RF Safety MeasurementsRF Safety MeasurementsIOSH meeting Emley 4IOSH meeting Emley 4thth July 2013 July 2013

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What’s New?What’s New?

Measurement & Monitoring EquipmentMeasurement & Monitoring Equipment

Survey EquipmentSurvey Equipment– For making quantitative measurementsFor making quantitative measurements

‘‘Broadband’ instrumentsBroadband’ instruments

Frequency selective instrumentsFrequency selective instruments

Personal MonitorsPersonal Monitors– For monitoring worker exposure levels For monitoring worker exposure levels

Area MonitorsArea Monitors– Continuous monitoring Continuous monitoring

SurveySurvey EquipmentEquipment Provides a relatively accurate Provides a relatively accurate

assessment of field strength and can assessment of field strength and can be used to determine the level of be used to determine the level of compliance to a particular standard.compliance to a particular standard.

Requires a reasonable level of Requires a reasonable level of training before it can be used with training before it can be used with confidence.confidence.

Does not provide continuous Does not provide continuous monitoring against sudden monitoring against sudden equipment failure.equipment failure.

Types of Field Sensing ProbeTypes of Field Sensing Probe

Probes measure either the Electric Probes measure either the Electric and/or Magnetic field components.and/or Magnetic field components.

Usually broadband without frequency Usually broadband without frequency selection capability.selection capability.

Usually isotropic although anisotropic Usually isotropic although anisotropic probes used for leakage measurement.probes used for leakage measurement.

May provide “flat” or “shaped” response.May provide “flat” or “shaped” response.

ShapedShaped Response Response

‘‘Flat’ Flat’ ResponseResponse

Types of Field Sensing ProbeTypes of Field Sensing Probe

ICNIRP 1998

1

10

100

1000

10000

1 10 100 1000 10000 100000 1000000 1E+07 1E+08 1E+09 1E+10 1E+11 1E+12

Frequency (Hz)

E F

ield

Str

eng

th (

V/m

)

1

10

100

1000

10000

1 10 100 1000 10000 100000 1000000 1E+07 1E+08 1E+09 1E+10 1E+11 1E+12

Frequency (Hz)

E F

ield

Str

eng

th (

V/m

)

Shaped ProbesShaped Probes ShapedShaped probes have a frequency response probes have a frequency response

weighted in accordance with a given safety weighted in accordance with a given safety guideline or standard. Shaped probes read guideline or standard. Shaped probes read out in “% of Std.” e.g. ICNIRP rather than in out in “% of Std.” e.g. ICNIRP rather than in normal field units.normal field units.

Shaped Shaped probes are useful for multiple probes are useful for multiple emitter environments (multiple frequencies emitter environments (multiple frequencies with different permissible exposure levels).with different permissible exposure levels).

Shaped Shaped probes are also useful when probes are also useful when surveying unknown or classified surveying unknown or classified frequenciesfrequencies..

Freq. PWR STD. %STD(MHz) W/m2 W/m2

148 2.5 10 25 900 5.0 22.5 22 2100 20 50 40 Total = 27.5 W/m2 87%

30 100 300 1 3 10

MHz GHz

Po

we

r D

en

sity

(W

/m²)

50

10

40

30

20

Determining Compliance in a Determining Compliance in a Multi-Emitter EnvironmentMulti-Emitter Environment

Freq. PWR STD. %STD(MHz) W/m2 W/m2

148 10 10 100 900 5.0 22.5 22 2100 12.5 50 25 Total = 27.5 W/m2 147%

30 100 300 1 3 10

MHz GHz

Po

we

r D

en

sity

(W

/m²)

50

10

40

30

20

Determining Compliance in a Determining Compliance in a Multi-Emitter EnvironmentMulti-Emitter Environment

Magnetic Fields are measured with loopsMagnetic Fields are measured with loops

AmmeterAmmeter

Flux LinesFlux Lines

Flux lines passing Flux lines passing through loop generate through loop generate current. Maximum current. Maximum indication if flux lines are indication if flux lines are perpendicular to loop. perpendicular to loop.

Mutually orthogonalMutually orthogonalloops are used inloops are used inmost most survey probes.survey probes.

Magnetic field measurementsMagnetic field measurements

Narrowband operation compared to E-field probes.Narrowband operation compared to E-field probes. May generate large out-of-band responses.May generate large out-of-band responses.

Low frequency Electric fields are measured with platesLow frequency Electric fields are measured with plates..

Field lines strike perpendicular to Field lines strike perpendicular to plates and the resultant dieletric plates and the resultant dieletric current is measured.current is measured.

ELF MeasurementsELF Measurements

Electric fields are measured Electric fields are measured with a mutually orthogonal with a mutually orthogonal array of diode dipoles or array of diode dipoles or thermocouples. thermocouples.

Measurement of RF Electric FieldsMeasurement of RF Electric Fields

Thermocouples exhibit extremely good adherence to the Thermocouples exhibit extremely good adherence to the square of the field strength, and their output is relatively square of the field strength, and their output is relatively independent of ambient temperature. Typically have wide independent of ambient temperature. Typically have wide frequency operation (0.3 to 100GHz). Limited sensitivity, frequency operation (0.3 to 100GHz). Limited sensitivity, minimum measurement around 6 V/m (10minimum measurement around 6 V/m (10W/cm²). W/cm²). Relatively low overload level. Relatively expensive.Relatively low overload level. Relatively expensive.

Diode Based DetectionDiode Based Detection– Better “zero stability” and overload handling Better “zero stability” and overload handling

but higher temperature sensitivity.but higher temperature sensitivity.– Good at low field levels, when diode is in it’s Good at low field levels, when diode is in it’s

“square-law” region. A diode is a non-linear “square-law” region. A diode is a non-linear device, which when operated at higher device, which when operated at higher levels will detect peak rather than average levels will detect peak rather than average levels. Not recommended for multiple signal levels. Not recommended for multiple signal or pulsed signal environments.or pulsed signal environments.

Measurement of RF Electric FieldsMeasurement of RF Electric Fields

Types of Detectors - DiodeTypes of Detectors - Diode

Pow

er In

Voltage Out

(A2 + B2 + C2)

(A + B + C)2

“Square Law”Response

Measurement UncertaintyMeasurement UncertaintyThree basic approaches to measurement uncertainty: Three basic approaches to measurement uncertainty:

Direct comparison Direct comparison (or shared) risk approach - (or shared) risk approach - Measurement is taken as read. Uncertainty just quoted. Measurement is taken as read. Uncertainty just quoted. May lead to poor quality measurements.May lead to poor quality measurements.Additive approach Additive approach - Maximum permissible exposure - Maximum permissible exposure level is inset from the given safety guideline level by the level is inset from the given safety guideline level by the amount of uncertainty. Can be overly restrictive.amount of uncertainty. Can be overly restrictive.Hybrid Approach Hybrid Approach - If uncertainty is kept inside a given - If uncertainty is kept inside a given limit e.g. 4dB, the measurement can be taken as read. If limit e.g. 4dB, the measurement can be taken as read. If uncertainty exceeds the stated limit then the additional uncertainty exceeds the stated limit then the additional uncertainty is applied to the recorded value.uncertainty is applied to the recorded value.

Measurement UncertaintyMeasurement Uncertainty10. Uncertainty

NBM550 & EF0391 (Broadband equipment)

Source of uncertainty Uncertainty Value (dB)

Probability Distribution Divisor Standard Uncertainty(dB)

Variation due to probe isotropy

1 Rectangular 1.73 0.58

Variation in linearity response

0.5 Rectangular 1.73 0.29

Variation due to frequency response

1.25 Rectangular 1.73 0.72

Calibration uncertainty 1.5 2 (k=2) 0.75

Variation due to temperature

0.2 Rectangular 1.73 0.12

Repeatability 2.0 Normal 2 (k=2) 1.0

Combined standard uncertainty

1.58

Expansion factor 1.96

Expanded uncertainty 3.1

Expanded uncertainty (%) 50%

See EN50413 for further information.See EN50413 for further information.

Frequency Selective Survey MeterFrequency Selective Survey Meter

Combines the features of a hand-held spectrum analyser and an isotropic probe.

Utilises an active antenna ideal for high sensitivity applications.

Expensive compared to conventional broadband equipment.

Limited frequency range 100kHz/27MHz to 6GHz.

Frequency Selective Survey MeterFrequency Selective Survey Meter

Frequency Selective Survey MeterFrequency Selective Survey Meter

Induced Current MeasurementsInduced Current Measurements Induced body current & contact current measurements Induced body current & contact current measurements

occasionally may be relevant for frequencies up to 110MHz.occasionally may be relevant for frequencies up to 110MHz.

RF induces voltage in ungrounded conductorPerson acts as current path to ground by touching charged conductor

RF induces voltage flowin personCurrent flows to ground

Most UK telecoms and broadcast Most UK telecoms and broadcast companies utilise personal monitors companies utilise personal monitors as part of a RF safety programme. as part of a RF safety programme. simple to usesimple to use have a wide operating frequency have a wide operating frequency

range (100 kHz to 100 GHz) range (100 kHz to 100 GHz) shaped alarm thresholdshaped alarm threshold expensiveexpensive not accurate enough to be used not accurate enough to be used

for quantitative readingsfor quantitative readings

Personal monitorsPersonal monitors

TThe small print:he small print:PolarizationPolarization - - Personal monitors should have the Personal monitors should have the

ability to detect all polarizations equally as an ability to detect all polarizations equally as an acute failure or near field applications will not have acute failure or near field applications will not have a predictable polarization.a predictable polarization.

Detection Angles - Personal Detection Angles - Personal monitors should have the monitors should have the widest possible detection widest possible detection angles, typically 45 to 90 angles, typically 45 to 90 degreesdegrees.

Personal monitorsPersonal monitors

The small print:The small print:Directivity. Most personal monitors are not Directivity. Most personal monitors are not

omnidirectional, by function of their design they do omnidirectional, by function of their design they do not detect rf emissions to the rear of the unit. not detect rf emissions to the rear of the unit. Accuracy should not be affected by presence of Accuracy should not be affected by presence of the human body.the human body.

Dosimetry. Personal monitors are not required to Dosimetry. Personal monitors are not required to be true dosimeters - however some models do be true dosimeters - however some models do have data logging capability.have data logging capability.

Personal monitorsPersonal monitors

Area MonitorsArea MonitorsThere are two main applications for area monitors:There are two main applications for area monitors:

Constant monitoring of an area near high power Constant monitoring of an area near high power systems such as radar. Typically used for systems such as radar. Typically used for occupational applications.occupational applications.

Constant monitoring Constant monitoring and dataloggingand datalogging of of relatively low power signals near cell sites, radio relatively low power signals near cell sites, radio masts etc. Typically used in public areas.masts etc. Typically used in public areas.

What to use?What to use?

For general public measurements:- Telecoms & broadcast – usually Electric field only- Frequency Range?- Measurement Sensitivity?- Broadband or frequency selective? - Measurement uncertainty often less critical as

recorded values will be tiny percentages of permissible exposure levels.

What to use?What to use?For occupational measurements:

- Frequency range?- Broadband or frequency selective?- Measurement range?- Overload (CW & Peak)?- Electric and/or Magnetic field?- Time or spatial averaging? - Meter memory?- Diode or thermocouple detectors? (Simple diode

detectors not recommended for multiple signal or pulsed signal environments)

Measurement equipmentMeasurement equipmentThe small print:The small print:

- Frequency sensitivity Frequency sensitivity - IsotropyIsotropy- LinearityLinearity- Out of band responseOut of band response- Temperature responseTemperature response- Unwanted response to the E or H componentUnwanted response to the E or H component- Minimum measurement distanceMinimum measurement distance- Calibration standardCalibration standard

No automatic need for measurement or No automatic need for measurement or monitoring equipment. Risk assessment can be monitoring equipment. Risk assessment can be by calculation.by calculation.

Measurement or monitoring equipment should be Measurement or monitoring equipment should be backed up with appropriate training.backed up with appropriate training.

Measurement or monitoring equipment should be Measurement or monitoring equipment should be used as part of a RF safety programme.used as part of a RF safety programme.

Note requirements of EN50499 ‘Determination of Note requirements of EN50499 ‘Determination of workers exposure to electromagnetic fields’ etc.workers exposure to electromagnetic fields’ etc.

Occupational ExposureOccupational Exposure

Measurement & monitoring equipment can be Measurement & monitoring equipment can be an important part of an RF safety programme an important part of an RF safety programme but do not forget common sense and a basic but do not forget common sense and a basic risk assessment, you need to identify;risk assessment, you need to identify;- output power from source(s)output power from source(s)- distance to source(s)distance to source(s)- exposure periodexposure period- operating frequencyoperating frequency

Occupational ExposureOccupational Exposure

Safe?Safe?Unsafe?Unsafe?Near Hz detector

blah V2/m2 %*~!!

And finally…….And finally…….

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