short quiz 3 Flashcards
If a parallel-wire transmission line is left ____, the electric and magnetic fields escape from the end of the line and radiate into space.
open
The radiation from a transmission line can be greatly improved by _____ the transmission line conductors so that they are at a right angle to the transmission line.
bending
The magnetic fields no longer cancel and, in fact, aid one another. The electric field spreads out from conductor to conductor. The result is an ______.
antenna
_________ occurs if the segment of transmission wire converted to an antenna is one-quarter wavelength long at the operating frequency.
optimum radiation
it can be defined as an electrical conductor or system of conductors used either for radiating electromagnetic energy or for collecting electromagnetic energy
antenna
An antenna is basically a _______ that converts electrical alternating current oscillations at a radio frequency to an electromagnetic wave of the same frequency
transducer
For transmission of a signal, radio-frequency electrical energy from the transmitter is converted into ___________ by the antenna and radiated into the surrounding environment (atmosphere, space, water).
electromagnetic energy
________ occurs when the electromagnetic signal intersects the antenna, where the electromagnetic energy is converted into radio-frequency electrical energy and fed into the receiver.
Reception
In two-way communication, the same antenna can be and often is used for both ______ and _______.
transmission, reception
A common way to characterize the performance of an antenna is the _______, which is a graphical representation of the radiation properties of an antenna as a function of space coordinates.
radiation pattern
The simplest pattern is produced by an idealized antenna known as the ________.
isotropic antenna
An isotropic antenna, also called an _________, is a point in space that radiates power in all directions equally.
omnidirectional antenna
The actual radiation pattern for the isotropic antenna is a _______ with the antenna at the center.
sphere
The electric and magnetic fields radiated from an antenna form the ___________, and this field is responsible for the transmission and reception of electromagnetic energy through free space.
electromagnetic field
An antenna, however, is also part of the electrical circuit of a transmitter (or a receiver); because of its distributed constants, it acts as a circuit containing ________, _________, and _______.
inductance, capacitance, resistance
The electric and magnetic fields about the antenna are in ______ and ______quadrature.
space, time
The region in space in which the reactive component of the field predominates is called the ___________, and beyond this region is the radiating field predominates.
reactive-near field region
That region in which the radiating field predominates is further subdivided into the _________ and the _____.
radiating near-filed region
radiating far-field region
what are the three regions that composed the space surrounding the antenna?
reacting near-field
radiating near-field
radiating far-field
Reactive near-field region
For a very short dipole, or equivalent radiator, the outer boundary is commonly taken to exist at a distance ________ from the antenna surface.
𝑹 =𝝀/𝟐𝝅
For an antenna focused at infinity, the radiating near-field region is sometimes referred to as the _______ on the basis of analogy to optical terminology
Fresnel region
The ______ on an antenna, however, moves at a velocity considerably less than that of the radiated energy in free space because the antenna has a dielectric constant greater than that of free space
RF energy
it increases the dielectric constant and lowers wave velocity.
stray capacitance
The change in velocity resulting from stray capacitance is called _______ because the ends of the antenna are made farther apart electrically than they are physically.
end effect
End effect is counteracted by making the physical length about ____ percent shorter than the electrical length,
5
𝑳 =𝟎. 𝟗𝟓𝒄/𝟐𝒇
This formula is accurate for all practical purposes in
determining the physical length of a _________
antenna at the operating frequency.
half-wavelength
The half-wave antenna is the (longest,shortest) resonant length of antenna.
shortest
The purpose of the antenna is to dissipate as much energy as possible in the form of _________. The energy dissipated by the radiation resistance, therefore, is the useful part of the total power dissipated.
radiation
In the halfwave antenna, the radiation resistance is (large,small) compared to the ohmic resistance, and most of the available energy is radiated.
large
Because the half-wave antenna has different conditions of voltage and current at different points, and because impedance is equal to the voltage across a circuit divided by the current through it, the _____ will vary along the length of the antenna.
impedance
The impedance of the half-wave antenna usually is considered to be the impedance as seen by the transmitter at the input terminals. This impedance consists of both ______ and ____.
resistance
reactance
If the antenna is cut to a length of exact resonance, the reactance is _____ and the impedance is purely resistive
zero
if the antenna is longer or shorter than resonance, reactance is _____.
present
When the antenna is made (longer, shorter), capacitive reactance is present; when the antenna is made (longer,shorter) inductive reactance is present.
shorter
longer
If the transmitter is feeding a __________, a power loss is caused by the reactive component of the antenna impedance.
nonresonant antenna
If the frequency of the transmitter is changed, the __________ of the antenna also changes.
electrical length
If the frequency is made somewhat (lowered,higher), the electrical length is made (greater,shortened), and inductive reactance is added to the impedance.
higher
greater
If the frequency is (lowered,highrer), the electrical length is (greater,shortened), and capacitive reactance is added to the impedance.
lowered
shortened
An __________or _________ is defined as a mathematical function or graphical representation of the radiation properties of antenna as a function space coordinates.
antenna radiation pattern
antenna pattern
what are the radiation properties
power flux density
radiation intensity
field strength
directivity phase or polarization
A trace of the received electric (magnetic) field at a constant radius is called the amplitude _______.
field pattern
a graph of the spatial variation of the power density along a constant radius is called an amplitude ________.
power pattern
it (in linear scale) typically represents a plot of the magnitude of the electric or magnetic field as a function of the angular space.
field pattern
it (in linear scale) typically represents a plot of the square of the magnitude of the electric or magnetic field as a function of the angular space.
power pattern
it (in dB) represents the magnitude of the electric or magnetic field, in decibels, as a function of the angular space
power pattern
HPBW means …
half Power Beamwidth
Various parts of a radiation pattern are referred to as _____.
lobes
it is a “portion of the radiation pattern bounded by regions of relatively weak radiation intensity.”
radiation lobe
it (also called main beam) is defined as “the radiation lobe containing the direction of maximum radiation.” In some antennas, such as split-beam antennas, there may exist more than one of this.
major lobe
it is any lobe except a major lobe. All the lobes with the exception of the major can be classified as ________
minor lobe
it is is “a radiation lobe in any direction other than the intended lobe.” Usually it is adjacent to the main lobe and occupies the hemisphere in the direction of the main beam.
side lobe
it is “a radiation lobe whose axis makes an angle of approximately 180◦ with respect to the beam of an antenna.” Usually it refers to a minor lobe that occupies the hemisphere in a direction opposite to that of the major (main) lobe.
back lobe
what are the three electric field component for an amplitude pattern of an antenna
𝐸𝑟, 𝐸𝜃, 𝐸∅
In the far field, the radial __ component for all antennas is zero or vanishingly small compared to either one, or both, of the other two components.
𝐸𝑟
it is defined as “a hypothetical lossless antenna having equal radiation in all directions.” Although it is ideal and not physically realizable, it is often taken as a reference for expressing the directive properties of actual antennas.
isotropic radiator
it is one “having the property of radiating or receiving electromagnetic waves more effectively in some directions than in others. This term is usually applied to an antenna whose maximum directivity is significantly greater than that of a half-wave dipole.”
directional antenna
it is defined as one “having an essentially nondirectional pattern in a given plane (in this case in azimuth) and a directional pattern in any orthogonal plane (in this case in elevation).”
Omnidirectional antenna
it is defined as “the plane containing the electric-field vector and the direction of maximum radiation,”
E-plane
it is defined as “the plane containing the magnetic-field vector and the direction of maximum radiation.”
H-plane
The measure of a plane angle is a _____. One _____ is defined as the plane angle with its vertex at the center of a circle of radius r that is subtended by an arc whose length is r.
radian
Since the circumference of a circle of radius r is C = 2𝜋r, there are __ rad in a full circle.
2𝜋
The measure of a solid angle is a _________. One ______ is defined as the solid angle with its vertex at the center of a sphere of radius r that is subtended by a spherical surface area equal to that of a square with each side of length r.
steradian
Since the area of a sphere of radius r is A = 4𝜋r2, there are ____ sr in a closed sphere
4𝜋
The ______ of a pattern is defined as the angular separation between two identical points on opposite side of the pattern maximum.
beamwidth
One of the most widely used beamwidths is the _______, which is defined by IEEE as: “In a plane containing the direction of the maximum of a beam, the angle between the two directions in which the radiation intensity is one-half value of the beam.”
Half-Power Beamwidth (HPBW)
Another important beamwidth is the angular separation between the first nulls of the pattern, and it is referred to as the ______.
First-Null Beamwidth (FNBW).
