What is the difference between antenna gain, directivity, and efficiency?
Gain and Directivity
Directivity is a theoretical property of the antenna's radiation pattern. It describes how well the antenna concentrates energy in a particular direction regardless of how much total power is actually radiated. A very directive antenna can still have low gain if most of the input power is lost to ohmic heating in the antenna structure.
Gain accounts for the real losses in the antenna: conductor losses (resistive heating in the metallic structure), dielectric losses (heating in any dielectric substrate or radome), and mismatch loss (power reflected from the antenna back to the source). The total antenna efficiency η = ηr × ηc × ηd, where ηr is radiation efficiency, ηc is conductor efficiency, and ηd is dielectric efficiency.
Realized gain further includes the impedance mismatch between the antenna and its feed line: Grealized = G × (1 - |Γ|²), where Γ is the reflection coefficient at the antenna input. For a well-matched antenna (S11 < -10 dB): mismatch loss < 0.5 dB and Grealized ≈ G. For a poorly matched antenna (S11 = -3 dB): mismatch loss = 3 dB.
Frequently Asked Questions
Which do I use in a link budget?
Use realized gain for the most accurate link budget because it includes all losses. If the antenna is well-matched (VSWR < 2:1): use gain (the mismatch loss is < 0.5 dB and often included as a separate line item). Never use directivity in a link budget unless you separately account for all efficiency factors.
How do I measure gain?
Compare the antenna under test to a reference antenna with known gain (gain transfer method): G_DUT = G_ref + (P_DUT - P_ref). Standard gain horn antennas with calibrated gain are commonly used as references. Alternatively, use the three-antenna method with three unknown antennas to determine all three gains.
What efficiency is typical?
Well-designed metallic antennas at microwave frequencies: η > 90% (loss < 0.5 dB). Microstrip patch antennas on lossy substrates: η = 70-90%. Electrically small antennas: η = 10-50% (dominated by radiation resistance vs loss resistance). Low-loss waveguide-fed horns: η > 95%.