EMC Standards Activities

Don Heirman, Associate Editor As your Society's Vice President for Standards, I oversee three subcommittees devoted to Standards activity. One committee is the Standards Development Committee (SDCom) which is chaired by Steve Berger. Steve and I appreciate the article provided by Elya Joffe as below on recent committee activities. And, we appreciate the editing support for this article provided by Hugh Denny, chair of the Standards Education and Training Committee (SETCom). We reiterate Elya's call for volunteers to assist us with Standards, whether as a member of the SDCom, SETCom, or the last of my three subcommittees reporting for Standards, the Standards Advisory and Coordination Committee (SACCom), chaired by Elya Joffe. Get involved and make a difference!

Revision of IEEE Standard 473: IEEE Recommended Practice for an Electromagnetic Site Survey (10 kHz to 10 GHz)

by Elya B. Joffe, Member – Standards Develoment Committee (SDCom)

A. IEEE Standard 473:1991 at a Glance

IEEE STD 473 describes a systematic approach for performing an EM site survey, commencing from preparation of the planning of the test until completion, data analysis and documentation of the measurement results. The Standard outlines procedures and information regarding the methods for conducting EM site surveys particularly for EMR Bio-effects and equipment effects applications. The Standard is due for reaffirmation or revision in 2001. It is proposed that the Standard be revised to reflect current needs and applications, emerging technologies and available measurement techniques and instrumentation.

B. Why the Need for Revision?

Inasmuch as the Standard extensively covers a broad spectrum of considerations in conducting EM site surveys, a detailed review indicates that an extensive revision is necessary. The following are some examples of developments which merit revision of the Standard:

1. Increased density of radio sites, with an overwhelming number of transmitters collocated on site.
2. Expanded radio services have been assigned higher frequency bands, due to lack of available spectrum. Thus, the maximum frequency applied of the Standard (10GHz) appears to be non-representative of current and future spectrum usage.
3. ISM (industrial, scientific and medical) applications render environments, once at ambient levels, highly "EM polluted," posing potential problems to radio services.
4. Communication, command and control sites are often located near pulsed sources. In addition, many communication services utilize pulsed waveforms.
   
   The proposed revision, therefore, will:
   
5. Expand the scope of the Standard
6. Reflect modern test and measurement equipment and procedures
7. Incorporate techniques to measure time domain characteristics of pulsed waveforms, spread spectrum signals, etc.
8. Update the information data base as necessary

(1) Revise/extend the scope of the Standard

Currently, the Standard's scope covers EMR Bio-effects and EM environment for electronic equipment, but little focus is given to the most serious problem in dense RF areas, namely: antenna to antenna coupling. As the transmitter sites become more and more crowded, the need for site compatibility becomes ever increasing. However, performing an EM site survey for site compatibility considerations requires that specific parameters such as modulation waveforms, signal bandwidth, spectral distribution, signal waveform characteristics, field polarization, etc, be measured. The existing Standard addresses these parameters only to a limited extent.

Due to recent changes in frequency allocations, the upper frequency range needs to be extended from the current 10GHz to 40GHz.

(2) Updating the Test Equipment and Test Methods

The test techniques are based on test equipment available two decades ago. For example:

• The current Standard refers primarily to "E-Field Strength Meters." This limits significantly the spectral data that can be extracted from the measurements.
• "Saving" data is still specified using... paper (plotters) rather than magnetic media (e.g. floppy disks...).
• No time domain measurement equipment is specified. In particular – the use of digital oscilloscopes, proper detectors and LNAs, etc, is not incorporated.
• Measurement equipment needs and usage limitations in high ambient field levels are not addressed.

Additional issues which need to be updated include:

• Antenna/sensor related issues:

- Field strength probes vs. conventional antennae

- Use of directional antennas for spatial resolution of interference

- Use of both horizontal and vertical polarization measurements or circular polarization in order to obtain full data

- Calibration of the antenna for correct survey

- Use of monopole antennas (not only loop antennas) for lower frequency measurements (in the HF/shortwave band)

• Procedure related issues

- Addition of proper warnings and procedures for performing measurements in high ambient environments, and safety precautions for the same, e.g., warnings regarding avoidance of overload or saturation conditions which may be detrimental to measurement results and often fatal for the equipment performance, etc.

- Procedures for selecting sweep rates, considering time domain characteristics of the RF sources

- Methodology for full-system calibration of test equipment

(3) Incorporate measurement techniques for time domain characteristics of pulsed waveforms

With the increased use of high power pulsed RF sources, e.g., cellular base stations, radar emitters, etc. in the near vicinity of communication, command and control facilities, for either commercial or military applications, the extraction of the time domain characteristics becomes crucial for completion of the RF compatibility analysis. Suppose that an EM site survey is to be conducted in order to investigate the possibility of installation of a new pulsed radio communication service in an existing radio communications facility. The communication system is a TDMA (time division multiple access system) operating at a low duty cycle, with a high immunity to interference (error correction encoding, etc). Having completed the survey, it is found that the dominant source is a radar at a nearby airport, the spectrum of which almost completely overlaps the frequency band allocated to the new system.

Normally, two approaches are employed in this case:

• Frequency culling, where the frequency overlap (in and out of band) is considered. If no overlap is observed, the probability of interference is, obviously, reduced.
• Amplitude culling, where the amplitude of the signal coupling into the receiver from the radiating source, is performed. This analysis considers also the path losses, the front end characteristics of the receiver, etc. Obviously, whenever there is some frequency overlap, either directly or due to spurious signals, some interference may be probable.

In the discussed case, having reviewed the spectral data obtained by measurement on site, it was found that a significant frequency overlap ("frequency culling") existed. Therefore, amplitude culling was conducted, and it was found that the RF interference signal at the input of the receiver was 30-40dB above its sensitivity level. Based on the measured spectral data alone, the radar, being a broadband emitter, could have potentially blocked the receiver. This would have caused a severe degradation to the receiver performance. In many "normal" cases, this would have prohibited the installation of the equipment on site.

Thus, a major objective of the proposed effort is to incorporate time domain measurements. Such data will enable the performance of "statistical culling," in other words, a statistical analysis of the probability of interference between the system on site is performed. Of course this process has benefits only where pulsed systems are concerned, but today, in the "digital age," this is, in fact, the reality. Such measurements were not possible in the past, but can reliably be conducted with modern test equipment.

(4) Need for a format for site survey documentation

In the performance of site surveys, one of the most important issues is the proper documentation of the survey results. A methodology for recording the data, organizing it, and finally interpretation of the data, are necessary in order to effectively complete the site survey.

An appendix outlining the recommended format for the site survey documentation will be added to the Standard.

C. Call for Volunteers

Would you be interested in being involved in this activity? If you are

• involved in RF site studies
• experienced in running "on site" RF measurements
• familiar with statistical analysis techniques
• or simply have interest in this issue

and would like to contribute to this activity, please do not hesitate to let us know of it.

Our goal is to prepare a useful, up to date standard. Would you like to be part of it?

For further information, please contact Elya B. Joffe at: eb.joffe@ieee.org  EMC

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