Broadband EM Field Measurement Takes a Leap Ahead
By Bob Johnson, Narda Safety Test Solutions

Four decades ago, when Narda Microwave essentially created the market for equipment dedicated to measuring nonionizing radiation, there were no international standards for radiation safety, and the question of “what level of exposure is safe” wouldn’t become a hot topic for debate for years to come. The arrival of the first standards created a contentious environment that remains today. Through all of this, the mission of broadband instruments has remained essentially the same: To capture “all” signals and present their strength to the user as an aggregate value, usually expressed in volts per meter. What has changed is the ease with which these measurements can be made, their accuracy, measurement data quantity and quality, the sophistication of data analysis -- and a massive improvement in instrument portability. In other words, pretty much everything but the measurement itself. The NBM-550 broadband electromagnetic field measurement system just introduced by Narda exemplifies just how far this equipment has come.

Narda has expanded its capabilities tremendously in the last 6 years, with the acquisition of the radiation measurement systems business of Wandel & Goltermann, and the Italian test equipment company PMM. This resulted in some redundancy in the product line. However, it also presented opportunity to combine the best of each of the three design methodologies to produce a family of instruments that the individual entities alone might not have achieved. The NBM-550 (Figure 1) represents this achievement, and (along with some other instruments soon to follow) will replace the company’s current 8700, EMR, and 8053 Series instruments.

To ensure that the NBM Series that would incorporate the features needed by most users, Narda engineers talked to hundreds of customers throughout North America, Europe, and Asia before designing these instruments. The most obvious results of this input are enhancements to the instrument display, comprehensive PC-based analysis and data management software, and the addition of an optional GPS receiver that functions as integral part of the measurement system. However, there are a variety of other enhancements as well.
The previous instruments had 4-line x 20 character displays (or less), which limited the amount of information that could be provided to the operator. The NBM Series instruments have a 4-in. backlit monochrome LCD with 240 x 320 pixel resolution. While a color TFT LCD might seem a logical choice, monochrome displays remain far superior when viewed in direct sunlight or through sunglasses.

The enhanced display allows a much larger amount of information to be simultaneously displayed. The most common functions are handled by soft keys on the display with dedicated buttons below them on the instrument body. The display can show results as actual, minimum, maximum, average, and maximum average, and units are selectable as V/m, A/m, W/m2, and mW/cm2. It will also display the reading as a “% of Std” Total field strength can be displayed with readings from each axis (when used with a probe with separate axes), and the unit continuously records and stores 8 hr. of measurement data, which can be viewed simultaneously with maximum, minimum, average, and maximum average field levels on the display. The instrument has the gist of key standards and guidances in firmware, which allows it to compute results obtained at specific frequencies, determine the percentage of standard’s allowable level that is present, and show that percentage on the display.

The GPS feature satisfies a longstanding request from users: the ability to integrate geographic coordinates into the measurement and documentation process, without the need to bring along a stand-alone GPS receiver and entering them manually. The Narda GPS receiver and antenna connect direct to the instrument body, and operates from the main NBM-550 keypad. Coordinates become part of the data record, a valuable and precise tool for verifying measurement location. GPS accuracy in enhanced mode is 3 m.

The NBM-550 “GPS bundle” includes the receiver, cable, and hardware that allow the receiver to be mounted on the meter (Figure 2). It also includes an audio recorder with VU meter (used with the standard integral microphone) for adding one voice comment up to 30 s long for each measurement result. The audio becomes part of the data file and is transferred to the supplied PC software (described later) for inclusion in reports.

The “bundle” also includes conditional logging capability that records data only when the energy level falls below or rises above a user-defined threshold, or only once at the upper or lower threshold points. The “GPS Option Set” is field-installable by the user, and need not be part of the initial purchase.

The most time-consuming element of the measurement survey is often not making the measurements but reporting the results in sufficient detail to satisfy the requirements of a RF radiation safety program. To that end, the enhanced software (NBM-TS) supplied with the NBM Series not only makes the process easier, but more complete as well.

A picture being worth 1,000 words, the software allows satellite photos from services such as Google Earth and Microsoft Virtual Earth to be integrated into survey reports based on the coordinates supplied by the GPS receiver (Figure 3). The software also manages remote control of the NBM Series instruments and performs firmware upgrades. Narda crafted the software to have the look and feel of Microsoft Vista, with which it will surely be used in the coming years. Nevertheless, the NBM-TS software runs equally well on WindowsXP.

Fewer, But Better, Probes
With this announcement, Narda has the reduced the number of available probes but increased their capabilities. Frequency coverage is very broad (as great as 27 MHz to 60 GHz) and 10 individual models are available. When the unit is turned on, it performs a series of diagnostic tests of its own capabilities and those of the probes, which eliminates the need to place the probes in an RF-radiation-free environment at start-up.

Each new probe is calibrated at multiple frequencies and the calibration information is stored in the probe and once connected is transferred to the meter. The new probes are also more rugged than their predecessors and calibration is recommended only once every 2 yr., versus every year for older units.

All of the electric and magnetic field probes, flat or shaped response (E-field) probes, and diode and thermocouple detectors can be connected directly to the meter or via fiber optic cable for remote operation at up to 60 ft. away. This removes the instrument and the person operating it from the area of measurement, providing a more “pristine” environment. Shaped probes, first patented by Narda in 1986, let the operator quickly determine total field strength and its relation to international standards. Other probes allow field strength of each axis to be read separately for polarization measurements.

The “flat response” type probe is designed to be equally responsive to energy at every frequency within its rated frequency range. It provides the operator with a single value (expressed as V/m, A/m, or mW/cm2), the total of all emissions it has detected within its frequency range. This level of detail is acceptable for the majority of applications, such as when a safety officer wants to determine if the subject environment complies with an applicable standard, but not which emitters are contributing to the total.

However, if more detail is required, Narda offers shaped response probes whose frequency response is optimized to achieve varying levels of sensitivity that mirror a particular standard (or guidance) as closely as possible. For example, many of the world’s major guidances and standards set electric-field limits for maximum human exposure at lower frequencies (around 1 MHz) at 614 V/m (1000 W/m2). At higher frequencies, from 10 to 400 MHz, the maximum exposure limits are typically much more stringent: 61.4 V/m (10 W/m2), a difference of 20 dB (a 100-time reduction in power). Narda’s “shaped” probes for this application are thus 100 times more sensitive at 100 MHz as they are at 1 MHz.

Since they are designed to be representative of standards or guidances, shaped probes display total field strength not in V/m or W/m2, but as a percentage of the energy allowed by a standard. At a multiple emitter site, a result of 15% of standard is simple to understand. The total detected field strength of each emitter has produced a total energy level that is 15% of the maximum allowable energy level (at its operating frequency and rated output power level).

Some of the other features of the NBM-550 include:

• 5,000-result measurement memory
• Time averaging up to 30 min
• Individual or continuous spatial averaging
• Spatial averaging with up to 24 locations
• Audible warnings: Hot spot, programmable limit
• Calibration reminder
• Selectable backlight duration: 5, 10, 30, 60 s and full on
• Storage for multiple user-created test configurations
• Programmable auto-off
• Hold button for freezing the current result
• Powered by user replaceable NiMH AA rechargeable batteries
• Headphone jack
• Low-battery and calibration interval warnings
• Timer logging up to 24 h with a sampling rate of 1 s to 6 min. in 11 steps
• External trigger

The NBM-550 alone weighs 1.3 lb. (550 g) and with typical a probe and GPS 1.6 lb. (740 g). The instrument measures 1.7 x 3.8 x 11 in, (45 x 98 x 128 mm). Without GPS, it operates for a maximum of 20 h (no backlight), 12 h with backlight continuously on and without GPS, and 10 h with GPS continuously on but without backlight onIt is supplied with a hard case, battery charger, rechargeable 2500 mAh NiMH batteries, shoulder strap, bench-top tripod, NBM-TS software, calibration certificate, manual, and USB cable. More information can be obtained at www.narda-sts.us or by e-mail to nardasts@l-3com.com.

NARDA SAFETY TEST SOLUTIONS
www.nardamicrowave.com
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