Antenna gain is measured by comparing how much power an antenna concentrates in a specific direction against a known reference. The most common approach uses a calibrated reference antenna and a straightforward power comparison, though several methods exist depending on your accuracy requirements and available equipment. The result is expressed in decibels, typically as dBi (referenced to an ideal isotropic radiator) or dBd (referenced to a standard dipole antenna).
What Antenna Gain Actually Tells You
Gain describes how effectively an antenna focuses energy in a particular direction compared to a reference. It doesn’t mean the antenna adds power. Instead, it redirects energy that would otherwise spread in all directions and concentrates it into a narrower beam. A higher gain number means more energy is focused toward your target.
Two units dominate antenna gain specs. dBi compares your antenna to an isotropic radiator, a theoretical point source that radiates equally in every direction. dBd compares it to a half-wave dipole antenna, which itself has a gain of 2.15 dBi. Converting between the two is simple: subtract 2.15 from a dBi value to get dBd, or add 2.15 to a dBd value to get dBi. This matters because manufacturers sometimes use whichever unit makes their antenna look better on a spec sheet. An antenna listed at 5 dBi is the same as one listed at 2.85 dBd.
The Gain-Transfer Method
The gain-transfer method (also called the gain-comparison method) is the most accessible way to measure antenna gain. You need two things: the antenna you want to test and a reference antenna with a known, calibrated gain value. Standard gain horns are the most common reference antennas for this purpose.
The procedure works like this. First, set up your reference antenna and a source antenna at a fixed distance apart, with the source transmitting a known signal. Measure the received power at the reference antenna. Then swap the reference antenna for the antenna you’re testing, keeping everything else identical: same distance, same transmitted power, same frequency, same cable. Measure the received power again. The gain of your test antenna equals the known gain of the reference antenna plus the difference in received power (in dB) between the two measurements.
This method cancels out most of the variables that would otherwise complicate your calculation. You don’t need to know the exact transmitted power, cable losses, or path loss, because those factors stay constant between the two measurements. What you’re left with is a clean comparison.
The Three-Antenna Method
When you don’t have a calibrated reference antenna, the three-antenna method lets you determine gain using three uncalibrated antennas. You measure the transmitted and received power for each possible pair of the three antennas (pair AB, pair AC, pair BC), giving you three equations with three unknowns. Solving them yields the gain of each antenna individually.
This method is how calibration laboratories establish the gain of reference antennas in the first place. It requires more careful measurement and more time, but it eliminates the need for any pre-calibrated standard. NIST and other standards bodies use variations of this technique combined with extrapolation measurements to calibrate the gain standards that everyone else relies on.
Using the Friis Transmission Equation
If you know the transmitted power, received power, distance between antennas, and the gain of one antenna, you can calculate the gain of the other using the Friis transmission equation. In plain terms, this equation says the received power equals the transmitted power, multiplied by both antenna gains, multiplied by a factor that accounts for signal spreading over distance.
That distance factor is called free space path loss, and it depends on both the physical separation between antennas and the signal’s wavelength. At higher frequencies (shorter wavelengths), the path loss increases for the same physical distance. To use this approach, you measure received power with a known source antenna at a measured distance, then solve for the unknown gain. The math is straightforward, but in practice this method is sensitive to reflections and multipath interference, so your test environment matters a lot.
Getting the Test Environment Right
The biggest source of error in antenna gain measurement isn’t the math or the equipment. It’s reflections. Any signal that bounces off the ground, walls, or nearby objects before reaching the receiving antenna corrupts your measurement. You have two main options for controlling this.
An anechoic chamber is a room lined with radio-absorbing material that eliminates reflections. This gives the cleanest measurements but requires a significant investment. For many professional antenna labs, this is the standard environment. An outdoor range on flat, open ground can also work, particularly if you elevate both antennas on towers to minimize ground reflections, or use a ground-reflection range that deliberately incorporates a single known reflection into the measurement.
Minimum Distance Between Antennas
Your two antennas need to be far enough apart that the test antenna’s radiation pattern has fully formed. Too close, and you’re measuring in the “near field,” where the signal’s amplitude and phase vary in complex ways that don’t represent how the antenna performs at real-world distances. The minimum separation, called the Rayleigh distance or far-field distance, is calculated as 2D² divided by the wavelength, where D is the largest dimension of the antenna’s aperture.
For a dish antenna with a 1-meter diameter operating at 10 GHz (wavelength of 3 cm), the minimum distance is about 67 meters. For a small Wi-Fi antenna at 2.4 GHz, it might be less than a meter. Getting this distance wrong is one of the most common mistakes in gain measurement, and it consistently produces readings that are too low.
Near-Field Scanning
For large, high-gain antennas where the far-field distance would be impractically long (hundreds of meters or more), near-field scanning offers an alternative. A small probe antenna is moved across a grid very close to the test antenna, measuring both the amplitude and phase of the signal at each point. Software then mathematically transforms this near-field data into the far-field radiation pattern and gain.
The probe is typically a simple open-ended waveguide or small horn with a gain roughly 25 to 30 dB below that of the antenna being tested. It scans across a rectangular grid slightly larger than the physical aperture of the test antenna, recording complex (amplitude and phase) data at each grid point. This technique is the standard approach for calibrating large gain standards and is widely used in aerospace and satellite antenna testing. The tradeoff is that it requires more expensive equipment (the probe positioning system and phase-coherent measurement hardware) and more complex data processing.
Equipment You Need
The core instrument for most antenna gain measurements is a vector network analyzer, or VNA. This device sends a signal through one port and measures what comes back or what arrives at another port, capturing both amplitude and phase information. You connect the VNA’s output port to the source antenna via a coaxial cable, and the receiving antenna feeds back into another port. The VNA then gives you a direct reading of the power ratio between the two.
Beyond the VNA, you’ll need:
- Calibrated coaxial cables with known loss characteristics at your operating frequency. Cable losses that aren’t accounted for will directly subtract from your measured gain.
- A reference antenna with a certified gain value if you’re using the gain-transfer method. Standard gain horns are available commercially with calibration certificates traceable to national standards.
- An antenna positioner or turntable if you need to measure gain across different angles rather than just peak gain.
- Absorber material to reduce reflections from nearby surfaces, cables, and mounting hardware.
For simpler or lower-frequency work, a signal generator and spectrum analyzer can substitute for a VNA. You lose phase information, which rules out near-field scanning, but for basic far-field gain-transfer measurements this setup works fine and costs less.
Industry Standards for Antenna Measurement
The primary reference document is IEEE 149-2021, titled “IEEE Recommended Practice for Antenna Measurements.” This is a comprehensive revision of the original 1979 standard and covers procedures for measuring gain, directivity, radiation efficiency, and radiation patterns. If you’re performing measurements for compliance, certification, or any formal purpose, this document defines the accepted methodology. Following its recommended practices also helps you avoid systematic errors that might not be obvious from a simpler description of the measurement process.