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Publication Number: FHWA-HRT-10-037
Date: October 2010

Step Frequency Ground Penetrating Radar Characterization and Federal Evaluation Tests

Results

Measured emission data were collected to determine whether SF GPR emissions levels were below the emissions mask that was proposed in the background section of this report. General measurements were used to make an overall assessment of emission levels using a resolution bandwidth (RBW) of 1 MHz and a video bandwidth (VBW) of 3 MHz. More detailed measurements were made in narrow frequency ranges corresponding to critical Federal systems using appropriate RBW and VBW settings. The operating spectrum for the SF GPR was 140–3,000 MHz, while the measured spectrum used for the emission measurements was 50–6,000 MHz.

This section provides a summary of general measurement results to illustrate system compliance with the proposed emissions mask. In instances where measurements indicate frequencies at which the system did not meet NTIA criteria, information is provided to describe ambient phenomena that were often present. To further address any noncompliant phenomena observed, supplemental follow-up data from an FCC-certified laboratory are provided in appendix C. Figure 19 through figure 36 provide follow-up data collected after final system tuning was complete. The three notch configurations, A1, A2, and A3, were tested during the follow-up testing in September 2009, while only one notch configuration, A1, was tested during the initial testing in March 2009.

Figure 7 through figure 10 correspond to measured emissions from the SF GPR when its emissions were unnotched. Figure 11 through figure 14 correspond to measured emissions responses from the SF GPR when its emissions were notched. Measured responses from the SF GPR with notched emissions were typically compliant with a few isolated exceptions.

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Figure 7. Graph. EIRP peak measurement values for horizontal measurement antenna orientation and maximum transmitter emission orientation without emission notching.

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Figure 8. Graph. EIRP peak measurement values for vertical measurement antenna orientation and maximum transmitter emission orientation without emission notching.

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Figure 9. Graph. EIRP RMS measurement values for horizontal measurement antenna orientation and maximum transmitter emission orientation without emission notching.

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Figure 10. Graph. EIRP RMS measurement values for vertical measurement antenna orientation and maximum transmitter emission orientation without emission notching.

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Figure 11. Graph. EIRP peak measurement values for horizontal measurement antenna orientation and maximum transmitter emission orientation with emission notching.

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Figure 12. Graph. EIRP peak measurement values for vertical measurement antenna orientation and maximum transmitter emission orientation with emission notching.

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Figure 13. Graph. EIRP RMS measurement values for horizontal measurement antenna orientation and maximum transmitter emission orientation with emission notching.

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Figure 14. Graph. EIRP RMS measurement values for vertical measurement antenna orientation and maximum transmitter emission orientation with emission notching.

Unnotched SF GPR responses exhibited several frequencies that exceeded compliance criteria in the original emissions data from March 2009, while original notched emissions data rarely exceeded emission criteria. Emissions data collected during follow-up testing after system tuning in September 2009 illustrated that system performance met NTIA criteria in both notched and unnotched operating configurations with only a few exceptions. In instances where exceptions occurred in follow-up data and emissions exceeded NTIA criteria, the reasons were addressed (provided in this section).

Emissions data collected for the proposed compliant operating configuration (see figure 11 through figure 14) were obtained using the following methodologies corresponding to each figure. Data presented in figure 11 show the EIRP level received by emissions measurement antennas oriented horizontally while measuring peak emissions from the SF GPR with a notched emissions spectrum. Data in figure 12 show the EIRP level received by emissions measurement antennas oriented vertically while measuring peak emissions from the SF GPR with a notched emissions spectrum. Data in figure 13 show the EIRP level received by emissions measurement antennas oriented horizontally while measuring RMS emissions from the SF GPR with a notched emissions spectrum. Data in figure 14 show the EIRP level received by emissions measurement antennas oriented vertically while measuring RMS emissions from the SF GPR with a notched emissions spectrum.

In figure 11, (which generally illustrates compliant peak measurements), there are values that are a small number of decibels above the threshold criteria of 1,530 and 1,560 MHz. Other values exceeding the criteria corresponded to locations where significant ambient measurement values are present. In figure 12, peak measurements are also typically compliant. Frequencies of approximately 1,555 MHz are a small number of decibels above the threshold criteria. Frequencies in the 1,040-MHz range that exceed criteria correspond to substantial ambient emissions. Figure 13 and figure 14 present RMS data that are generally compliant as well. Several SF GPR measurement peaks correspond to ambient measurement spikes in figure 13 and figure 14 and therefore do not indicate compliance issues. In the frequency range of 1,500–1,550 MHz, there are RMS values that are a small number of decibels above the threshold criteria.

Appendix C provides additional SF GPR data that were collected at an FCC-certified laboratory after attenuation adjustments were programmed into the system in an effort to bring the remaining system emissions into compliance. Figure 19 through figure 34 illustrate that the peak and RMS emissions greater than 140 MHz had few emissions that did not meet NTIA criteria for the adjusted configuration used during the additional emission measurements in September 2009. Below 140 MHz, peak values exceeded compliance criteria at several frequencies primarily due to unintentional emissions caused by radio frequency (RF) switches in the SF GPR. Unfortunately, there is not a practical way to reduce these unintentional emissions using current SF GPR technology. Detailed measurements are provided in figure 35 and figure 36 to illustrate low frequency phenomena occurring between 50 and 200 MHz. A clear transition between the characteristics of intentional emissions above 140 MHz can be observed relative to unintentional emissions below 140 MHz.

The following list provides potential reasons why SF GPR emissions exceeded NTIA criteria for some tested frequencies in September 2009 test data (follow-up data plots corresponding to these listed descriptions are provided in appendix C):

  • The test report in appendix E indicates that during emission measurements, some SF GPR antenna array elements were as close as 6.88 ft (2.1 m) from the measurement antenna when it was rotated into the 90-degree position (see figure 2). For frequencies between 960 and 1,600 MHz (far field frequencies for this scenario), the free space loss should be related to the distance by 20*log(R). Therefore, the correction factor for a 6.88-ft (2.1-m) distance versus a 9.84-ft (3-m) standard distance is 20*log(2.1/3) = 3.1 dB. This correction factor can be applied to data from 960 to 1,600 MHz for figure 19 to figure 34. Most emission issues in this frequency range are addressed by this correction. This correction factor applies to SF GPR emission measurements made at the FAA Technical Center as well.

  • Ambient emissions exceeded NTIA criteria at several localized frequencies between 50 and 3,000 MHZ in RMS data presented in figure 27 through figure 34. These ambient emissions accounted for localized features that exceeded NTIA criteria in SF GPR data in figures 26 through 33.

  • Isolated peaks that exceeded NTIA criteria in figure 23, figure 25, figure 26, figure 28, figure 30, and figure 34 appear to be related to intermittent ambient phenomena that occurred during SF GPR emission measurements. This hypothesis is supported by measurement results obtained using similar measurement configurations that do not include these isolated peaks.

  • Unintentional peak and RMS emissions below 140 MHz exceeded NTIA criteria, as shown in figure 35 and figure 36. These unintentional emissions were caused by monolithic microwave-integrated circuit (MMIC) switching devices, and they occurred each time the antenna array switched between antenna array elements. These emissions were a side effect of the operation of the MMIC devices within the SF GPR and were common in all RF solid state switches.

Data collected in narrow frequency bands corresponding to critical Federal systems were not analyzed for this report because this data are anticipated to be evaluated individually by appropriate Federal agencies. The original measurement data from this test are available in electronic form to accompany this report. Figure 17 and figure 18 in appendix B provide the data file matrix corresponding to critical Federal system data collected in March 2009. The data file matrix describing all other data files collected in March 2009 is provided in figure 15 and figure 16 in appendix B. Procedures for data file import are provided in appendix D.

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