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Questions and Answers
What is the function of an antenna in addition to radiating or receiving energy?
What type of radiator has equal radiation in all directions?
What type of wire is required to create radiation if charge is moving with constant velocity?
What is the range of frequencies within which the performance of an antenna conforms to a specific standard?
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What is the angular separation between two identical points on an amplitude pattern of an antenna?
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What is the minimum radial distance to the far field region for an antenna of maximum length D?
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What is an aggregate of individual discrete radiating elements (antennas) in an electrical and geometrical arrangement?
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What is the radiation lobe containing the direction of maximum radiation?
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The maximum directivity of an elementary electric dipole with uniform current distribution is
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The ratio of the gain to the directivity of an antenna is defined as the
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The units of Intensity are
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The polarization of an electric field with two components orthogonal to each other and to the direction of propagation, of the same amplitude, and with a 90 deg phase difference is
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The ratio of the available power at the terminals of a receiving antenna to the power density of a plane wave incident on the antenna is referred to as the
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If the current I on an antenna is increased in 10 times, its electric field at the observation point is
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The maximum directivity of a linear dipole as the length of the dipole increases from very small to one wavelength
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The radiation resistance of a very small loop of radius lambda/25, with idealized uniform current distribution is
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The maximum effective area of a very small loop at a frequency of 300 MHz is about
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Antenna array, which radiation maximum radiation is along the axis of the array, is referred to as
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Study Notes
Antenna Fundamentals
- An antenna can radiate or receive energy, and also direct energy in some directions and suppress in others.
Types of Radiators
- Isotropic radiator: radiates equally in all directions.
- Directional radiator: radiates more in some directions than others.
Radiation and Antenna Characteristics
- Radiation can be created by bent wires or infinite length wires, but not smooth wires.
- Bandwidth: the range of frequencies within which an antenna's performance conforms to a specific standard.
- Beamwidth: the angular separation between two identical points on an amplitude pattern of an antenna.
- Far-field region: the region where the radiation pattern of an antenna is applicable.
Antenna Arrays
- An array is an aggregate of individual discrete radiating elements (antennas) in an electrical and geometrical arrangement.
- Major lobe: the radiation lobe containing the direction of maximum radiation.
- Minor lobe: the radiation lobe opposite to the major lobe.
Power Patterns and Directivity
- Directivity: the ratio of the radiation intensity of an antenna to the radiation intensity of an isotropic source.
- Power pattern: a graphical representation of the radiation intensity of an antenna.
- HPBW (Half-Power Beam Width): the angular separation between the points on an amplitude pattern where the power radiation density is half its maximum value.
Antenna Characteristics
- Directivity (Do): the maximum directivity of an antenna, related to its maximum effective area.
- Effective area (Ae): the maximum effective area of an antenna, related to its maximum directivity.
- Radiation resistance (Rr): the resistance of an antenna to radiation.
- Input impedance (Zin): the impedance presented by an antenna at its terminals.
Electric Field and Radiation Density
- Electric field: the strength of the electric field at a given point in space.
- Radiation density: the power per unit area at a given point in space.
- The electric field and radiation density of an antenna are related to its current and distance from the observation point.
Dipole Antennas
- Dipole antennas: a type of antenna consisting of two conductive elements.
- Half-wavelength dipole: a dipole antenna with a length of half a wavelength.
- Radiation resistance of a small dipole: proportional to the square of its length and inversely proportional to the square of the wavelength.
- Directivity of a linear dipole: increases as the length of the dipole increases from very small to one wavelength.
Loop Antennas
- Loop antennas: a type of antenna consisting of a closed loop of conductive material.
- Radiation resistance of a small loop: proportional to the fourth power of its radius and inversely proportional to the square of the wavelength.
- Directivity of a small loop: equal to 1.5, the same as that of an infinitesimal linear dipole.
Antenna Design
- To increase the bandwidth of a wire dipole antenna, its diameter should be increased.
- To decrease the bandwidth of a wire dipole antenna, its diameter should be decreased.
- The maximum directivity of an antenna is related to its maximum effective area and beam solid angle.
Antenna Arrays
- End-fire array: an antenna array with a radiation maximum along the axis of the array.
- Broadside array: an antenna array with a radiation maximum normal to the axis of the array.
- Directivity of a linear, broadside, uniform array of isotropic elements can be calculated as D0 = 2N(d/lambda).
- Directivity of a linear, end-fire, uniform array of isotropic elements can be calculated as D0 = 4N(d/lambda).
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Description
This quiz covers the basics of antennas, including their function and types of radiation. It also explores the concept of radiation in different directions and isotropic radiators.
