Antenna reference for directive gain: Which reference antenna is used to define directive gain in antenna engineering?

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:

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).

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