Survey Instruments

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Contents

  1. Survey system components
  2. System design approaches
    bulletProbes and meters calibrated as a set
    bulletProbe calibration information stored in probe
    bulletProbe calibration information stored in meter
    bulletProbes with amplifiers; components calibrated independently
  3. Sensor designs
    bulletElectric Field Probes
    bulletMagnetic Field Probes

Survey System Components

 

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

 

  1. A broadband probe—the sensor

  2. A meter

Although there are a couple of special designs where the sensor and probe are built into the same package, this approach is rare because it results in a dedicated system with limited flexibility.

Most probes are isotropic, or omni-directional.  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 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.

 

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 that will be discussed yield different results when it comes to:

 

bulletCost
bulletCalibration
bulletOperation with the probe separated from the meter
bulletOperation at low frequencies
bulletInterchangeability of system components

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

Configuration Advantages Disadvantages
Probes without amplifiers
bulletLess expensive
bulletSmaller, lighter probes
bulletProbes & meters calibrated as a set; cannot be interchanged
bulletDifficult to use at low field levels with a cable due to "cable flex" problems
Probes with amplifiers
bulletProbes and meters are calibrated independently, can be interchanged
bulletProbe extension cables practical above 10 MHz
bulletMore expensive probes
bulletLarger, heavier probes
Probes with amplifiers & fiber optic cables
bulletProbes and meters are calibrated independently, can be interchanged
bulletFiber optic cables can be used at all frequencies to separate probe from meter
bulletFiber optic system and amplifier requires a battery in the probe
bulletMost expensive design
Integrated probe & meter package
bulletLeast expensive design
bulletSmallest package
bulletSensor close to body can compromise performance
bulletDedicated instrument; cannot change sensors

System calibration depends on system design. 

bulletCalibrated 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 to calibrate each component individually.  The downside is that you cannot swap probes or meters of the same exact model and maintain calibration accuracy.
bulletIndependently 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.

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. 

bulletElectric 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 hundred MHz) a surface area sensor design 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.
bulletMagnetic 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 eliminate out-of-band pick-up and sensitivity to the electric field component.  See  measurement Artifacts for more information.

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Site last modified: 2/28/2007