A Practical Guide to Antenna Specifications

Reading an antenna datasheet — what actually matters

Antenna datasheets range from one-page summaries with a handful of specifications to 20-page technical documents filled with S-parameter plots, 3D radiation patterns, and environmental test results. Most of the information is useful only in specific contexts. For system integration, you need to understand five core specifications: frequency range, gain, VSWR, beamwidth, and polarisation. Everything else — efficiency, group delay, phase centre, power handling — matters only when your application demands it.

This guide walks through each specification, explains what it means in practice, and identifies the common ways manufacturers present data that can mislead buyers who do not read carefully.

Frequency range: the fine print

The frequency range specifies the band over which the antenna meets its other specifications — typically VSWR ≤ 2:1. But not all frequency ranges are created equal. A specification of “698-2700 MHz, VSWR ≤ 2:1” means the antenna is designed to work across that entire range, but the gain, pattern, and efficiency may vary significantly from one end to the other.

Check the plots. A responsible datasheet includes VSWR vs frequency, gain vs frequency, and efficiency vs frequency. If the VSWR is 1.5:1 at 800 MHz but 2.0:1 at 2600 MHz, the antenna performs better at lower frequencies. If you are deploying at 2600 MHz, you are getting the worst performance the antenna can deliver within its specified range.

Multi-band vs wideband. A “multi-band” antenna has distinct resonant frequencies where it performs well (e.g., 900 MHz and 2400 MHz), with poorer performance between them. A “wideband” antenna maintains consistent performance across the entire range. If your system operates at two widely separated frequencies, a multi-band antenna is usually more efficient than a wideband design.

Marketing vs engineering. Some datasheets specify frequency range as “typical” rather than “guaranteed.” Others specify the range where VSWR ≤ 3:1 instead of 2:1, making the bandwidth appear wider than it practically is. Always ask for the test standard — and if it is not specified, assume the worst case.

Gain: dBi, dBd, and the confusion

Antenna gain is a ratio of power density in the antenna’s peak direction to that of a reference antenna. The reference matters — and it is not always the same.

dBi (decibels relative to isotropic) uses an isotropic radiator — a theoretical point source that radiates equally in all directions — as the reference. This is the most common specification in modern datasheets and is used throughout this guide.

dBd (decibels relative to a dipole) uses a half-wave dipole as the reference. A dipole has 2.15 dBi of gain, so dBd = dBi – 2.15. An antenna specified as “5 dBd” has 7.15 dBi of gain. If you compare a dBi specification with a dBd specification without converting, you will underestimate the dBd antenna’s actual gain by 2.15 dB.

The peak gain trap. Datasheets typically specify peak gain — the maximum gain in any direction. This is useful for directional antennas but can be misleading for omnidirectional antennas. An omnidirectional antenna with “5 dBi gain” might have a peak gain of 5 dBi at 30° elevation, but only 0 dBi at the horizon. If your link is at the horizon — typical for mobile devices — you are getting 0 dBi, not 5 dBi. Always check the elevation pattern, not just the peak gain number.

Realised gain vs directive gain. Directive gain ignores losses in the antenna’s matching network and conductors. Realised gain includes those losses. A directive gain of 5 dBi with 70% efficiency gives a realised gain of 3.5 dBi. If the datasheet does not specify which gain is reported, assume it is directive gain — the higher, more impressive number. Ask for realised gain if efficiency matters to your application.

VSWR and return loss: two ways to say the same thing

VSWR (voltage standing wave ratio) and return loss both describe how much power is reflected back from the antenna. They are mathematically equivalent — a VSWR of 2:1 corresponds to a return loss of 9.5 dB. The choice of metric is largely conventional: VSWR is more common in legacy specifications, return loss is more common in modern datasheets and VNA measurements.

What is acceptable? VSWR ≤ 2:1 (return loss ≥ 9.5 dB) is the standard specification for most commercial antennas. This means less than 11% of the power is reflected — acceptable for most applications. VSWR ≤ 1.5:1 (return loss ≥ 14 dB) is required for high-performance systems where every decibel matters. VSWR ≤ 3:1 (return loss ≥ 6 dB) is acceptable for wideband antennas covering multiple octaves, where 25% reflection is an unavoidable compromise.

Where is it measured? VSWR is measured at the antenna’s feed point. Once you add a cable, the VSWR at the transmitter end changes. A well-matched antenna with a long, lossy cable may show a good VSWR at the transmitter — but the cable is dissipating the reflected power as heat, not improving the antenna’s performance. Always measure VSWR at the antenna feed point for accurate characterisation.

Beamwidth: the shape of coverage

Beamwidth is the angular width of the antenna’s main lobe, measured between the half-power (-3 dB) points. It is specified separately for the horizontal (azimuth) and vertical (elevation) planes.

Narrow beamwidth = high gain = long range. A 10° beamwidth antenna concentrates energy into a tight beam, achieving 15+ dBi gain. But it must be pointed precisely — a 5° pointing error can reduce gain by 3 dB. These antennas are used for point-to-point links and radar.

Wide beamwidth = low gain = area coverage. A 180° beamwidth antenna (hemispherical pattern) radiates in one direction with moderate gain (3-8 dBi). A 360° azimuth antenna (omnidirectional) radiates in all horizontal directions with low gain (0-5 dBi). These are used for base stations, access points, and mobile devices.

Check both planes. An antenna with 65° horizontal beamwidth and 30° vertical beamwidth is a sector antenna for cellular base stations. An antenna with 360° horizontal and 10° vertical beamwidth is a ceiling-mount Wi-Fi access point. The beamwidth in each plane determines the coverage shape, and both matter for system design.

Polarisation: matching the wave

Polarisation describes the orientation of the electric field as the wave propagates. The three common types are vertical (electric field perpendicular to the ground), horizontal (parallel to the ground), and circular (electric field rotates as the wave propagates).

Linear polarisation match. A vertically polarised antenna transmitting to a horizontally polarised receive antenna will suffer 20-30 dB of polarisation mismatch loss — essentially, the link will not work. Both antennas must use the same polarisation, or the receive antenna must be dual-polarised to capture both.

Circular polarisation advantage. Circularly polarised antennas can receive signals of any linear polarisation with at most 3 dB of mismatch loss. This is why GNSS antennas use circular polarisation — satellites can be at any orientation relative to the receiver. The trade-off is that circularly polarised antennas are more complex and expensive than linearly polarised designs.

Dual polarisation for MIMO. Modern cellular and Wi-Fi antennas often use dual polarisation (typically ±45° slant) to support MIMO. Two co-located antennas with orthogonal polarisations are decorrelated, enabling spatial multiplexing without the physical separation required for single-polarisation MIMO. When evaluating a MIMO antenna, check the isolation between polarisation ports — it should be at least 15 dB.

Environmental and mechanical specifications

• IP rating: IP67 for outdoor (dust-tight, temporary immersion). IP68 for harsh environments (continuous immersion). IP20 for indoor only.
• Operating temperature: -40°C to +85°C is standard for outdoor. Check the radome material — polycarbonate becomes brittle below -20°C; ABS degrades above +60°C.
• Wind load: specified in Newtons at a given wind speed (typically 150 km/h). Important for tower mounting — a large antenna with high wind load may require structural reinforcement.
• Connector type: N-type for outdoor (weatherproof), SMA for indoor, TNC for vibration-prone environments. Ensure the connector matches your radio — adapters add loss and failure points.
• Mounting hardware: pole clamp, wall bracket, or integrated magnet. Check the pole diameter range and the maximum torque specification.

Reading an antenna datasheet is an exercise in understanding what is specified, where it is measured, and what is omitted. The five core parameters — frequency, gain, VSWR, beamwidth, and polarisation — define the antenna’s electrical performance. The environmental specifications define its mechanical limits. Together, they tell you whether the antenna will work in your system, for how long, and under what conditions. Read carefully, ask for the plots, and verify in situ.