Reducing test times in antenna and RCS applications

Antenna technology is in a constant state of change as performance requirements get tougher and systems become more complex. This has two important consequences. First, more testing - and more test data - is required to validate and verify system performance. Second, that testing must be completed in less time and at lower cost to help ensure timely and profitable completion of in-spec systems.

This dynamic and demanding situation suggests the need for test tools that provide new capabilities and offer technical and economic benefits. One such tool is a new class of standalone microwave receiver that is based on a high-performance network analyser design.

On the technical side it can reduce test times, increase test coverage, boost productivity and contribute to improved system quality.

This article looks at the challenges of modern antennas, describes the new microwave receiver and demonstrates faster measurement times in near-field, far-field and radar cross-section (RCS) example scenarios.

Ongoing changes in testing requirements are driven by changing technologies available to antenna designers - and by the resulting higher-performance designs that are enabled by these new technologies.

For example, the advent of active transmit/receive modules provides new capabilities in shaping the amplitude and phase distribution of microwave energy across the aperture of an antenna. This has enabled not only higher-performing antenna designs but also the ability to dynamically change antenna performance by changing the transmit/receive module states.

As designers apply these new technologies, the resulting antenna designs are becoming increasingly complex. With greater complexity comes a need for greater quantities of test data to completely characterise antenna performance.

For example, thorough testing of near-field characteristics requires an imposing number of measurement points:

• Simple case: Three test ports, one polarisation, 64 electronic beam states and five frequencies in a 100x100 matrix yields 9.6 million test points;
• Complex case: Three test ports, one polarisation, 256 electronic beam states and 62 frequencies in a 100x100 matrix totals 476.16 million points.

The numbers are only slightly less daunting for far-field and RCS measurements. In all cases, it’s clear that faster measurement speeds will have a profound effect on total test times.

Of course, the potential for improvement in simple cases may be limited by factors such as probe and positioner velocities.

In complex cases, which are the trend, the advantages over existing solutions are impressive.

Before the 1980s, test engineers used dedicated microwave receivers for antenna test applications. In 1985, some began using microwave network analysers as the receiver: new technology brought greater stability, accuracy, repeatability and reliability to that generation of instrumentation.

What was novel and innovative in the ’80s has since become common practice in many antenna test facilities. Recently, the industry seems to have come full circle: high-performance network analysers have become part of dedicated microwave receivers designed specifically for antenna/RCS measurements.

One example is the Agilent N5246A microwave receiver, which is derived from the recent PNA-X family of vector network analysers. The new receiver lacks the sources, couplers and test ports of the VNA.

However, it includes five simultaneous receiver channels, data acquisition speeds of up to 400,000 data points per second on each of the five channels and a 500 Mpt data buffer.

In addition to fast data acquisition time, key specifications for this type of receiver include measurement sensitivity and frequency agility. These capabilities provide several advantages, not the least of which is a speed advantage when dealing with the imposing numbers of measurement points described above.

When evaluating dedicated microwave receivers, a variety of additional capabilities will be useful in antenna/RCS applications. One example is multiple simultaneous receivers, which eliminate the need for PIN switches - and so reduce test time - when evaluating multichannel devices such as monopulse antennas.

Another example is versatile sweep control that provides ascending, descending, arbitrary and random frequency changes. A reverse-sweep capability allows dual-directional scans, which will help minimise near-field data-acquisition and scanning times.

User-selectable bandwidth enables trade-offs between measurement sensitivity and data acquisition time (eg, lower sensitivity produces faster measurements), which is especially useful in near-field testing. Pulsed measurement modes are useful for active-array antennas and other pulsed applications.

Data from a variety of typical test scenarios will highlight the speed advantages of the new-generation receiver. The comparisons come from measurements made with the N5246A and the HP/Agilent 8530A, which was derived from the 8510A network analyser. Three examples are presented: near-field, far-field and RCS.

The example scenario is: testing an active-array monopulse antenna with three test ports (sum, delta azimuth and delta elevation); measuring co-polarised response at multiple frequencies in the X-band on a 100x100 sampling grid; and using typical numbers of beam states and frequencies.

Total measurement time includes data acquisition time plus frequency switching, retrace and instrument overhead times.

Table 1 compares the results from older- and newer-generation solutions. To enable estimates for other test configurations, each column includes average-per-point data acquisition times (total time divided by total number of points).

Table 1: Near-field measurement scenarios with total test times and per-point average times.

As expected, total measurement time increases along with the complexity of each configuration. However, the new-generation receiver provides a 46x to 142x improvement in total test time compared with the older receiver.

One explanation is the wider bandwidth of the new model. Its sensitivity of -89 dBm at 100 kHz IF bandwidth and -81 dBm at 600 kHz IF bandwidth is adequate for near-field testing due to the close proximity of the probe and the associated processing gain obtained from the near-field transform.

Given the speed of the new receiver, probe velocity will be a limiting factor in most of the scenarios here. This is not the case with the older, slower receiver.

Today, complexity continues to increase in near-field measurements of active antennas, with 1024 electronic beam states and 60 to 100 test frequencies being common. As this trend continues, faster acquisition speed will provide a larger economic advantage for operators of near-field test ranges.

On a far-field test range, two factors will limit the maximum achievable measurement speeds: the frequency agility of the remote signal generators and the maximum rotational rate of the positioner.

For simple far-field test scenarios, virtually any combination of source and receiver will be much faster than the maximum positioner velocity (typically 3 rpm). As a result, upgrading to a faster receiver will provide only modest improvements in total test time.

Table 2 shows a variety of far-field measurement scenarios. For low-complexity measurements, there is little or no difference in total measurement times for the old or new test systems. As complexity increases, there is an improvement in total test time with the newer configuration. However, this is due primarily to the 600 µs frequency agility of the updated remote source (although a 1 ms worst case was used to compute the values in the table). When using the new microwave receiver and the highly agile signal generator, system integrators have seen a 2x to 3x improvement in total test time.

Table 2: Far-field measurement scenarios with total test times and per-point average times.

For complex far-field measurements requiring more than 10 test frequencies, upgrading to a faster remote signal source will provide the greatest improvement in total test time and provide the best gains in productivity.

The example scenario is an RCS imaging application in which full polarisation matrix data will be acquired. Cross-range acquisition is ±30 degrees; the angular increment is either 0.1 or 0.25 degrees; and down-range resolution varies from 801 to 16,001 points.

The new measurement configuration is compared with two older approaches that provided -89 dBm or -113 dBm sensitivity. It’s important to note that older receivers generally required test engineers to make a conscious trade-off between measurement sensitivity and frequency agility (ie, sweep speed). The new receiver uses mixer-based downconversion to provide good measurement sensitivity while maintaining very fast frequency agility.

Table 3 shows the relative difference between the old and new receivers. With -89 dBm measurement sensitivity, the new receiver is 35x faster than the older solution.

Table 3: RCS measurement scenarios (full polarisation matrix) with total test times and per-point average times.

At -113 dBm sensitivity, the new solution is 40x faster than the old receiver. On an RCS range, this will result in significantly lower data acquisition times and contribute to improved productivity.

The numbers make it clear: the new-generation measurement receiver provides faster measurement speed and shorter total test time, helping test ranges address the contradictory challenge of gathering more test data while reducing total test times.

This can lead to shorter development time, faster time-to-market, higher quality and lower cost-of-test, even as antenna designs and technologies continue to evolve. Ultimately, this combination of technical and economic benefits will contribute to greater competitiveness in a dynamic and demanding marketplace.