
Vertical half wave dipole
23
Q
a key configuration of a half wave dipole where:
1. When viewed from above, the radiation pattern is almost circular
2. Radiation is strongest perpendicular to the antenna
A
Horizontal half wave dipole
24
Q
The approximate radiation resistnace of the half wave dipole
A
72 ohms
25
Characteristic impedance of free space
120 ohms
26
Of then expressed in decibels using the logarithmic formula: 10logD, where D = 120/radiation resistance
Antenna directivity
27
A higher directivity means the antenna _______________ in a specific direction
Focuses energy better
28
In reality, Earth’s surface affects _________ due to reflection, absorption, and diffraction
Wave propagation
29
The radiation patten that assume free space condition on half wave dipole means _______ with the propagation of waves
No ground of obstacles interfere
30
In ground effects on a half wave dipole, when a dipole antenna is placed above the ground, part of the radiated wave travels _________, while another part reflects _______
directly into space (direct wave); off the ground (ground reflected wave)
31
The impact on Radiation pattern on half-wave dipole, at certain angles, the direct and reflected waves reinforce each other which is also called ___________, where it is increasing the radiation intensity
Constructive interference
32
The impact on radiation pattern of half wave dipole, at other angles, they cancel out or also called as ________, which redues radiation in some directions
Destructive interference
33
For the ground effects on a half wave dipole, the image antenna represents the ______
virtual effect of reflection
34
For the ground effects on a half wave dipole, the Direct wave and ground reflected wave interfere to modify the ____________
radiation pattern
35
In a half wave dipole, if the antenna is one quarter wavelength above the ground, the ________ disappears, and the field strength _______ in the upward direction
lower lobe
doubles
36
In half wave dipole, if the antenna is one-half wave length above ground, The main radiation lobe shifts to ____________ due to ground reflections
30 degrees from the horizontal
37
Ground reflections affect all antenna and can either __________
Enhance or reduce signal strength
38
For a vertical antenna in half wave dipole, radiation resistance _________ at around 73 ohms at sufficient height
Fluctuates but stabilizes
39
For a horizontal antenna in half wave dipole, radiation resistance starts _______
very low and increases as height increases
40
At heights above one-half wavelength in half wave dipole, reflections have _________ and resistance becomes _________
less effect;
stable
41
Antenna height affects radiation pattern due to
Reflections from the ground
42
Reflected waves can interfere with the ________, either amplifying or weakening the signal.
Direct waves
43
Radiation resistances fluctuates at _________ but stabilizes beyond half wave length height
Lower heights
44
A monopole antenna with one quarter wave length, mounted vertically, and grounded
Grounded Antennas
45
What is the other term of grounded antenna?
Marconi antenna
46
The functions of a grounded antenna are similar to a
Hertz Antenna
47
Grounded antenna uses __________ waves
Ground-reflected
48
Requires a conductive ground surface for efficiency
Grounded antenna
49
The voltage and current standing waves of a quarter-wave grounded antenna mimic a ________
half-wave ungrounded antenna
50
_______ play a crucial role in antenna radiation
Ground waves
51
One of the observations of the radiation pattern of grounded antenna is that the lower half of each lobe is ________
Canceled by ground reflections
52
One key observations of radiation pattern of grounded antennas is that the horizontal radiation increases __________
For better area coverage
53
one key observation of radiation pattern of grounded antennas are optimum radiation occurs at
Five-eights wavelength
54
Increases the electrical length of an antenna without changing its physical size
Antenna loading
55
The methods of antenna loading are
Loading coils
Top loading
Folding techniques (L or T shape)
56
Inductors added in series to increase the effective length of an antenna
Loading coils
57
The effects of loading coils is that it cancels out ________
Capacitive reactance
58
The effects of loading coils is that it increases ________
Radiation efficiency
59
The effect of loading coils is that it allows antenna to behave like a _______
Resonant circuit
60
It involves placing a metallic array or spokes at the top of the antenna
Top loading
61
The wheel increases the ________ to ground, reducing the overall antenna capacitance
shunt capacitance
62
One benefit of top loading is that it increases the __________ at base
Standing wave current
63
One benefit of top loading is that it enhances ____________
Radiation resistance and efficiency
64
One benefit of top loading is that it reduces voltage at __________
The antenna base
65
The grounded antennas and loading techniques are widely used in
AM radio transmission
Mobile communication towers
Maritime and military communication
Low frequency communication systems
66
It is formed when two or more antenna elements combine to function as a single antenna
Antenna arrays
67
Antenna arrays enhances __________ and concentrates _________ within a smaller geographic area
Directivity; radiated power
68
Types of elements in antenna arrays
Driven elements
Parasitic Elements
69
This type of element in antenna array is directly connected to the transmission line and powered by the source
Driven elements
70
This type of element in antenna array is not connected to the transmission line and receive energy through mutual induction
Parasitic elements
71
Commonly known parasitic elements
Reflector and director elements
72
it is an element that is longer than driven element and acts like a concave mirror
Reflector element
73
This element also reduces signal in its direction, increases it in the opposite direction
Reflector element
74
This element is shorter than driven element and acts like a convex lens
Director element
75
This element also increases signal in its direction, reduces it in the opposite direction
Director element
76
The radiation directivity can be increased in ________ planes
Horizontal or vertical planes
77
The radiation directivity depends on _______ and whether they are _______.
Element placement; driven
78
If not driven, pattern depends on whether the elements are __________; otherwise, depends on the _________ of the feeds
Directors or reflectors; relative phase
79
The simplest antenna array where it uses several resonant dipoles of equal size in parallel and collinear. All of its elements fed in phase from the same source
Broadside array
80
Broadside array radiates at _______ to the plane of the array
Right angles
81
Broadside array has minimal ________ direction
Radiation in-plane
82
The directivity of a broadside array can be increased by _______
Adding more elements
83
This array is similar to broadside array but without crisscrossed transmission lines
End-fire array
84
In an end-fire array, fields are ____________ with the plane of the array
additive in line
85
End-fire array produces a ___________ radiation pattern
More directional
86
It is capable of operating over a wide bandwidth (3 MHz - 30 MHz)
Nonresonant array or The Rhombic Antenna
87
Nonresonant array is ideally suited for _________ transmission
HF transmission
88
Nonresonant array is constructed using ________ several wavelengths long
Four nonresonant elements
89
Nonresonant array is mounted _______ and placed ________ or more above ground
Horizontally and placed half-wavelength
90
The terminating resistor of a nonresonant array abosrbs about ___________
1/3 of the total input power
91
The maximum efficiency of a nonresonant array
67%
92
Two conductively connected elements, forming a full wavelength current path
Folded dipole
93
The impedance is _______ that of a half-wave dipole
4 times (example 288ohms for two wires, 648ohms for three)
94
The advantage of a folded dipole is that it has higher __________
input impedance
95
One advantage of a folded dipole is it has _________ where it improves performance over different frequencies
Wider bandwidth
96
Folded dipole can be made even broader using ______
Fat dipole moments
97
Consists of a folded dipole, which is the driven element, and additional parasitic elements such as reflector and director
Yagi-Uda Antenna
98
In a Yagi-Uda Antenna, reflector is ________ than dipole, wherein it enhances gain
5% longer
99
In a Yagi-Uda Antenna, directors are _________, wherein it focuses the signal forward
5% shorter than dipole
100
Elements in Yagi Uda Antenna are spaced _________ apart
0.1-0.2 wavelengths
101
The directivity of a Yagi Antenna is ________, improcing signal reception and transmission
7 dB to 9 dB
102
Yagi-uda Antenna is named after two Japanese scientists, who are they?
Shintaro Uda and Hidetsugu Yagi
103
Common uses of Yagi-Uda Antenna
VHF TV Antennas (54-216MHz) due to wide bandiwdth
Wireless communication and radio applications
104
In Yagi-Uda Antenna, bandwidth can be increased by using __________ of different lengths
Multiple folded dipoles
105
Consists of two dipoles placed at right angles (90 degrees) to each other
Turnstile Antenna
106
The signals in the dipoles in the Turnstile Antenna are _________
90 degrees out of phase
107
The configuration of a turnstile antenna creates a ________ that is nearly omnidirectional
Combined radiation pattern
108
A class of frequency independent antennas developed in 1957 at the Uniersity of Illinois by V.H. Rumsey, J.D. Dyson, R.H. DuHamel, and D.E. Isbell.
Log-periodic Antenna
109
Designed to provide a wide operating bandwidth
Log-periodic antenna
110
The Log-periodic antenna’s radiation resistance and pattern _______ across a range of frequencies
Remain stable
111
The bandwidth ratio of log-periodic antenna is
10:1 or greater
112
Log-periodic antenna is used in the following applications
TV
HF & VHF communications
Military communications
113
Log periodic antenna has a ________ of dipoles in varying sizes
Repeating geomtric pattrn
114
The dipole lengths and spacing of a log periodic antenna follow a __________.
Constant ratio
115
The transmission line in the log-periodic antenna is _________ at the small end
Crisscrossed
116
The lengths of the dipoles and their spacing in log-periodic antenna are related in such a way that ___________ have a constant ratio to each other
Adjacent elements
117
For a typical design, the design ratio of log-periodic antenna is _____ while alpha is _____
0.7, 30 degrees
118
Log periodic antenna can be _______ or _______
Unidirectional or bidirectional
119
Log periodic antenna has __________ directive gain, especially in large arrays
moderate to hig hdirective gain
120
Input impedance changes repetitively as a function of frequency, aligning with the
Logarithm of frequency
121
Log-periodic antennas get their name from how their impedance ___________ when plotted against the ____________
Varies periodically; logarithm of frequency
122
Log-periodic antenna input impedance fluctuates but __________ in a sinusodial pattern
Not necessarily
123
Other parameters in log periodic antenna that also vary with frequency are
Radiation pattern
Directivity
Power gain
Beamwidth
124
Log periodic antenna do not require a _______
Terminating resistor
125
Common applications of log-periodic antenna
HF & VHF communications
High-gain TV antennas (high performance antennas)
126
Consists of a single-turn coil of wire that carries RF current and is much shorter than one wavelength
Loop antenna
127
The simplest loop antenna is a _________ with ________ dipoles connected together
Polygonal shape with elemental dipoles
128
Loop antenna is surrounded by a _______, the loop’s radiation pattern is _________ of its exact shape
Magnetic field; independent
129
The radiation pattern of a loop antenna is similar to that of a
Short horizontal dipole
130
Multi-turn loops or multiple wire turns have _________, which is proportional to the square of the number of turns
Higher radiation resistance
131
Loop antennas exhibit ________ polarization, similar to dipoles
Linear polarization
132
Polarization depends on ________
Loop orientation (vertical or horizontal loop)
133
Common uses of loop antenna
Mobile communications
VLF communications
134
The direction of the received signal in loop antennas can be found by ________ until a null or zero value appears, indicating the signal’s origin
Rotating the loop
135
Consists of multiple antennas or antenna arrays working together as a single unit
Phased Array Antenna
136
Phased array antennas allows beamwidth & direction control electronically, __________ the antennas
Without physically moving
137
Phased array antenna eliminates the need for ___________ of antennas
Mechanical rotation
138
The phased array antenna operates based on interference among electromagnetic waves:
Constructive interference
Destructive interference
139
This type of interference among electromagnetic waves is when signals reinforce each other
Constructive interference
140
This type of interference among electromagnetic waves is when signals cancel each other out
Destructive interference
141
Types of Phase Array Antennas
Single High Power Transmitters
Multiple Low Power Transmitters
142
This type of phased array antennas distributes power to many antennas via power splitters & phase shifters.
Single High Power Transmitter
143
A computer controls the ________ & _______ at each antenna in a single high power transmitter
Signal strength & time delay
144
This type of phased array antenna uses separate low power devices for each antenna element
Multiple Low Power Transmitters
145
With multiple low power transmitters, this parameter control signal timing between antennas of phased array antennas
Phase shifters
146
A broadband VHF/UHF antenna designed for applications requiring
circular polarization
Helical Antennas
147
Helical antennas Can be used as a ___________ to
enhance gain & directivity.
single antenna or combined in an array
148
Operating frequency of Ultra High Frequency Antenna
0.3 GHz - 3 GHz
149
Operating Frequency of Microwave Communications
1 GHz - 100 GHz
150
Designed to be highly directive
UHF and Microwave Antennas
151
The beamwidth of an antenna (UHF and Microwave) ________ as its gain _______
decreases; increases
152
Microwave antennas typically have half-power beam widths of
1 degree or less
153
Not all radiated energy is directed in
the main lobe; some energy is radiated
in ________
minor lobes, known as side lobes
154
Side lobes in Microwave antennas can interfere with nearby ______, reducing system
efficiency
microwave paths
155
The ratio of maximum gain in the
forward direction to the maximum
gain in the backward direction.
Front-to-Back Ratio
156
Front-to-back ratio can be _________ in real world installations due to reflections from nearby objects
20 dB lower
157
Front-to-back Ratio is important for ___________, where transmit and receive antennas are positioned opposite each other
repeater stations
158
Measures the coupling loss
between transmitting and
receiving antennas in the same
vicinity
Side-to-Side and Back-to-Back
Coupling
159
Side-to-Side and Back-to-Back
Coupling, Essential for ___________
preventing
interference
160
in this type of microwave systems, it uses highly directional (highgain) antennas
Point-to-point microwave communication
161
The transmitting antenna focuses energy into a _______, significantly increasing effective radiated power while the receiving
antenna, similar to a telescope, enhances the received power by
concentrating signals.
narrow beam
162
preferred choice for high-gain
microwave communication.
Parabolic reflector antennas
163
most common type of antenna for
microwave transmission and reception and also provides extremely high gain and directivity
Parabolic Reflector Antenna
164
The parabolic reflector antenna consists of two key components which are
Parabolic Reflector
Feed Mechanism
165
A key component of a parabolic reflector antenna where a passive surface that reflects radio waves into a focused, in-phase beam
Parabolic Reflector
166
a type of key component of the parabolic reflector antenna where it houses the primary antenna (usually a dipole or dipole array) that radiates electromagnetic waves toward the reflector.
Feed Mechanism
167
A _____________ is a key component of a parabolic antenna, which
is shaped like a dish. This shape allows it to efficiently reflect and direct
electromagnetic waves
parabolic reflector
168
the point where all incoming parallel waves (like radio or
light waves) reflect and meet.
The focus
169
The parabolic reflector works by first, when a signal (wave) is transmitted from the focus, it reflects off the __________ and exits as __
parabolic surface and exits as parallel waves
170
The parabolic reflector works by if used as a receiver, it works in reverse----signals from a specific direction _____________
get focused at the focus
171
The principle of how a parabolic reflector works is used in
Satellite dishes
Radio telescopes
Flashlight reflectors
Car headlights
172
this refers to how tightly focused the radiated energy is. A
narrower ______ means better directionality and signal strength
beamwidth of a parabolic antenna
173
Parabolic reflectors focus _______ using the properties of a parabola.
electromagnetic waves
174
defines how efficiently the reflector directs energy.
aperture ratio in parabolic reflector antenna
175
determines the spread of the signal—smaller _________ mean more focused
signals.
beamwidth
176
What is a parabolic reflector antenna?
A parabolic reflector antenna is a type of antenna that uses a parabolic shape to focus signals.
177
What is the efficiency of a parabolic antenna represented by?
Parabolic Antenna Efficiency (η)
178
Why isn't all the energy reflected by a parabolic reflector?
Some energy is absorbed by the dish, diffracted around the edges, and blocked by the feed mechanism.
179
What is the typical efficiency value of a parabolic antenna?
η = 0.55 (55%)
180
What does power gain measure in a parabolic reflector?
Power gain measures how much the parabolic reflector amplifies the signal compared to an isotropic antenna.
181
What are the components of the power gain formula?
f = frequency (Hertz), c = speed of light (3×10^8 m/s)
182
How is power gain often measured?
In decibels (dB)
183
What adjustment is made for 100% efficiency in power gain?
Add 2.66 dB to the result.
184
What is the relationship between power gain and wavelength?
Power gain is inversely proportional to the square of the wavelength (λ^2).
185
What happens to power gain as the wavelength decreases?
Power gain increases.
186
What is the effective area of a parabolic antenna?
The area expressed in terms of square wavelengths.
187
What is capture area in receiving antennas?
The portion of the antenna that effectively captures energy.
188
What causes the capture area to be smaller than the physical dish size?
* Spillover losses * Blockage from the feed mechanism * Inefficiencies in the reflector’s surface
189
What does aperture efficiency (illumination efficiency) represent?
How well the antenna captures and reflects energy.
190
What does the term 'k' represent in the context of parabolic antennas?
Aperture efficiency.
191
What is the aperture number (f/D)?
The ratio of the focal length to the dish diameter.
192
How does the aperture number affect the beam width?
* Larger f/D ratio = narrower beam * Smaller f/D ratio = wider beam
193
What does reciprocity mean in parabolic antennas?
A parabolic antenna behaves the same way for transmission and reception.
194
What do you get when calculating transmit and receive power gain?
Nearly the same result.
195
Radiates electromagnetic energy and directs it toward the parabolic reflector; considered the primary antenna in a parabolic setup
Feed Mechanism
196
Primary type of parabolic antenna feed where the antenna is placed at the focus of the paraboloid, reflecting energy into a beam.
Center Feed
197
Reflects stray emissions back toward the parabolic reflector, aligning energy with the desired beam direction.
Spherical Reflector (in Center Feed)
198
Uses a small horn antenna placed at the focus of the paraboloid to emit directional electromagnetic waves.
Horn Feed
199
A horn that flares in one direction only.
Sectoral Horn
200
A horn that flares in both width and height.
Pyramidal Horn
201
A horn with circular flaring, forming a cone shape.
Conical Horn
202
Feed type named after an 18th-century astronomer, using a hyperboloidal subreflector to direct signals toward the main reflector.
Cassegrain Feed
203
Redirects signals toward the main reflector, making them appear to originate from the focus.
Subreflector (in Cassegrain Feed)
204
Enables weak signal reception and use of long transmission lines by allowing preamplifier placement near the feed without blocking reflected waves.
Advantages of Cassegrain Feed
205
A truncated cone attached to a circular waveguide, used as a standalone antenna or feed for a parabolic dish.
Conical Horn Antenna
206
Determines antenna gain and is optimized with flare angle for reflector illumination.
Length of Truncated Cone (in Conical Horn Antenna)
