Measurements and Monitors

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Transcript Measurements and Monitors

RF Safety Measurements
IOSH meeting Emley 4th July 2013
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What’s New?
Measurement & Monitoring Equipment

Survey Equipment
– For making quantitative measurements
‘Broadband’ instruments
Frequency selective instruments

Personal Monitors
– For monitoring worker exposure levels

Area Monitors
– Continuous monitoring
Survey Equipment
Provides a relatively accurate
assessment of field strength and can
be used to determine the level of
compliance to a particular standard.
Requires a reasonable level of
training before it can be used with
confidence.
Does not provide continuous
monitoring against sudden
equipment failure.
Types of Field Sensing Probe
 Probes
measure either the Electric
and/or Magnetic field components.
 Usually broadband without frequency
selection capability.
 Usually isotropic although anisotropic
probes used for leakage measurement.
 May provide “flat” or “shaped” response.
Types of Field Sensing Probe
E Field Strength (V/m)
10000
 ‘Flat’ Response
1000
100
10
1
1
10
100
1000
10000
100000 1000000 1E+07
1E+08
1E+09
1E+10
1E+11
1E+12
Frequency (Hz)
ICNIRP 1998
E Field Strength (V/m)
10000
 Shaped Response
1000
100
10
1
1
10
100
1000
10000
100000 1000000 1E+07
Frequency (Hz)
1E+08
1E+09
1E+10
1E+11
1E+12
Shaped Probes
Shaped probes have a frequency response
weighted in accordance with a given safety
guideline or standard. Shaped probes read
out in “% of Std.” e.g. ICNIRP rather than in
normal field units.
 Shaped probes are useful for multiple
emitter environments (multiple frequencies
with different permissible exposure levels).
 Shaped probes are also useful when
surveying unknown or classified
frequencies.

Determining Compliance in a
Multi-Emitter Environment
50
Freq.
PWR
STD.
(MHz)
W/m2
W/m2
148
2.5
10
900
5.0
22.5
2100
20
50
Total = 27.5 W/m2
Power Density (W/m²)
40
30
%STD
25
22
40
87%
20
10
30
100
MHz
300
1
3
GHz
10
Determining Compliance in a
Multi-Emitter Environment
50
Freq.
PWR
STD.
(MHz)
W/m2
W/m2
148
10
10
900
5.0
22.5
2100
12.5
50
Total = 27.5 W/m2
Power Density (W/m²)
40
30
%STD
100
22
25
147%
20
10
30
100
MHz
300
1
3
GHz
10
Magnetic field measurements
Magnetic Fields are measured with loops
Flux Lines
Ammeter
Flux lines passing
through loop generate
current. Maximum
indication if flux lines are
perpendicular to loop.
Mutually orthogonal
loops are used in
most survey probes.
 Narrowband operation compared to E-field probes.
 May generate large out-of-band responses.
ELF Measurements
Low frequency Electric fields are measured with plates.
Field lines strike perpendicular to
plates and the resultant dieletric
current is measured.
Measurement of RF Electric Fields
Electric fields are measured
with a mutually orthogonal
array of diode dipoles or
thermocouples.
Thermocouples exhibit extremely good adherence to the
square of the field strength, and their output is relatively
independent of ambient temperature. Typically have wide
frequency operation (0.3 to 100GHz). Limited sensitivity,
minimum measurement around 6 V/m (10W/cm²).
Relatively low overload level. Relatively expensive.
Measurement of RF Electric Fields
 Diode
Based Detection
– Better “zero stability” and overload handling
but higher temperature sensitivity.
– Good at low field levels, when diode is in it’s
“square-law” region. A diode is a non-linear
device, which when operated at higher
levels will detect peak rather than average
levels. Not recommended for multiple signal
or pulsed signal environments.
Power In
Types of Detectors - Diode
“Square Law”
Response
(A + B + C)2
(A2 + B2 + C2)
Voltage Out
Measurement Uncertainty
Three basic approaches to measurement uncertainty:
 Direct comparison (or shared) risk approach Measurement is taken as read. Uncertainty just quoted.
May lead to poor quality measurements.
 Additive approach - Maximum permissible exposure
level is inset from the given safety guideline level by
the amount of uncertainty. Can be overly restrictive.
 Hybrid Approach - If uncertainty is kept inside a given
limit e.g. 4dB, the measurement can be taken as read.
If uncertainty exceeds the stated limit then the
additional uncertainty is applied to the recorded value.
Measurement Uncertainty
10. 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 (%)
See EN50413 for further information.
50%
Frequency 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 Meter
Frequency Selective Survey Meter
Induced Current Measurements
 Induced body current & contact current measurements
occasionally may be relevant for frequencies up to 110MHz.
RF induces voltage in
ungrounded conductor
Person acts as current path to
ground by touching charged
conductor
RF induces voltage flow
in person
Current flows to ground
Personal monitors

Most UK telecoms and broadcast
companies utilise personal monitors
as part of a RF safety programme.
 simple to use
 have a wide operating frequency
range (100 kHz to 100 GHz)
 shaped alarm threshold
 expensive
 not accurate enough to be used
for quantitative readings
Personal monitors
The small print:
 Polarization - Personal monitors should have the
ability to detect all polarizations equally as an
acute failure or near field applications will not have
a predictable polarization.
 Detection Angles - Personal
monitors should have the
widest possible detection
angles, typically 45 to 90
degrees.
Personal monitors
The small print:
Directivity. Most personal monitors are not
omnidirectional, by function of their design they do
not detect rf emissions to the rear of the unit.
Accuracy should not be affected by presence of
the human body.
Dosimetry. Personal monitors are not required to
be true dosimeters - however some models do
have data logging capability.
Area Monitors
There are two main applications for area monitors:
 Constant monitoring of an area near high power
systems such as radar. Typically used for
occupational applications.
 Constant monitoring and datalogging of
relatively low power signals near cell sites, radio
masts etc. Typically used in public areas.
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?
 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 equipment
The small print:
-
Frequency sensitivity
Isotropy
Linearity
Out of band response
Temperature response
Unwanted response to the E or H component
Minimum measurement distance
Calibration standard
Occupational Exposure
No automatic need for measurement or
monitoring equipment. Risk assessment can be
by calculation.
 Measurement or monitoring equipment should be
backed up with appropriate training.
 Measurement or monitoring equipment should be
used as part of a RF safety programme.
 Note requirements of EN50499 ‘Determination of
workers exposure to electromagnetic fields’ etc.

Occupational Exposure
Measurement & monitoring equipment can be
an important part of an RF safety programme
but do not forget common sense and a basic
risk assessment, you need to identify;
- output power from source(s)
- distance to source(s)
- exposure period
- operating frequency
And finally…….
Near Hz detector
blah  V2/m2 %*~!!
Safe?
Unsafe?
Any Questions?
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