How to Read an Antenna Datasheet: VSWR, Return Loss, Gain and Efficiency
Two antennas can both claim “3 dBi gain” and “VSWR < 2.0” and still perform very differently once they are inside your product. A datasheet is only useful if you know what each number measures — and what it leaves out. This guide walks through the figures you will meet on almost every antenna datasheet and shows how to compare them fairly.
VSWR and return loss: how well the antenna accepts power
Both numbers describe the same thing: how closely the antenna’s impedance matches the 50 Ω system it is connected to. When the match is imperfect, part of the power sent to the antenna is reflected back toward the radio instead of being radiated.
VSWR (Voltage Standing Wave Ratio) is written as a ratio such as 1.5:1 or simply 1.5; a perfect match is 1.0. Return loss expresses the same reflection in decibels; a larger number is better. You can convert between them through the reflection coefficient Γ: Γ = (VSWR − 1) / (VSWR + 1), and return loss = −20 log₁₀ Γ.
| VSWR | Return loss | Power reflected | Mismatch loss |
|---|---|---|---|
| 1.2 | 20.8 dB | 0.8% | 0.04 dB |
| 1.5 | 14.0 dB | 4.0% | 0.18 dB |
| 2.0 | 9.5 dB | 11.1% | 0.51 dB |
| 2.5 | 7.4 dB | 18.4% | 0.88 dB |
| 3.0 | 6.0 dB | 25.0% | 1.25 dB |
The table explains why “VSWR ≤ 2.0” is such a common specification: at that point about 89% of the power is still accepted by the antenna, and the loss is only half a decibel. Wideband cellular antennas that must cover 698 MHz all the way to 4 GHz often specify up to 3.0 at the band edges, which is normal for that bandwidth.
Two cautions. First, check the frequency range the VSWR figure applies to — a single number quoted at the centre frequency tells you little about the band edges. Second, a good VSWR only proves that power is not reflected. It does not prove the power is radiated: a lossy antenna, or even a 50 Ω resistor, shows an excellent VSWR. That is why efficiency matters.
Efficiency: how much power actually leaves the antenna
Radiation efficiency is the ratio of radiated power to the power delivered to the antenna, covering conductor and dielectric losses; “total efficiency” also includes the mismatch loss above. It is given as a percentage or in dB: 50% is −3 dB, 70% is −1.5 dB, 80% is about −1 dB.
For small embedded antennas in IoT devices, 40–60% across the cellular low band is a realistic, good result; external antennas typically reach 60–80%. Efficiency is the number that most directly predicts real-world performance, because it feeds straight into the device’s Total Radiated Power (TRP) and Total Isotropic Sensitivity (TIS) — the figures carriers test during certification.
Gain: where the radiated power goes
Gain combines efficiency with directivity — how strongly the antenna concentrates its radiation in a particular direction: gain = efficiency × directivity. An antenna does not create power; higher gain in one direction always means less in another.
- dBi is gain relative to an ideal isotropic radiator; dBd is relative to a half-wave dipole. 0 dBd = 2.15 dBi, so always confirm the unit before comparing two datasheets.
- dBic is used for circularly polarised antennas such as GNSS patches.
- Peak gain is the single best direction on the 3D pattern. Average gain is the mean over the whole sphere and, in dB, is numerically the same as total efficiency.
- A high peak gain is valuable for a fixed point-to-point link. For a tracker, handheld or vehicle device whose orientation is unknown, a smooth omnidirectional pattern with good efficiency usually matters more than a high peak figure.
Regulatory limits are also written in terms of radiated power (EIRP), so a very high-gain antenna may force you to reduce transmit power to stay within the limit.
The rest of the sheet
- Polarization: linear (vertical or horizontal) for most cellular, Wi-Fi and ISM antennas; right-hand circular (RHCP) for GNSS. A mismatch between transmit and receive polarization costs signal.
- Axial ratio: for circularly polarised antennas, how close the polarization is to a perfect circle. Lower is better; 3 dB or less at zenith is a common target for GNSS.
- Impedance: almost always 50 Ω. It must match your radio module and cable.
- For active GNSS antennas: LNA gain, noise figure, supply voltage and current. The noise figure of the first amplifier largely sets the sensitivity of the whole receive chain.
- Ingress protection (IP67, IPX7), operating temperature, cable type and length. Cable loss is real: several metres of thin coaxial cable can cost 1–2 dB or more at cellular frequencies, and considerably more at 5–6 GHz.
Measured where?
The most important question is one the datasheet cannot fully answer: under what conditions were these numbers measured? Figures for an external antenna are normally taken on a defined ground plane or in free space. Figures for an embedded antenna are taken on the manufacturer’s evaluation board. Inside your enclosure — next to a battery, a display, a metal bracket or a different size of PCB — resonance shifts and efficiency changes.
Treat the datasheet as a way to shortlist candidates, then verify the final choice inside your actual product with a passive measurement in an anechoic chamber, followed by active OTA (TRP/TIS) testing. Santa Fe Technologies provides chamber test reports on request and can tune or customise an antenna for your enclosure.
Quick checklist
- Does the VSWR / return loss figure cover every band you need, not just the centre frequency?
- Is efficiency stated, per band, and is it peak or average?
- Is gain in dBi or dBd, and is it peak or average?
- Were the measurements made on a ground plane, in free space or on an evaluation board — and how close is that to your product?
- Are cable and connector losses included?