Antenna reference for directive gain: Which reference antenna is used to define directive gain in antenna engineering?
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AHalf-wave dipole
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BElementary doublet
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CIsotropic
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DInfinitesimal dipole
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EShort monopole over ground
Answer
Correct Answer: Isotropic
Explanation
Introduction / Context:Directive gain compares an antenna's maximum radiation intensity to that of a reference antenna with the same total radiated power. Understanding the reference standard is key to correctly interpreting gain specifications in datasheets and link budgets.
Given Data / Assumptions:
- Directive gain (not power gain) is defined relative to an isotropic radiator.
- The isotropic radiator radiates uniformly in all directions (4π steradians).
Concept / Approach:By definition, directive gain D = U_max / (P_rad / (4π)), where U_max is the maximum radiation intensity and P_rad is total radiated power. The denominator corresponds to an isotropic radiator's uniform intensity. Hence, the isotropic antenna is the reference for directive gain.
Step-by-Step Solution:
Recall that dBi (decibels relative to isotropic) is the common unit for gain.A half-wave dipole's directivity is about 1.64 (2.15 dBi), which illustrates that the dipole is not the reference; it has gain above isotropic.Verification / Alternative check:Standards and textbooks define directive gain with the isotropic reference, while power gain may also be reported relative to isotropic (dBi) or to a dipole (dBd = dBi − 2.15 dB).
Why Other Options Are Wrong:
Half-wave dipole / elementary doublet / infinitesimal dipole: real or canonical antennas, but not the reference for directive gain.Common Pitfalls:
Confusing dBi (isotropic reference) with dBd (dipole reference).Final Answer:
Isotropic