National Bureau of Standards Library. £-01 Admin. Bldg. OCT 6 1981 191108 ~&h /CO sm 8 NBS TECHNICAL NOTE 658 6 $ , a £i "ffAU O* U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards Development of Electric and Magnetic Near-Field Probes tx NATIONAL BUREAU OF STANDARDS The National Bureau of Standards' was established by an act of Congress March 3. 1901. The Bureau's overall goal is to strengthen and advance (he Nation's science and technology and facilitate their effective application for public benefit. To this end. the Bureau conducts research and provides: (1) a basis for the Nation's physical measurement system. (2) scientific and technological services for industry and government. (3) a technical basis for equity in trade, and (4) technical services to promote public safety. 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THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and accessibility of scientific information generated within NBS and other agencies of the Federal Government; promotes the development of the National Standard Reference Data System and a system of information analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services to ensure that the NBS staff has optimum accessibility to the scientific information of the world. The Office consists of the following organizational units: Office of Standard Reference Data — Office of Information Activities — Office of Technical Publications — Library — Office of International Relations. 1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, DC. 20234. 3 Part of the Center for Radiation Research. 3 Located at Boulder, Colorado 80302. « Part of the Center for Building Technology. I 1 1975 \° Development of Electric and Magnetic $ f Near-Field Probes Frank M. Greene Electromagnetics Division Institute for Basic Standards ,7t National Bureau of Standards Boulder, Colorado 80302 y T2 * °*en> of U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary NATIONAL BURE.AU OF STANDARDS Richard W Roberts Director Issued January 1975 Library of Congress Catalog Card Number: 74-600200 National Bureau of Standards Technical Note 658 Nat. Bur. Stand. (U.S.), Tech Note 658, 53pages (Jan. 1975) CODEN: NBTNAE For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. 013.46:658) $1.10 CONTENTS Page 1. INTRODUCTION 2 2. THE ELECTRIC NEAR-FIELD PROBES 4 2.1 General 4 2.2 Electrical Characteristics 6 2.3 The Response of a Short Dipole Antenna 9 2.4 Dipole Antenna Calibration and Measurement Errors 12 3. THE MAGNETIC NEAR-FIELD PROBES 17 3.1 General 17 3.2 Electrical Characteristics 19 3.3 The Response of a Small Loop Antenna 21 3.4 Loop Antenna Calibration and Measurement Errors-- 2 2 4. THE ELECTRIC-DIPOLE RESPONSE OF A LOOP ANTENNA 26 5. PARTIAL -RESONANCE EFFECT IN A LOOP ANTENNA 32 6. ERRORS DUE TO FIELD HARMONICS 36 7. THE NBS CONDUCTING-PLASTIC TRANSMISSION LINE 38 7.1 Introduction 38 7.2 Conducting-Plastic Materials 39 7.3 Construction of the Conducting-Teflon Transmis- sion Line 40 7.4 Electrical Characteristics of the Conducting- Teflon Line 41 Conductor Resistance 41 Mutual Capacitance 41 Propagation Function 41 Attenuation Function 42 Phase Function 42 7.5 Transmission Line Attenuation 42 8. CONCLUSIONS 46 9. REFERENCES 47 in LIST OF FIGURES Page Figure 1. Construction details of the 5-cm and 10-cm dipole antennas 5 Figure 2. Schematic diagram of the portable dipole probe- 7 Figure 3. Equivalent circuit of a short dipole antenna 8 Figure 4. Increase in the effective length and apparent self -capacitance of a short dipole antenna with electrical half-length 13 Figure 5. Increase in the effective length of the 2. 5-cm, 5-cm and 10-cm dipole antennas with frequency-- 14 Figure 6. Increase in the apparent self -capacitance of the 2. 5-cm, 5-cm and 10-cm dipole antennas with frequency 15 Figure 7. Calibration of the 10-cm dipole probe for USAFSAM, Brooks AF Base (f = 10 MHz) 16 Figure 8. Construction details of the 3.16-cm and 10-cm diameter loop antennas 18 Figure 9. Schematic diagram of the portable loop probe 20 Figure 10. Equivalent circuit of a small loop antenna 20 Figure 11. Relative response of the 1-cm, 3.16-cm and 10-cm diameter loop antennas 24 Figure 12. Calibration of the 10-cm diameter loop probe for USAFSAM, Brooks AF Base (f = 10 MHz) 25 Figure 13. Equivalent lumped circuit of an electrically small, singly loaded loop antenna showing the electric- dipole and magnetic-dipole responses-- 26 Figure 14. Worst-case measurement error of a circular-loop antenna due to its electric-dipole response 30 Figure 15. Worst-case measurement error of the 1-cm, 3.16-cm and 10-cm diameter loop antennas due to their electric-dipole response 31 IV . LIST OF FIGURES (Continued) Page Figure 16. Simplified equivalent circuit of a small loop antenna 3 2 F i gu r e 17. Increase in the response of a loop antenna due to partial resonance 34 Figure 18 Increase in the response of the 1-cm, 3.16-cm and 10-cm diameter loop antennas due to partial resonance 35 Figure 19 . Attenuation and phase-shift of the conducting- plastic transmission line 45 LIST OF TABLES Page Table I. Dimensions of the 5-cm and 10-cm Dipole Antennas 4 Table II. Approximate Values of the 5-cm and 10-cm Dipole Capacitances 9 Table III Increase in the Effective Length of a Dipole with Electrical Length 10 Table IV. Increase in the Quantity C4/C a of a Thin Dipole with Electrical Length 11 Table V. Dimensions of the 3.16-cm and 10-cm Loop Antennas 17 Table VI. Approximate Self -Resonant Frequencies of the Loop Probes 33 v : DEVELOPMENT OF ELECTRIC AND MAGNETIC NEAR-FIELD PROBES Frank M. Greene ABSTRACT This publicat ion describes the development and design of small el ectric and magnetic near-field probes for measuring haza rd-level fields up to 20,000 V/m and 100 A/m, respe ctively. They were originally de- signed to be used over the frequency range from 10 to 30 MHz, and consis t of short dipole antennas and small, single-turn , balanced, loop antennas to meas- ure the electric- and magnetic-field components, respectively . The probes are intended for use by various researcher s in their electromagnetic, radiation -exposure programs for determining the effects of hazard- level, non-ionizing, EM fields on living tissue, ele ctro-explosive devices, and vola- tile fuels. In order to later extend the use of the probes to frequencies above 30 MHz, a detailed analysis was made of several types of measurement errors likely to be encountered. The principal errors result from a variation with frequency in: the effective length and impedance of the dipoles; and the electric-dipole response and partial resonance of the loops. Cor- responding corrections are given for each type of error as a function of the operating frequency from 10 to 1000 MHz, and as a function of the physical and electrical sizes of the probes. As a result of the analysis, the dipoles can now be used for measurements at frequencies up to 750 MHz, and the loops to 75 MHz with an estimated uncertainty of 0.5 dB.