Future-Ready Near-Field Technique Will Transform Antenna Measurements
What you’ll learn:
- The importance of measuring an antenna’s radiation properties.
- How an innovative near-field technique can address measurement issues in electronically steered arrays.
Antennas are a necessity in any wireless communications or sensing device. Modern antennas in radar, cellular, or satellite communications come integrated with circuitry to control transmission direction, interference, eavesdropping protection, etc. This enables a radar system to clearly determine the location of targets and improves the capacity of communication links and networks.
These antennas are also called electronically steered arrays (ESAs). They’re arrays of passive antenna elements, e.g., patch antennas, combined with a feed network (or beamformer) that enables control of amplitude and phase per array element. Thus, RF transmissions can be steered into desired directions. Implementations vary significantly in frequency range, power characteristics, array design, etc. (Fig. 1).
Measuring an antenna’s radiation properties is important; for example, to comply with regulations or system specifications, or to verify the system’s mission-readiness. The measured signal propagates over the air. Compared to measuring conductively, i.e., tapping the signal using probe tips or connectors and cables or waveguides, radiated measurements are usually more complex and costly. That’s because they need measurement antennas and anechoic chambers to reduce interference from other signals which also introduce more measurement uncertainty.
Measuring spatial characteristics requires rotating the antenna under test or the probing antenna, using positioners, turntables, or robots. Unfortunately, this comes with mechanical complexities, such as minimizing position uncertainty and phase variations when moving cables.
Most antennas are meant to overcome a significant distance between transmitter and receiver. As a result, there’s interest in determining characteristics for such distances in what’s known as the “far-field zone.”
Figure 2 shows three common arrangements to obtain far-field characteristics: measuring with sufficient separation between the antenna and the probe; collimating the electromagnetic waves with a reflector to create far-field conditions at closer range; or scanning a surface close to the antenna, in its so-called “near field,” and transforming the data to the far-field characteristics.
These arrangements have their individual tradeoff of size, complexity, speed, and cost.1 Generally, however, their overall economics are challenging, especially for larger antennas, and may prevent more thorough testing.
Innovative Near-Field Measurement Technique to the Rescue
Researchers at NI and TU Dresden found a disruptive approach that enables the test engineer to measure an antenna’s radiation with very compact equipment, at a fraction of the time required with conventional methods, and with the same accuracy that’s achievable with these incumbent methods.4,6 The setup’s compactness would make it ideal for testing ESAs in the field. One could avoid putting large devices in larger chambers or taking the antennas out of these devices for measurements.
An array of probe antennas is placed very close to the antenna under test (Fig. 3). Bringing the antenna and probe close together makes the setup small. Again, “close” means “near-field.”
Figure 4 helps illustrate the benefit of being in the near field—e.g., for an antenna with a diameter of 1 m and a frequency of 1 GHz, the radiated near field, where conventional near-field scanners would operate, begins about 1.13 m away from the antenna, and the far field at 6.7 m. What’s more, in the experiments, very good results were obtained with the probe array positioned in the reactive near field, that is, with an even more compact setup.
In contrast to near-field scanners, the novel technique requires no mechanical movement. This greatly simplifies the mechanics and avoids early wear-and-tear of cables, significantly improving operational stability and measurement repeatability. Also, avoiding movement leads to fast measurements, especially those of radiation patterns and the spatial distribution of other RF parameters.
A linear mapping transforms near-field data to far-field equivalents. To do that, one needs at least as many probes as there are array elements in the ESA. One may measure the field on all of these probes simultaneously using just as many instrumentation channels. This is the quickest route, but only practical for a small number of elements.
One may switch through the probes using a single instrument channel. This is easiest and enables accurate measurements of the antenna’s transmission characteristics. To make a single-channel setup scale well to various array sizes would require combining the near-field probe array with a beamforming network. This gives best control of the fields generated by the probe array and, thus, also enables test engineers to measure an ESA’s receive characteristics.
Proof-of-Concept
We demonstrated the measurement approach in a proof-of-concept based on commercial mmWave communications technology.4 Figure 3 shows the setup with a 64-element array of passive probes and a 64-element dual-polarized ESA featuring NXP MMW9014K beamformers. Another ESA that we measured had 16 single-polarized elements and used Anokiwave AWMF-0108 beamformers. The distance between the arrays was 5 mm.
For comparison, we made measurements directly in the far field in a Bojay anechoic chamber using NI RFmx and OTA measurement software.2 In all measurements, we used an NI PXIe-5831 mmWave vector signal transceiver (VST) with 18 ports.
Figures 5 through 7 show how well our measurements correlate with conventional methods. Figure 5 shows this for the 64-element ESA’s transmit patterns for two different beam configurations at 25.875 GHz.4 Figure 6 illustrates measurement of receive patterns of the 16-element ESA at 28 GHz. Here, we also compare implementations with four signal generators and one that uses a single generator plus beamformer on the probes’ side.7,8
Figure 7 shows wideband modulated measurements of the error vector magnitude (EVM) for the same 16-element array at 28 GHz, based on a 3GPP 5G NR frequency range 2 signal.5 Results for three different beam directions are in good agreement with those from conventional methods.
For these results, the near-to-far-field mapping parameters need to be calibrated before any measurements can be transformed into far-field characteristics. Once known and stored, the parameters are valid for processing measurements over a long period. This is because the mapping depends on those “passive” properties of the antenna design that shape the electromagnetic field over the air (i.e., mechanical layout and choice of material of the parts facing the radiated scenery). Thus, the mapping parameters are independent of the effects of any active circuitry in the antenna under test.
Calibration of the mapping is described in detail in References 4 and 6. It requires measuring a reference ESA one time. Unlike a “golden unit”-type approach, by design of the process, we don’t need to worry about longer-term drift or it failing over time. Still, the mapping parameters depend on the ESA design.
Consequently, this near-field probe array method is great if measurements for a particular ESA design need to be made repeatedly. Such situations arise, for example, in automated validation during development, in production testing, and to verify mission-readiness over the lifetime of the system.
A New Antenna Measurement Technique
Antenna arrays are an important part of modern communication and radar systems. For applications at higher frequencies, they’re indispensable.
In this article, we described a novel near-field antenna measurement approach that leverages NI’s PXI platform and VST instrumentation. The new approach promises test engineers can use compact measurement setups that avoid moving parts for ease of operation. This suits it particularly to perform measurements repeatedly for extensive test coverage in validation, production, or in the field.
By avoiding movement during measurements, the technique is very fast. Of course, the method works for CW-based radiation pattern measurements as well as under modulation conditions.
References
1. A. Laundrie, “Evaluating Antenna Testing Options,” Microwave Journal, vol. 68, issue 1, pages 18-28, January 2025.
2. Achieve Ultrafast 5G mmWave OTA Validation - NI
3. Instrument Innovations for mmWave Test - NI
4. M. Laabs, E. Zakutin, M. Obermaier, D. Plettemeier, E. Bürger, T. Deckert, V. Kotzsch, M. V. Bossche, “OTA Near-Field Measurement Approach for Electrically Steered Antenna Arrays,” IEEE Transactions on Antennas and Propagation, vol. 72, issue 1, pages 44-53, January 2025.
5. M. Lohning, T. Deckert, V. Kotzsch, and M. V. Bossche, "A Novel OTA Near-field Measurement Approach Suitable for 5G mmWave Wideband Modulated Tests,” IEEE MTT-S Int. Microw. Symp. Dig., June 2022.
6. M. Obermaier, T. Deckert, M. Laabs, D. Plettemeier, “Compact mmWave Measurement System for Array Antennas in a Production Environment,” International Journal on Microwave and Wireless Technology, January 2025.
7. M. Obermaier, J. Lange, T. Deckert, M. V. Bossche, D. Plettemeier, “RX Characterization of mmWave Antenna Arrays using an Active Probe Array Structure,” Proc. 54th Eur. Microw. Conf. (EuMC), 2022.
8. T. Deckert, L. Kaimann, M. Piana, M. Vanden Bossche, M. Obermaier, D. Plettemeier, “Beamforming Multi-Element Near-Field Probe Scales to Measuring Large Phased Arrays,” to appear in Proc. German Microwave Conference (GeMiC), March 2025.