EDM-1, EDM-2, EDM-3 and EDM-15. One of the intentions of the NBS project was to develop an instrument design that could be produced by commercial manufacturers; experience showed that the EDM series of meters could not be commercialized readily. The overall and continuing objective of this project is to design and fabricate portable, isotropic monitors for making accurate field intensity surveys of rf radiation. To satisfy this objective it was decided to first check the adequacy and accuracy of existing field intensity meters (FIM's) including conventional radio receivers and the newer rf radiation hazard meters. The next step was to develop satisfactory procedures for measuring the relatively strong fields near various types of transmitting antennas, over a wide frequency range. The frequency range of greatest concern in this project is 500 kHz to 1 GHz, which covers the majority of high-power emitters such as AM, FM, VHF and UHF television, communications, and other transmitters used for broadcasting. The usable frequency range of the EFM-5 monitor extends from 100 kHz to 4 GHz but the response is not uniform (flat) over this large a frequency range. The rf probe program at NBS has dealt with both the theoretical and experimental aspects of quantifying fields, including the design and fabrication of prototype instruments. The EFM-5 described here is a useful radiation monitor for measuring ambient fields in the environment and also for checking rf leakage radiation in the vicinity of industrial and consumer devices. The types of instrumentation used in the past to measure these two different situations can be summarized as follows: (1) A tunable field strength meter or scanning spectrum analyzer, using an antenna which must be oriented for the local field polarization. These receivers generally have high sensitivity and are not designed as portable instruments for making quick surveys. For example, they are unsuitable for checking the leakage rf field of industrial sources and most other near-zone situations. (2) A microwave "hazard meter" which is designed to test the high level fields close to radar antennas and similar emitters, or to test for leakage fields as mentioned above. The frequency of interest is generally above 300 MHz and the measurable levels generally exceed 20μW/cm2, which is equivalent to a free-space electric field strength of about 9 V/m. This type of instrument is useful near a radiating source but has been used for other applications due mainly to the lack of more suitable instrumentation. As background information, a brief discussion is given here of the design philosophy and use of a "conventional" field intensity meter (FIM). A single component of electrical field strength is measured by attaching some sort of electrical dipole antenna to a tunable radio receiver. The receiver acts as a frequency-selective voltmeter and the antenna is generally calibrated separately. That is, it is necessary to apply a multiplying factor to the receiver dial indication. The required conversion factor between the antenna pickup (volts) and the electric field strength (V/m) is known as the "antenna factor." It must be determined experimentally for each antenna used, at each orientation angle and signal frequency, by immersing the antenna in a known (standard) field. Figure 1 is a functional block diagram of a typical FIM, consisting basically of a superheterodyne communications receiver with certain added features. This type of field measurement system is characterized by high sensitivity and sharp selectivity. The measurements are made as a function of signal frequency, for each field polarization desired. By contrast, a different approach is used for measuring electric field intensity with a so-called "hazard meter." This type of non-tunable rf meter is characterized by low sensitivity and the use of broadband antennas having a "flat" response over a wide frequency range. Also, the "wide-open" measurement system generally employs an isotropic or nondirectional type of pickup antenna [1]. The first technical phase in the NBS program was a study of the near-zone measurement problem in order to devise instruments which could furnish repeatable, accurate and convenient measurements near sources of leakage radiation. It is desired to quantify rf fields without distorting them, while providing adequate shielding to the measuring device and telemetering link (or transmission cable). As a result, breadboard models of several types of rf monitors were built and tested. Improved receiving antennas were designed to make meaningful measurements without requiring time-consuming multiple orientation of the pickup antenna. The advent of an isotropic rf probe invented at NBS made it possible to perform rapid field surveying [2]. An investigation was performed at NBS to find a device or phenomenon that could provide a useful sensor of EM fields of complicated structure. Most of the early attempts revealed severe limitations for designing a general-purpose probe. For example, thermal rf sensors based on absorbed heat energy or temperature rise in a lossy material could not be made sufficiently rugged and stable unless the response time was excessive. It was concluded early in the program that the preferred type of rf sensor consists of a short dipole antenna and semiconductor detector, which has a fast response time and good sensitivity. As explained later, the EFM-5 isotropic probe is based on the use of three orthogonal dipoles, three diode detectors, and special signal processing to obtain the "total magnitude" of the electric field at the measurement point. However, it is advantageous to first review some of the early probe development work at NBS. Brief descriptions are given here of five types of rf probes which were evaluated in addition to the preferred choice using short dipoles and miniature diodes. The five types are as follows: (1) Color change in liquid crystals: These probes relied on observing the change in color of liquid crystal material, caused by temperature rise from absorption of rf energy. The probes were found to have a slow reaction time to changes in field level and were difficult to correct for variations in ambient temperature. They also had low sensitivity with only a small dynamic range, and the calibration was not stable over a long period of time. (2) Resistance change of a lossy dielectric: This type of sensor also operates on the basis of temperature rise due to absorption of rf energy. The lossy material tested was plastic impregnated with carbon, using resistance change as an indicator of temperature. However, similar to number (1) above, the sensitivity was inadequate and the time constant was excessive. (3) Glowing gas probe: Research was done at NBS on a "crossed dipole" probe with a gas tube at the center gap of the dipole antennas. Strong rf fields produced emission of light or change of resistance in noble gases. Progress toward realizing a useful measuring instrument was disappointing, due mainly to lack of stability and repeatibility, but also to lack of sensitivity and difficulty in handling the radioactive materials required for generating free electrons in the gas tubes. The sensors tested at NBS consisted of neon (Ne) gas inside a miniature glass bulb located at the center of a set of short dipoles. The metal dipole wires had sharp points at the center gap to enhance the E field. Light from the glowing gas was observed visually, but could have been guided by a fiberoptic bundle to a photodetector. Tests with a 5 cm dipole indicated that the Ne gas would not ignite until the rf power density was nearly 100 mW/cm2. After ignition, the glow disappeared when the field was reduced to about 10 mW/cm2. Thus the sensitivity was too low, although means could probably be provided for pre-ignition of the gas, such as with a separate RC oscillator. In one such device a high-resistance transmission line was used, and the Ne tube was caused to flash at a low frequency rate in the absence of an rf field. The frequency of oscillation could then be calibrated as a function of rf field level. (4) Incandescent bulb probe: Another type of rf sensor functions in terms of the short-circuit current developed at the center of a receiving dipole. In tests at NBS, the dipole was center-loaded with a miniature incandescent bulb having approximate dimensions 2 mm long by 1 mm diameter. The response time of such a sensor was found to be about 1 millisecond. The cold resistance of the filament was about 75 s, with a white-hot resistance of about 400 . The intensity of infrared and visible radiation increased as a function of the rf-induced dipole current. The radiation was guided through a glass fiber-optic bundle to a solidstate photodetector. The phototransistor output voltage, after subtracting the "dark" value, was found to increase approximately in proportion to the fourth power of the incident electric field value. The glass bulb had a lens-style envelope for better transfer of radiation into the fiber light guide. Tests indicated that the usable dynamic range was very small. The range achieved, between "dark" current and full brilliance of the bulb, was only about one decade (20 dB) in E field. It would thus be difficult to provide any burnout protection for the sensor bulb. (5) Thermocouple sensor probe: Several versions of rf probes using short dipoles center-loaded with thermocouple heaters were evaluated at NBS. In these probes the heater element and "hot" thermocouple junction were encased in a miniature glass bead, which in turn was enclosed in an evacuated glass bulb. The dc output of each thermocouple was proportional to (E)2, although over only a limited range in field strength. The total dynamic range between minimum-discernible field strength and burnout level was about 30 dB. In addition, the probes were susceptible to changes in ambient temperature and the sensor response time was quite slow. The radiation protection guide recommended by the American National Standards Institute (ANSI) for exposure to rf radiation is 10 mW/cm2, in the 10 MHz to 100 GHz frequency range [3]. This corresponds to free-space equivalent electric (E) and magnetic (H) field strengths of approximately 200 V/m and 0.5 A/m, respectively. Higher levels are permitted for short time durations if the power density averaged over a six-minute period does not exceed 10 mW/cm2 (100 W/m2). According to the ANSI C95.1 standard, "Radiation characterized by a power level tenfold smaller will not result in any noticeable effect on mankind. Radiation levels which are tenfold larger than recommended are certainly dangerous" [3]. General purpose instrumentation for monitoring exposure to rf radiation should also be capable of measuring weak fields [4]. The EFM-5 monitor measures accurately from a level of only 1 V/m up to 1000 V/m (0 to 60 dBV/m). This is equivalent to a plane-wave power density range of 0.000265 to 265 mW/cm2. Recently, instruments employing isotropic probes have become available commercially which permit practical measurement of either plane waves or complex fields, including the near zone of large antenna arrays. Most conventional FIM's with directive antennas cannot reliably measure EM fields with reactive near-field components, multipath reflections, unknown field polarization, complicated modulations, and large field gradients. The EFM-5 monitor was developed by NBS for measuring these complicated E fields at frequencies from 200 kHz to 1000 MHz. 1.2 Definitions of Field Intensity Units The mathematical relationships between the field intensity units of a plane-wave field, using RMS values for E and H, are given by magnitude of the power density in the radiated field, W/m2, magnitude of the electric field, V/m, Plane-wave conditions generally exist in the far zone of a transmitting antenna. In this case, the E and H field vectors are orthogonal to each other, and both are perpendicular to the direction of propagation. In addition, the ratio of the magnitudes, E/H, is a constant, given by the free-space wave impedance Z。. In this plane-wave case, E and H have the definite relation to average power density given by eq (1). For an isotropic E-field radiation monitor, the total magnitude of the electric field is defined by (and measured in terms of) the root-sum-of-squares (RSS) value of three mutually-orthogonal E-field components, as follows: |