Antennas and Transmission Lines Flashcards
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Read the first 6 Antennas and Transmission Lines flashcards as text
A technician measures a Standing Wave Ratio (SWR) of 1:1 at the transmitter end of a transmission line. What does this measurement indicate?
Answer: The impedance of the antenna and transmission line are perfectly matched.
An SWR of 1:1 is the ideal condition, indicating that there are no standing waves on the line. This occurs when the load (antenna) impedance is exactly equal to the characteristic impedance of the transmission line, allowing for maximum power transfer from the transmitter to the antenna with no reflected power.
Which of the following best describes the polarization of an electromagnetic wave?
Answer: The orientation of the electric field vector with respect to the Earth's surface.
The polarization of a radio wave is conventionally defined by the orientation and movement of its electric field (E-field) vector as it travels through space. For example, a vertical antenna produces a vertically polarized wave because its electric field is perpendicular to the Earth's surface.
What is the approximate theoretical feed-point impedance of a quarter-wavelength vertical antenna (Marconi) when installed with a perfect ground plane system?
Answer: 36 ohms
A theoretical quarter-wave vertical antenna over a perfect, infinitely conductive ground plane has a feed-point impedance of approximately 36.5 ohms. This is a fundamental characteristic used in matching the antenna to a transmission line, which often requires a matching network for optimal performance with 50-ohm coaxial cable.
A coaxial cable is specified as having a velocity factor of 0.66. What does this signify?
Answer: The signal travels through the cable at 66% of the speed of light in a vacuum.
The velocity factor (VF) of a transmission line is the ratio of the speed at which a signal propagates through the line compared to the speed of light in a vacuum. It is always less than 1 because the dielectric material in the cable slows the wave's propagation. A velocity factor of 0.66 means the signal travels at 66% of the speed of light.
What is the primary function of a balun in an RF antenna system?
Answer: To interface between a balanced system and an unbalanced system.
The term "balun" is a portmanteau of "balanced to unbalanced." Its primary purpose is to connect a balanced system (like a dipole antenna or twin-lead feedline) to an unbalanced system (like a coaxial cable), ensuring proper current flow and preventing the feedline from radiating.
An antenna is specified as having a gain of 3 dBd. What does this mean?
Answer: The antenna has twice the power gain in its favored direction compared to a half-wave dipole antenna.
The unit "dBd" means "decibels relative to a dipole antenna." A gain of 3 dB represents a doubling of power. Therefore, an antenna with a 3 dBd gain has a radiated power in its direction of maximum radiation that is twice the power of a standard half-wave dipole antenna fed with the same input power.