How to Read an Antenna Datasheet: Gain, VSWR, Impedance, Pattern

Datasheets in four numbers

Most antenna datasheets can be summarised in four primary numbers: gain, VSWR, impedance, and the radiation pattern. Here is what each actually tells you and where the small print quietly hides.

Gain

What it means

Gain (in dBi or dBd) describes how efficiently an antenna concentrates energy in a preferred direction compared to a reference antenna. Higher is not automatically better. A high-gain antenna typically has a narrow vertical beamwidth, which is great on a tall mast with a predictable path, but awkward on a mobile vehicle that tilts in traffic.

What to check

Ask: what beamwidth does this gain imply, and does that suit my application? A 6 dBi omni with a 12-degree vertical beamwidth is excellent at 20 metres height; terrible at 3 metres on a van roof.

VSWR

What it means

VSWR measures impedance match between the antenna and the feeder. A perfect match is 1:1. Most quality professional antennas specify 1.5:1 or better across their working band.

What to check

Look not only at the headline figure but at the curve: does it hold across the full working range, or only at the centre frequency? Ask: what is the worst-case VSWR at the edges of the band you will actually use?

Impedance

Nearly all professional UHF and VHF antennas specify 50 ohms, to match standard RF equipment. Exceptions are rare and usually intentional. A non-50 ohm antenna will need a matching network, which is a design consideration in its own right. If a datasheet does not state impedance, ask before you order.

Radiation pattern

What it means

The radiation pattern tells you where the energy actually goes. Two plots matter: the horizontal (azimuth) pattern and the vertical (elevation) pattern.

What to check

A vertical omni with good azimuth coverage can still have a narrow elevation beam that aims its energy somewhere unhelpful. Ask: does this pattern match the geometry of my install? Datasheets that only show one plane are hiding something.

Small print that earns its keep

• Polarisation: Specified how? Vertical for most applications, RHCP for GPS, slant for cellular.
• Power handling: Watts continuous, not peak, unless otherwise stated.
• Connector type: N-type, TNC, SMA, 7/16 DIN. Know before you order.
• Temperature range: Do the operating and storage ranges cover your site?
• Dimensions and weight: Relevant for mounting and wind loading calculations.
• Certification: Any relevant markings (CE, UKCA, EN standards).

A one-minute sanity check

Before ordering, read the datasheet looking for three things:

1. Does the working frequency range include all the bands the application needs, with headroom?
2. Is the VSWR specified and plotted across that full range?
3. Does the pattern fit the install geometry and the user's movement profile?

If any answer is uncertain, our engineering team will review the datasheet with you on request. Email sales@renair.co.uk.