Figure 1 - Nearfield Measurement System
Microwave leakage is measured by terminating either the transmitter or the receiver into a load and performing a nearfield measurement scan. The leakage is most easily evaluated by first transforming the leakage signal into the far-field equivalent. The equivalent farfield leakage is then compared with the levels obtained in the normal test configuration. Figure 2 shows the derived farfield leakage effect on the E plane cut with the source cable to the antenna under test (AUT) loaded. A similar test would be performed to assess the leakage with the receiver cable at the probe loaded.
Figure 2 - E-Plane Cut / Leakage (AUT Loaded)
The leakage signal is seen to be below 80 dB down from the main beam peak, and at least 40 dB down from the side lobe peaks. The resulting effect on gain uncertainty can be readily computed from the vector contributions of the desired signal versus the leakage signal. The contributions to uncertainties in side lobes at 40 dB down are less than +0.1 dB. If 50 or 60 dB side lobes were of interest, the leakage signal, being only 20 to 30 dB down, would affect side lobe measurement uncertainties by up to +1.0 dB.
Loose connectors, bad coax cables, and components with poor isolation can all result in excessive leakage. Wrapping coax connections with copper tape and cable or component substitution are typical approaches used to control leakage sources.
Alternately, if the residual leakage signal from a scan with the AUT input terminated into a load is repeatable, the leakage can be coherently subtracted, resulting in a significant reduction in its effect on side lobe measurement accuracy.
As seen in figure 1, a minimal amount of absorber was used as we were interested in eliminating the need for an anechoic chamber. Emerson and Cuming AN74 flat absorber was placed on the back wall, on the face of the Y axis carriage, and on the probe fixture. In addition, a small piece of AN72 was wrapped around the probe. The back wall absorber had a slight effect on the results, while the Y-axis carriage and probe absorber is most important. Figure 3 shows a comparison of the amplitude variation with the baseline absorber configuration versus one with all absorber removed (vert. offset 1 dB for clarity).
Figure 3 - AUT/Probe Separation Test
The absorber is seen to reduce the peak-to-peak signal variation as well as lead to a fairly smooth sinusoidal response. The peak-to-peak amplitude variation of about 1.4 dB corresponds to a signal to reflection level of about 22 dB. This is consistent with the specified AN-74 return loss of 24.5 dB at the test frequency of 9.338 GHz. The period of the sinusoidal response corresponds to 1/2 wavelength at the test frequency.
Because of the multipath environment, a self comparison test with varying AUT/probe distances produced large side lobe changes. Figure 4 shows the variation in farfield E-plane cuts with the AUT distance varied from 4 lambda to 4 1/2 lambda in 1/8 lambda intervals. Changes on the order of 5-10 dB are noted in side lobes at the -30 dB level. Similar variations were observed in the H-plane side lobes. A close inspection shows that the two patterns at 4 lambda and 4 1/2 lambda are essentially identical. This suggests the interfering signal goes through one full cycle when the direct signal is only changed 1/2 cycle, thus a round trip reflected signal is evident. With the AUT being a flat waveguide phased array, its face acts as a flat aluminum plate reflecting the scattered energy from the probe and Y-axis absorber directly back to the probe to interfere with the direct signal. A similar analogy exists for parabolic antennas when the multipath path length remains constant during a scan.
Figure 4 - E-Plane Cut vs. AUT/Probe Distance
Figure 5 - E-Plane Cut Averaged Results
Similar large side lobe differences were noted with self-comparisons done with the AUT and probe rotated together to 90 degrees, and then to 180 degrees. Clearly the test configuration would not support reasonable side lobe accuracies without further improvements in the absorber or suppression of the multipath effect.
Figure 6 shows a grey scale representation of the measured nearfield amplitude and phase. The checkerboard characteristic of the phase plot is caused by the 90 degree difference in measured phase at alternate nearfield X,Y points.
Figure 6 - Nearfield Amplitude / Phase
Figure 7 shows the results of the first volumetric scan compared to the averaged result of the 8 scans. The close correlation is self evident.
Figure 7 - Averaged Result vs. Volumetric Scan
Figure 8 - Self Comparison Test 2 Scans @ 1/4 Lambda (Multipath Suppressed)
The side lobe differences of the self comparison tests performed above allow an estimation of the side lobe uncertainty contribution due to the residual multipath errors not completely cancelled with the volumetric scanning technique. A side lobe change of 1 dB at the -25 dB level corresponds to an error signal approximately 25 dB down from the side lobe peak, or about 50 dB down from the main beam peak. The volumetric scan is seen to reduce the multipath contribution by on the order of 25 dB. In addition, the phase data can be reversed in the coherent addition so that the multipath result is enhanced and the antenna pattern is suppressed. This provides a way of directly observing the multipath(Fig. 10).
Advantages of the volumetric scanning technique ( over simply taking two scans spaced 1/4 lambda apart and averaging them) include reduced acquisition time and reduced data storage and manipulation requirements. Additional work is being performed by NSI to study alternate scanning techniques vs. hardware configurations to optimize the effectiveness of the technique. The general applicability to other types of antennas is also being studied.
Figure 9 - Self Comparison Test 3 Scans @ 0,90,180 Degrees (Multipath Suppressed)
Figure 10 - Multipath vs. Desired Signal
Figure 11 - E-plane Cut Nearfield vs. Farfield
Figures 11 and 12 show the excellent correlation between the customer measured farfield data and the NSI calculated farfield from nearfield measurements taken in 1988. The nearfield data was taken using the multipath suppression technique described earlier in an office environment with minimal absorber. Side lobes are generally within 1 to 2.5 dB between the two techniques at levels down 30 to 35 dB from the main beam peak.
The residual side lobe uncertainty estimate for the nearfield measurements is about +1.5 dB at the 30 dB level. No farfield uncertainty estimates were available for the farfield test data, however, typical uncertainties for a good farfield range would be around + 2 dB, and clearly the differences in the side lobes measured with the two techniques falls within the bounds of the combined uncertainties.
Figure 12 - H-plane Cut Nearfield vs. Farfield
2. D. Slater and G. Hindman, "A Low Cost Near-Field Antenna Measurement System", 1989 Antenna Measurement Techniques Association Symposium, Monterey, Ca. Oct 9-13, 1989.