AntennasAnswers to RF Cafe Quiz #16
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All RF Cafe quizzes would make perfect fodder for employment interviews for technicians or engineers - particularly those who are fresh out of school or are relatively new to the work world. Come to think of it, they would make equally excellent study material for the same persons who are going to be interviewed for a job.
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Note: Many answers contain passages quoted in whole or in part from the text.
1. What does dBi, the most often used unit for antenna gain (or directivity), stand for?
c) Decibels of gain relative to an isotropic radiator
An isotropic radiator is theoretically a point source (dimensionless), and therefore distributes the input power uniformly across the entire spherical volume surrounding it. Directivity concentrates the input power in a preferred direction, leaving less power to be radiated in the not-preferred directions.
2. For which region of space does antenna gain normally apply?
b) Far field
Almost without exception, specified antenna gain refers to the far field. The common sense proof is that if you had two antennas with a gain of, say, 10 dBi, and placed them face-to-face, you would not realize a gain of 20 dB in signal power while the path loss would be negligible.
One exception would be NFC (near field communications) antennas which are designed to use combinations of inductive and/or magnetic coupling to transfer the signal.
Elevation Pattern Azimuth Pattern
c) 1/2-wave dipole
See Antenna Patterns page for more patterns.
4. What is the free space impedance that an antenna "sees?"
d) 120p Ω (≈377 Ω)
Z0 = = 120p Ω (μ0 = 4p 10-7 Henries/m, ε0 = 8.854 x 10-12 Farads/m)
5. What is the name given to the point where the RF input signal interfaces to the antenna?
This is where the signal conductors physically attach to the antenna's radiating element structure.
6. Where is the approximate transition point between near field and far field?
a) λ / (2p)
The explanation is a bit complex, so please see this link on the Conformity site for details.
7. What is an isotropic radiator?
a) An antenna that radiates equally in all directions
8. Which type of antenna would typically have the highest directivity?
Depending on the size, degree of curvature, and the edge properties of the parabolic dish, gain (directivity, which is generally interchangeable with gain for high efficiencies) can be very high.
See Antenna Patterns page for gain ranges of various antenna types.
9. On a center-fed 1/2-wave dipole, where is the voltage potential the highest?
b) At the tips (see diagram to right).
Intuitively, at the tips of the antenna the current has nowhere to flow, so I = 0 there. The 1/2-wave dipole acts like a capacitor where the voltage lags the current by 90°. As a result, the voltage is at maximum magnitude at the tips.
10. Yagi antennas are constructed of which three types of elements?
c) Reflector, driven, and director
The Yagi, or Yagi-Uda, is constructed similar to the one shown to the right. One or more reflector elements are behind the driven element, and one or more director elements are in front of the driven element.
Design your own at the DXZone website.
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