EVALUATING NEAR-FIELD RANGE MULTI-PATH

Gregory F. Masters
Nearfield Systems Incorporated
1330 E. 223rd Street #524
Carson, CA 90745 USA
(310) 525-7000

ABSTRACT

Introduction

     -  Flight-line

     -  Engineering office

     -  Manufacturing area.

Near-field range evaluation assigns far-field uncertainties to its calculated far-field patterns. It is important and involved process which can take many weeks. Typically it begins with a pattern error budget table as shown in Table 1. Combined error is an RSS with the errors assumed to be independent and uncorrelated. A-30 dB side lobe level was chosen because the antenna to be tested has a first side lobe spec. at -30 dB. The elements of this table are discussed by Newell (1). Some errors in this table are assessed through analytical modeling and others are determines through measurements. In the design phase theses items are typically estimated. In the acceptance phase these items are evaluated.

1. Probe relative pattern.......... 0.20 2. Probe polarization ratio........ 0.05 3. Probe Gain...................... 0.00 4. Probe Alignment................. 0.00 5. Normalization Constant.......... 0.00 6. Impedance mismatch.............. 0.00 7. AUT alignment................... 0.00 8. Aliasing Error.................. 0.03 9. Measurement area truncation..... 0.05 10. Probe x-y position error........ 0.10 11. Probe z position error.......... 0.20 12. Multiple reflections............ 0.87* 13. Receiver amp non-linearity...... 0.00 14. System. phase error............. 0.15 15. Rec. dynamic range.............. 0.10 16. Room scattering................. 0.48* 17. Leakage and cross-talk.......... 0.07 18. Random Errors in amp and phase.. 0.22 RSS combination (dB)................ 1.03

Table 1 Error Budget for -30 dB Side lobe Measurement

Evaluation of the error budget is important in low scattering anechoic chambers as well as portable systems in noisier RF environments. The test case used in this paper is in a manufacturing or office environment.

Near-field Range Multipath Effects

	-AUT and Probe
	-AUT and Scanner/Support Structure
	-AUT and Walls (including Floor and Ceiling)

Multipath effects can be identified on a range by observing the far-field effect of changing near-field test parameters which should not affect the AUT's far-field pattern. These are known as "self-comparison" tests. Some of these parameters are:

	-AUT-to-probe separation
	-AUT-to-scanner separation
	-AUT-to-wall separation
	-AUT orientation to phi
	-AUT orientation in Az and El
	-AUT lateral movement

When self-comparison test are used to evaluate range performance for a particular antenna project, the characteristics of the antenna used for evaluation should be very similar to the project antenna. An engineering or breadboard model may serve this purpose well. The AUT used in this paper is a 10 in. diameter waveguide array.

To the extent that the same pattern cut is made through the AUT's pattern, observed pattern changes in the self-comparison tests indicate errors due to multipath. When these variations are larger than that allocated in the budget, additional effort is needed to meet the test requirements.

Two approaches are commonly used to reduce scattering effects. They are:

1. RF Absorber
2. Averaging Measurement

Approach 1 places RF absorber in areas suspected to be highly reflective such as positioners, antenna support structure, walls floor and ceilings. Approach 2 averages scans and/or coherently adds the result to suppress the multipath (2). This suppression approach has been shown to yield a 10-20 dB multipath reduction in an office environment with the little absorber required. There are advantages and disadvantages to both approaches (See Table 2).

Approach	Advantages	Disadvantages
RF Absorber	Shorter test Times	Dirty Envir. More Real Estate Less Portable
Averaging Measurements	Clean Envir. Less real estate More portable	Longer Test Times

Table 2. Using RF Absorber vs. Averaging Scans

Locating and Evaluating Near-field Range Multipath

Quiet-zone imaging involves using the near-field range to probe radiated energy and then converting it to an angular spectrum so that high-reflection area in the room can be located. An angular scattering map is created which can be used in conjunction with a device such as a theodolite to locate the source. Upon location various methods can be applied to reduce the levels such as:

	- adding absorber
	- rearranging scanner system w.r.t. room
	- relocating equipment in the room

Quiet-zone imaging is an effective method of evaluating absorber reflections in the chamber but is discussed in detail in another paper (3).

Self-comparison tests are done by making two measurements with different test parameters and comparing them. The resulting angular plot indicates the direction and level of the multipath signal.

Prior to performing self-comparison test certain other near-field error parameters must be identified and reduced to insure that the self-comparison test method will give repeatable results. Some of these test are:

	- Random error (Cable and receiver
	- Leakage (Cable and receiver)

Table 3 Self-comparison Repeatability Tests

Each test results in a far-field error level below the main beam peak. To evaluate the side lobe error effect of a particular contributor, we convert the Pattern-to-error (Signal-to-Noise) ratio to an uncertainty.

For example:

 -30 dB Side lobe (Signal)
	-62 dB Repeatability Error (Noise)
	32 dB Pattern-to-Error Ration (SNR)
	+0.22 dB Uncertainty due to Repeatability

Once the far-field error due to repeatability between scans is low enough, we are ready to begin the self-comparison tests. The results of the tests are shown in Table 4.

Error Source	FF. Error Level Below -30 db Side lobe
Leakage - Cable - Receiver Total leakage	-42 db -65 db -42 db
Random Amp and Phase - Cable - Receiver Total Random Error	-40 db -33 db -32 db
Total Repeatability Error	-32 db

Table 4. Self-comparison Repeatability Test Error Levels for -30 dB Side lobe Level

Testing

Test Parameter	.
Antenna	10 in. Diam Waveguide Array
Frequency	9.338 GHz
Probe	WR90 Open-ended Waveguide
Probe Separation	3 Wavelengths
Scan Size	13 x 14 in.
Scanner	NSI's 211T portable XY Scanner

Table 5 Self-comparison Test Configurations

Multipath Source	Near-field Scan Tests	Reduction Method
AUT-Probe	2 Scans with probe moved 1/4 in Z	Add probe absorber Avg. Scans
AUT-Scanner	2 Scans with Scanner moved 1/4 in Z	Add scanner absorber
AUT-Wall	2 Scans with AUT moved 1/4 in Z	Add absorber Avg. Scans

Table 6. Multipath Self-comparison Tests

In test #1 two near-field scans were taken with the probe at locations one quarter wavelength (/4) apart. The difference between the two far-field patterns is due to the multipath between the AUT and the object moved. A /4 separation will insure that the difference between the patterns reveals the peak multipath in the z-directions. This test is done twice, once in the bare configuration (no probe absorber) and the other with 4 in. pyramidal absorber on the probe.

Figure 1 shows the result of the measurements. The solid line is the antenna pattern with probe absorber, the higher of the dashed lines is the multipath without probe absorber, and the lower dashed line is the reduced multipath after adding absorber to the probe. Note that the multipath error is very angular dependent and therefore will affect the antenna differently in different directions. The high multipath on boresight is clearly due to the larger reflections off the probe mounting plate.

In the first case, the multipath level is only 5 dB down from the first side lobe, given rise to a 3.9 dB uncertainty. In the second case, the multipath level is 20 dB down, given rise to a 0.85 dB uncertainty (see Table 7).

Test #2 moves the scanning structure to identify its multipath. Two scans are again taken moving the scanner by /4 in Z. The probe-AUT-separation is kept constant however, in order to keep the probe coupling constant, the test is repeated twice with and without absorber on the scanner. The -30 dB side lobe error results are found in Table 7.

In test #3 the AUT is moved /4 in Z. The probe-AUT separation is kept constant by moving the probe also. The scanner was not moved however for simplicity, but the multipath effect from the scanner was so low in test #2 that it was ignored. Figure 3 and Table 7 show that the multipath effect from absorber behind the wall accounts for 0.48 dB uncertainty at the -30 dB side lobe. It should be noted that elimination of the wall absorber would have only increased the overall uncertainty by 0.3 dB to 1.3 dB. This is consistent with many such portable systems.

At this point, it was noted that the first side lobe test requirement was met and so no further absorber testing was necessary. If greater accuracy was required on side lobes further form scanner boresight, other things could have been done to reduce the multipath, such as replacing the AN-74 flat absorber with pyramidal and by adding absorber behind the AUT. In addition, increasing the distance between multipath sources will also reduce their effect. This can be done by increasing probe-AUT separation.

Figure 1 Multipath Effects from Probe Structure

Multipath Source	Before Reduction		After Reduction
	SNR (dB)	Unc (dB)	SNR (dB)	Unc (dB)
AUT-Probe	5	+3.9	20	+0.85
AUT-Scanner	3	+4.6	30	+0.27
Total Multiple Reflections (#12 Table 1)	--	+5.6	--	+0.87
AUT-Wall room Scattering (#16 Table 1)	13	+2.0	25	+0.48

Table 7 Multipath Effects Before and After Reduction

Figure 2 Multipath Effects from Scanner

Results

Figure 3 Multipath Effects from Walls

Conclusion

This method has a wide application in both minimizing absorber placement in the production and office environment, and in enhancing the low-scattering anechoic chamber.

References

1. Newell, A.C. Error Analysis Techniques for Planar Near-field Measurements, IEEE Trans. Antenna and Prop. Vol. 36, no 6, June 1988
2. Hindman, G and Slater, D. Error Suppression Techniques for Near-field Antenna Measurements, 1989 AMTA Symposium
3. Hindman, G. Anechoic Chamber Diagnostic Imaging, 1992 AMTA Symposium