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RF Safety Applications


Typical Survey Set

8715 RF meter and probe

This survey set is one of the author’s favorites. It consists of the small and simple-to-operate Model 8715 meter and the Model B8742D probe. This probe has a shaped-frequency response that closely follows the FCC’s Maximum Permissible Exposure limit for General Population/Uncontrolled exposure from 300 kHz to 3 GHz. The full-scale rating on the probe is 600 percent of the MPE limit. Simple math can be used to convert the readings to the MPE limit for Occupational/Controlled exposure.
The meter and probe are calibrated independently. The probe has an amplifier in the handle that is adjusted to give a normalized output for use with any meter. The higher output level makes the use of a cable practical. Probes without internal amplifiers generally cannot be used successfully with cables.

 

Survey Set versus Monitor

The sensors used in probes are also used in RF personal and area monitors. Personal monitors use two sensors rather than the three used in probes. The better monitors, such as the Nardalert XT, have a detection capability that is close to a hemisphere. RF personal monitors cannot detect RF energy from behind. In fact, RF-absorbent material is normally used behind the sensors so that energy that reflects off the body is not detected. The Nardalert XT uses a combination of the following three sensor technologies to cover its extremely broad bandwidth:


RF Survey Equipment

The vast majority of RF survey instruments are comprised of two major components:

Most probes are isotropic, or omnidirectional. Isotropic probes are designed with a set of three identical sensors placed at 54.7° off the center axis of the probe. This geometric arrangement is designed to yield the same results from all directions. In reality, a good probe design yields very similar results in most directions, with the direction of the handle being a notable exception. Although there are a few anisotropic (directional) probes on the market, they are of little value except to find leaks. Microwave oven instruments use anisotropic probes because the regulations for microwave ovens are emissions standards, not human exposure standards.
There are a couple of special designs where the sensor and probe are built into the same package, but this approach is rare because it results in a dedicated system with limited flexibility.

System Design Approaches

There are several tradeoffs that influence what approach is used in the design of the survey system. These system tradeoffs are largely independent of the basic design issues involving the probe sensors. The different design approaches yield different results when it comes to

Although there are many permutations, the basic survey system design approaches can be viewed as:

Configuration

Advantages

Disadvantages

Probes without amplifiers

  • Less expensive

  • Smaller, lighter probes

  • Limited interchangability

  • Difficult to use at low field levels with a cable due to "cable flex" problems

Probes with amplifiers

  • Probes and meters are calibrated independently, can be interchanged

  • Probe extension cables practical above 10 MHz

  • More expensive probes

  • Larger, heavier probes

Probes with amplifiers & fiber optic cables

  • Probes and meters are calibrated independently, can be interchanged

  • Fiber optic cables can be used at all frequencies to separate probe from meter

  • Fiber optic system and amplifier requires a battery in the probe

  • Most expensive design

Integrated probe & meter package

  • Least expensive design

  • Smallest package

  • Sensor close to body can compromise performance

  • Dedicated instrument; cannot change sensors

 

System Calibration

System calibration depends on system design.

  • Calibrated as a Set. A survey system that is “calibrated as a set” means that probe and meter variances are adjusted for within the meter. This is simpler and less expensive to do than it is to calibrate each component individually. The downside is that you cannot swap probes or meters of the same exact model and maintain calibration accuracy.

  • Independently Calibrated Components. Probes with integral amplifiers make it possible to “normalize” the output so that various probes and meters can be swapped while still maintaining calibration accuracy. Another approach is to store calibration data in a memory chip inside the probe even though an amplifier is not used. Some of the newest survey systems do not use amplifiers in the probe but accomplish interchangeability by providing a calibration factor with the probe. The calibration factor, probe model, and serial number are stored in the meter’s memory. The meter detects which probe is connected and automatically sets appropriate scales with readings adjusted by the calibration factor.

See Calibration for more detailed information on the calibration techniques of RF survey systems.

Sensor Designs

Isotropic probes are designed with a set of three identical sensors placed at 54.7° off the center axis of the probe. This geometric arrangement is designed to yield the same results from all directions. Probes are designed to detect either the electric (E) field or the magnetic (H) field.

  • Electric field probes. Most E-field probe sensor elements are based on a dipole with a detector. The detector is normally either a diode or a thermocouple. Some thermocouple probes operate in the traveling wave mode at very high frequencies rather than functioning as a dipole. These probes can be used to make accurate measurements up to 100 GHz and perhaps even higher. At lower frequencies (below a few hundred MHz), a surface area sensor can be used instead of a dipole. Probes with surface area sensors have a narrower bandwidth, but can operate well at lower frequencies with less interaction with the field and the surveyor due to the much lower impedance of this design. Surface area sensors use diode detectors.

  • Magnetic field probes. All magnetic, or H-field, probes use loops of wire with either a diode or thermocouple detector. The challenge in designing an H-field probe is eliminating out-of-band pickup and sensitivity to the electric field component. See Measurement Artifacts for more information.