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EMI in an RQZ:
the need for buffer zones
Carol Wilson, CSIRO Research Consultant
RFI2010, Groningen
Outline of presentation
Introduction – purpose of EMI buffer zone
analysis
Prediction method components
• Interference thresholds
• Emission levels
• Characterisation of EMI activities
• Prediction of attenuation
Examples of analysis
Results of buffer zone analysis
Close
Introduction – EMI buffer zones
Intentional radio transmitters  narrowband signals 
variety of mitigation techniques (including avoidance)
Noise from electrical equipment, machinery 
broadband interference  harder to mitigate
Need to define buffer zones around human activity to
avoid EMI in design of SKA array configuration
Challenges due to SKA requirements
• Frequency – 70 MHz to 25.25 GHz – large compared
to most current telescopes
• Physical extent of telescope (large number of separate
stations over a large geographic extent)
• Rigorous demands of radioastronomy
• Time frame (current -> ~2020 -> 50 years or more)
SKA three zone approach
Core (2.5 km radius, 50% of array) with highest levels of
protection (aim for ITU-R RA.769-2 continuum).
Intermediate (<180 km radius, 25% of array) with
protection relaxed by 15 dB from continuum thresholds.
Remote (> 180 km, 25% of array) with protection relaxed
by further 25 dB (corresponds to VLBI levels).
Core site chosen in a remote location
Intermediate and remote sites to be chosen based on
RFI, UV coverage, geophysics, logistics, etc
Buffer zones for separation from EMI needed for
intermediate and remote sites
Components of prediction model
Emission level – radioastronomy threshold =
attenuation required
Model expected usage patterns of equipment
Use propagation model to find distance at which
attenuation is exceeded
• Thresholds established by ITU-R Study Group 7
• Emissions – use published EMC/EMI standards
• Model of usage patterns - estimated
• Attenuation – need propagation model(s)
Defining radio quietness
ITU Recommendation RA.769-2
• Harmful interference levels assuming reception into 0 dBi sidelobe
• Values for continuum, spectral line and VLBI observations
• Continuum observation values used as basic protection levels for SKA core
Frequency
f
(MHz)
151.525
325.3
408.05
611
1 413.5
1 665
2 695
4 995
10 650
15 375
22 355
23 800
Assumed bandwidth
Df
(kHz)
2.95
6.6
3.9
6
27
10
10
10
100
50
290
400
Threshold interference levels
Input power DPH
(dBW)
Pfd SH D f
(dB(W/m2))
Spectral pfd SH
(dB(W/(m2  Hz)))
(7)
(8)
(9)
–199
–201
–203
–202
–205
–207
–207
–207
–202
–202
–195
–195
–194
–189
–189
–185
–180
–181
–177
–171
–160
–156
–146
–147
–259
–258
–255
–253
–255
–251
–247
–241
–240
–233
–231
–233
(7) Power level at the input of the receiver considered harmful to high sensitivity observations, DPH. This is expressed as the
interference level which introduces an error of not more than 10% in the measurement of DP.
(8) pfd in a spectral line channel needed to produce a power level of DPH in the receiving system with an isotropic receiving
antenna.
(9) Spectral pfd needed to produce a power level DPH in the receiving system with an isotropic receiving antenna.
Interference thresholds
Interference threshold levels from Recommendation ITU-R RA.769-2
-160
Spectral pfd (dB(W/(m2 Hz)))
-180
-200
-220
-240
Continuum
Continuum +15
-260
Line
VLBI
-280
-300
10
100
1000
Frequency (MHz)
10000
100000
Emission standards
Interference power as a function of frequency from:
• Road vehicles – CISPR standard 12:2005.
• Railways – European EN 50121-2: 2006.
• Household appliances and tools – CISPR standard 141:2003.
• Arc welders – CISPR standard 11:2004.
• Power lines - Australian standard AS/NZS 2344:1997.
Frequency dependence:
• Roads: increasing to 400 MHz, then constant
• Rail: decreasing with frequency
• Appliances/tools: increasing to 300 MHz, assumed
constant beyond
Categories
Farmsteads/individual dwellings = 7 appliances, 4
power tools and 1 vehicle.
Towns – n x dwellings, assumption that 1/3 of
households are “active” at any time.
Roads – minor (small number of vehicles per day) and
major (multiple vehicles for a substantial part of the
day)
Rail – lightly used (4 trains a day) and heavy
(continuous use)
Mines – based on town analysis for similar activity
Power lines – range of voltages
Propagation models
Some propagation models use specific terrain
For buffer zones, generic models needed
ITU-R Recommendation P.1546-3 used for EMI buffers
Inputs: frequency, terminal heights, time percentage, type
of path (land, sea, mixed)
• Interference source height: 2 metres (except power lines)
• Telescope height: 1 metre < 300 MHz, 15 metres > 300 MHz
Model used iteratively to find distance for required loss
Attenuation increases as function of frequency
Attenuation decreases with higher antenna heights
(discontinuity at 300 MHz due to telescope height)
Example – minor roads
At 300 MHz, vehicle emission level = 63 (dBμV/m)
= -102.5 dBW/Hz
At 300 MHz, the RA.769-2 threshold = -258 dB(W/m2·Hz).
Intermediate zone, relaxed by 15 dB, = -243 dB(W/m2·Hz)
= -269 dBW/Hz
The attenuation required = -269 – (-102.5) = 166.5 dB
For antenna heights 15 m (telescope) and 2 m (interferer),
at 300 MHz, distance = 12 km
Furthermore…
Example – minor roads (continued)
Allow 10 vehicles per day within 12 km for 5% of day.
0.05*24*60 = 72 minutes or 7.2 minutes per vehicle
At 100 km/hr, vehicle travels 12 km in 7.2 minutes
Relax limit to 10.5 km as shown
Repeat at other frequencies
12 km
12 km
10.5 km
Example – minor roads (continued)
Buffer zone for minor roads
14.0
Buffer zone distance (km)
12.0
10.0
8.0
6.0
4.0
Low antenna below 300 MHz
15 m antenna
2.0
0.0
0
100
200
300
400
500
600
Frequency (MHz)
700
800
900
1000
Example for remote zone (25 dB relaxation)
Buffer zone for towns of 1000 people (67 households)
7
Buffer zone distance (km)
6.5
6
5.5
5
4.5
4
3.5
3
0
100
200
300
400
500
600
Frequency (MHz)
700
800
900
1000
Buffer zones for intermediate and remote stations
Intermediate
Remote
Minor roads
10.5 km
3 km
Major roads
33.5
6.5
Local rail
10.5
3.5
Heavy rail
30.5
5.5
Farms
13.5
3
21
5
1000 people
27
6.5
5000 people
31
7
10000 people
37.5
8.5
Up to 8 km
Up to 1.5 km
Towns:
100 people
Power lines
Conclusions
• Core of SKA to be protected primarily by remote
location
• Buffer zones around human activity have been
defined for the intermediate and remote stations
of the SKA
• The methodology for calculation can be applied to
other scenarios (e.g. intermediate traffic roads)
• EMI buffer zones will be part of analysis in
optimising the SKA array configuration
Thank you!
Questions?
Carol Wilson, Research Consultant
[email protected]
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