Conformity Magazine - September 2007 - (Page 20)

expected values. This additional delay is most likely due to its internal cabling and balun. Results of Applying Time Gating Once the insertion data were time-gated and Fourier transformed back to the frequency domain, they were used to find the FSAF of these antennas. We used the free-space Friis equation to determine the antenna factor (AF). The difference due to the various distances and between the gated and ungated data for each antenna was investigated. Figure 5(a, b) shows the differences for the DRG, Figure 6(a, b) for the large DRG and Figure 7(a, b) for the biconicals. The difference data was normalized to the average of the gated data for each antenna. All three antennas exhibit similar improvements in repeatability for different distances with the removal of the ground-reflected signal. The DRG shows the best clustering due to its relatively short pulse waveform. The biconical shows the largest spread of repeatability. This is due to its long pulse waveform and the non-ideal time gating applied to the data at the larger distances. The ungated biconical data show the expected nulls from ground bounce across its frequency range. The biconical data were also compared to the near freespace values used in [1]. The modeled three meter free space insertion loss was used to determine AF. As shown in Figure 7(b) these modeled data compares well to the measured gated values. This comparison indicates that this method would fit well with current EMC standards methods. One additional benefit from the time-domain processing is a lower system noise floor. As seen in Figure 8, there is an improvement of about 20-30 dB in this level. Even the spurious FM and cell phone bands levels have been lowered. Conclusions The use of time-domain signal processing for determining the free-space antenna factors for several EMC antennas has been outlined. Results for different separation distances are compared. Valid results highly depend on correct time-gating to allow for the full direct signal waveform while removing the reflected signals. Reliable and repeatable data can be obtained using this method. Separation of the direct and reflected waveforms provide a unique perspective for this evaluation. Items that affect the outcome of this method include the ring-down time of the antenna and the relation of the direct to the reflected path times. These conditions are favorable when the ring-down time is relatively short compared to the difference in the direct to reflected path times. Favorable conditions allow for improved accuracy in the determination of free-space antenna factors. This method fits well with current EMC standards methods. Visit: www.elgar.com • Call: 858.458.0223 20 CONFORMITY SEPTEMBER 2007 Figure 7: Comparison of (a) ungated and (b) gated AF differences for the biconical antenna http://www.elgar.com http://www.elgar.com

Table of Contents Feed for the Digital Edition of Conformity Magazine - September 2007

Contents Editor’s Note FCC Extends Disability Access Requirements to VOIP Commission Fines Importer $3 Million Over Non-Compliant TVs FCC Gets Tough on Retailers Over Analog TV Tuner Labeling Analog Cellular Service Sunset Date Reaffirmed Commission Adopts Recommendations of Katrina Panel Free-Space Antenna Factors Through the Use of Time-Domain Signal Processing Editorial Correction ESD Open Forum An Update on Revisions to IEC 61000-4-2, the Basic Standard on ESD Practical Advice for Meeting the DFS Testing Requirements of the FCC The EMC Impact of Embedded Software ESD Design Buyer's Guide Tate Renominated to the FCC EPA Releases Report on U.S. Electronic Waste Additional Standards For The EU’s Toy Directive Medical Device Manufacturer Enters Consent Order Updated Guidance Document on Laser Products Issued FDA Offers Advice on Defective Heating Pads CPSC Issues China Product Safety Program Report Other CPSC Actions In The News IEC Standards Update Product Reviews Product News UL Standards Update From Our “You Can’t Make This Stuff Up” Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - September 2007

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