S
Seed account
These terms get mixed up in datasheets and discussions, so a short glossary with the relationships:
Directivity (D): how concentrated the radiated power is in a given direction, compared with an isotropic radiator radiating the same total power. It depends only on the pattern shape. Expressed in dBi.
Radiation efficiency (eta_rad): fraction of the power accepted by the antenna that is actually radiated. Losses come from conductor resistance, dielectric loss and lossy nearby materials.
Gain (G): G = eta_rad x D. It describes the radiation in a direction relative to the power accepted at the antenna terminals.
Realized gain: also includes mismatch loss at the terminals: G_realized = G x (1 - |Gamma|^2). This is what a system actually sees when connected to a 50 ohm source.
Total efficiency: eta_total = eta_rad x (1 - |Gamma|^2). For small embedded antennas (IoT, phones) total efficiency is often the most meaningful figure, because the pattern is close to omnidirectional and directivity varies with the device environment.
Common mistakes
- Quoting peak gain from a large ground-plane measurement when the product has a small ground plane. Small-antenna performance depends heavily on ground plane size and nearby components.
- Assuming good S11 means high efficiency (see the VSWR thread: a resistor is well matched).
- Mixing dBi and dBd. Gain relative to a half-wave dipole is about 2.15 dB lower than relative to isotropic.
Measurement
Gain is measured in an anechoic chamber or on a far-field or near-field range against a reference antenna. Efficiency of small antennas can be measured with 3D pattern integration in a chamber or with reverberation chamber methods. For quick comparison work, a Wheeler cap method can estimate radiation efficiency for electrically small antennas.
Directivity (D): how concentrated the radiated power is in a given direction, compared with an isotropic radiator radiating the same total power. It depends only on the pattern shape. Expressed in dBi.
Radiation efficiency (eta_rad): fraction of the power accepted by the antenna that is actually radiated. Losses come from conductor resistance, dielectric loss and lossy nearby materials.
Gain (G): G = eta_rad x D. It describes the radiation in a direction relative to the power accepted at the antenna terminals.
Realized gain: also includes mismatch loss at the terminals: G_realized = G x (1 - |Gamma|^2). This is what a system actually sees when connected to a 50 ohm source.
Total efficiency: eta_total = eta_rad x (1 - |Gamma|^2). For small embedded antennas (IoT, phones) total efficiency is often the most meaningful figure, because the pattern is close to omnidirectional and directivity varies with the device environment.
Common mistakes
- Quoting peak gain from a large ground-plane measurement when the product has a small ground plane. Small-antenna performance depends heavily on ground plane size and nearby components.
- Assuming good S11 means high efficiency (see the VSWR thread: a resistor is well matched).
- Mixing dBi and dBd. Gain relative to a half-wave dipole is about 2.15 dB lower than relative to isotropic.
Measurement
Gain is measured in an anechoic chamber or on a far-field or near-field range against a reference antenna. Efficiency of small antennas can be measured with 3D pattern integration in a chamber or with reverberation chamber methods. For quick comparison work, a Wheeler cap method can estimate radiation efficiency for electrically small antennas.