How to Perform Antenna Pattern Measurements with Anechoic Chambers and Field Tests

Measuring the radiation pattern of an antenna is essential for understanding its performance, verifying design specifications, and ensuring regulatory compliance. Whether for a small patch antenna on a consumer device or a large satellite reflector, the radiation pattern reveals how electromagnetic energy is distributed in space. Two primary methodologies dominate the field: anechoic chamber measurements, which provide a controlled, free-space-like environment, and field tests, which capture real-world operating conditions. Each approach has unique strengths and limitations, and the choice between them depends on factors such as accuracy requirements, antenna size, frequency range, budget, and the need for environmental realism. This article provides an authoritative, in-depth look at both techniques, covering fundamental concepts, step-by-step procedures, equipment considerations, and advanced topics, enabling engineers and technicians to select and execute the most appropriate method for their antenna measurement tasks.

Fundamentals of Antenna Radiation Patterns

Before diving into measurement methods, a solid grasp of radiation pattern fundamentals is necessary. An antenna radiation pattern is a graphical representation of the relative power radiated or received as a function of direction in space. Typically plotted in polar or rectangular coordinates, patterns are measured in two principal cuts: the azimuth plane (horizontal) and the elevation plane (vertical). Key parameters derived from the pattern include:

Gain – The ratio of the power radiated in a given direction to that of an isotropic radiator, often expressed in dBi.
Directivity – A measure of the focusing ability, independent of losses.
Beamwidth – The angular width at half-power points (-3 dB), typically specified for both main beam and sidelobes.
Sidelobe level – The relative power of the highest sidelobe compared to the main beam, critical in interference management.
Front-to-back ratio – The ratio of power radiated forward versus backward, important for directional antennas.
Cross-polarization discrimination – The ability to reject orthogonal polarization, vital in dual-polarized systems.

Accurate measurement of these parameters requires careful control of the test environment. The antenna under test (AUT) must be placed in the far-field region of the source antenna, where the radiated wave approximates a plane wave. The far-field distance is defined as R ≥ 2D²/λ, where D is the largest dimension of the AUT and λ is the wavelength. Violating this condition leads to amplitude and phase errors that distort the measured pattern. Both anechoic chambers and field test sites are designed to satisfy far-field criteria, though they do so in different ways.

Anechoic Chamber Measurements in Depth

An anechoic chamber is a room lined with radio-frequency absorbing material (RAM) to minimize reflections from walls, ceiling, and floor. The interior simulates free-space conditions, allowing repeatable, high-precision pattern measurements without external interference. Chambers come in several configurations:

Fully Anechoic Chambers (FAC)

FACs are entirely lined with RAM. They are the gold standard for antenna measurements, especially for low-gain antennas and testing of polarization purity. The quiet zone—the volume where reflections are negligible—must encompass the AUT. Such chambers are used for frequencies ranging from VHF to millimeter-wave bands.

Semi-Anechoic Chambers (SAC)

SACs have a conductive floor to simulate a ground plane, common in EMC testing but less ideal for free-space antenna patterns. For antenna work, a SAC can be used if the ground-reflected path is controlled via time-domain gating or specialized absorber treatment.

Compact Range Chambers

For larger antennas, a compact range uses a reflector to transform a spherical wave from a feed horn into a plane wave within a short distance. This allows far-field measurements in a chamber much smaller than the traditional far-field distance. Compact ranges are widely used for satellite antennas and phased arrays.

Equipment and Setup for Chamber Measurements

A typical chamber measurement system comprises:

Positioner – A motorized turntable or robotic arm that rotates the AUT through azimuth and elevation angles.
RF source and receiver – A vector network analyzer (VNA) or spectrum analyzer with tracking generator.
Reference antenna – A calibrated standard gain horn for absolute gain measurements.
Data acquisition computer – Software that controls the positioner, captures S-parameters, and computes the pattern.

Calibration Procedure

Before measurement, system calibration is critical. A typical one-port calibration at the VNA port removes cable losses and phase offsets. For gain measurement, the reference antenna is substituted for the AUT and its known gain is used to derive the absolute gain of the AUT using the substitution method. The calibration also sets the reference level for relative pattern measurements.

Measurement Steps

Mount the AUT on the positioner in the chamber’s quiet zone, oriented at the desired polarization.
Connect the AUT to the VNA port 1 (transmit) and position a fixed receive antenna (probe) at port 2, or vice versa for reciprocal measurement.
Set frequency, sweep settings, and IF bandwidth for adequate dynamic range.
Perform calibration (through, reflection, isolation, and reference antenna measurement if needed).
Execute a pattern cut: rotate the AUT in azimuth (0° to 360°) while recording transmission coefficient (S21) at each angle. Repeat for multiple elevation angles for a 3D pattern.
Apply polarization measurements by rotating the AUT or probe to capture co-polar and cross-polar components.
Post-process data: normalize to maximum, apply gain reference, and export to plotting tools.

Advantages and Limitations of Chamber Measurements

Advantages:

Controlled, repeatable environment free from weather, interference, and multipath.
High dynamic range (often >60 dB) enabling measurement of low sidelobes.
Indoor operation allows year-round testing.
Ability to measure polarization and phase accurately.

Limitations:

Cost and footprint: large chambers for high-gain antennas are expensive.
Absorber performance degrades at very low frequencies (below 100 MHz) due to thickness limits.
Cannot replicate real-world effects like ground reflections or installation platform interactions.
For extremely large antennas (e.g., radar arrays hundreds of wavelengths across), even compact ranges become impractical.

For more on chamber design, see the Keysight application note on antenna measurements.

Field Testing Techniques

Field tests measure the radiation pattern in the actual operating environment. They are indispensable for large antennas that cannot be moved (e.g., base station towers, satellite dishes, parabolic reflectors), and for verifying performance after installation. Field testing also captures interactions with nearby structures and ground effects.

Types of Field Test Ranges

Open Area Test Site (OATS)

An OATS is a flat, open area with controlled ground properties. A conductive ground plane (e.g., metal mesh) is often used to provide a stable reflection. The AUT is mounted on a mast, and a test source is placed at a distance satisfying the far-field condition. The test source can be raised to different heights to measure elevation cuts. OATS is common for EMC and cellular base station antenna measurements.

Elevated Range

Both the AUT and the test source are placed on towers or rooftops to minimize ground effects. The line-of-sight path is above intervening obstructions. This method is typical for microwave link antennas and radar systems. Pattern measurement is often performed by rotating the AUT while the distant source transmits a continuous wave signal.

Drone-Based Measurement

For large antennas such as phased-array radars or satellite dishes, drones carrying a RF probe or a small transmitter can fly through the antenna’s far-field region. The drone’s position is tracked via GPS or an optical system, and the received signal is correlated with position to map the pattern. This technique has gained popularity for on-site testing of large installed antennas. See the National Physical Laboratory guide on drone measurements for practical advice.

Field Test Measurement Steps

Site selection and preparation: Choose a location with minimal obstacles, multipath, and radio interference. Measure the distance from the AUT to the test source and ensure far-field conditions. For ground reflection control, use a clear line-of-sight and optionally install a ground screen.
Equipment setup: Place the transmit antenna at a fixed location (often on a small tower). Connect it to a signal generator. The AUT remains in place. Connect the AUT to a spectrum analyzer or receiver via a low-loss cable. For gain measurement, a reference antenna is measured first.
Calibration: Perform a path loss calibration using a known reference antenna. This accounts for cable losses and free-space loss, allowing absolute gain derivation. If using a drone, calibrate the drone’s RF probe off-line.
Pattern data acquisition: Rotate the AUT in azimuth (or move the drone around it) while recording received power. For elevation patterns, tilt the AUT or move the test source vertically. Use a data logger that time-stamps angle and power readings.
Post-processing: Apply corrections for ground reflections (image method), atmospheric loss, and cable movement effects. Subtract the reference to get absolute gain. Plot the pattern and extract key parameters.

Challenges and Mitigations

Multipath: Reflections from buildings, trees, or the ground can corrupt the pattern. Mitigate by using time-domain gating (if using a VNA) or narrowband CW signals with strong line-of-sight dominance. Elevated ranges reduce multipath.
Weather: Rain, fog, and temperature variations affect signal propagation and equipment. Perform tests in stable weather and protect electronics.
Interference: Co-channel interference from other transmitters can be avoided by using a clear frequency channel, bandpass filters, and differential measurement methods.
Ground reflection nulls: In OATS, the reflection from the ground interferes with the direct path, causing ripples in the pattern. Using a height-scanning technique (varying the AUT height) can average out these effects.

For a detailed comparison of field test methods, refer to the IEEE Antennas and Propagation Magazine article on outdoor RCS and antenna measurements.

Comparing Chamber and Field Measurements

Choosing between a chamber and field test depends on trade-offs that must be evaluated per project.

Factor	Anechoic Chamber	Field Test
Accuracy and repeatability	Excellent (typical ±0.2 dB)	Moderate (±1 dB or more due to environment)
Cost	High (chamber construction and RAM)	Lower (requires range and travel)
Antenna size	Limited by chamber dimensions	No practical limit
Real-world effects	Not captured	Captured (interactions, ground, obstacles)
Speed	Fast (automated indoors)	Slower (manual setup, weather delays)
Low-frequency capability	Poor (absorber size)	Good (long wavelengths over open ground)
Security	Controlled, no emissions outside	Emissions may be intercepted

In practice, the best approach often combines both: use chamber measurements for design validation and precise characterization, then perform field tests for final acceptance and installation verification. Many standards (e.g., IEEE, ETSI) require either method depending on the application.

Advanced Considerations

Near-Field to Far-Field Transformation

When far-field conditions cannot be met within a chamber (e.g., for large phased arrays), near-field scanning offers a solution. A probe measures the amplitude and phase of the field over a plane, cylinder, or sphere close to the AUT. Mathematical transformation (via Fourier optics) computes the far-field pattern. This method is widely used for satellite antennas and is the subject of Rohde & Schwarz application notes on near-field testing.

Time-Domain Gating in Field Tests

Using a VNA with time-domain capability allows separation of the direct signal from reflections. By gating the time response, the measured pattern can be cleaned of multipath artifacts. This technique dramatically improves accuracy in open-area test sites.

Phased Array and MIMO Antenna Measurement

Modern antennas with beamforming require multiple pattern measurements for different beam states. Over-the-air (OTA) testing in a chamber or field is essential. Advanced chambers with multi-probe arrays can simultaneously capture 3D patterns of phased arrays, as described in Microwave Journal articles on OTA testing.

Conclusion

Antenna pattern measurement is a critical practice in RF engineering, demanding careful consideration of the test environment. Anechoic chambers provide controlled, precise, and repeatable results ideal for design characterization, while field tests reveal the real-world behavior of installed antennas. Each method has its place: chambers for development and certification, field tests for commissioning and troubleshooting. Engineers should understand the trade-offs in cost, accuracy, size, and environmental realism. By combining both approaches when feasible, and leveraging advanced techniques like near-field scanning and time-domain gating, one can obtain comprehensive antenna performance data that ensures optimal system operation. As antennas continue to evolve with 5G, satellite communications, and radar, mastering these measurement methods remains an indispensable skill.