The Effects of Contents and Apertures on the Structure of Electromagnetic Fields in Enclosed Spaces

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The Effects of Contents and Apertures on the
Structure of Electromagnetic Fields in Enclosed
Spaces  

• C. Marvin, M.P. Robinson J. F. Dawson.
• University of York
• R. Kebel. Airbus

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Introduction

• Current practise is to prohibit the use of
  carry-on electronic devices with antennas and to
  restrict the use of other carry-on electronic
  devices in aircraft cabins when the doors are
  closed.
• Evidence of interference to aircraft systems is
  cited as the reason for these restrictions.

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Operating Issues

• As the number and variety of such devices
  increases, passenger acceptance of the
  restrictions is likely to decrease
• Passengers would welcome the facility to continue
  their communications and IT activities in flight
• Appropriate cabin design could make a seamless
  transition from the departure lounge to the cabin

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Electromagnetic Issues

• Many electromagnetic systems already operate in
  aircraft as long as they are designed in there
  need be no problem.
• Can other carry-on electromagnetic systems be
  incorporated?
• What is the electromagnetic environment in the
  aircraft cabin?

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Electromagnetic Field Properties

• The aircraft cabin is a conducting tube with
  apertures and contents.
• Comparable studies on missile bodies (GENEC) are
  tractable using full-wave or intermediate level
  models.
• The scale of a passenger cabin makes either of
  these approaches difficult and a statistical
  approach is required.

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Preliminary Measurements

• Press reports have speculated that the interior
  of trains with multiple mobile phone users may
  contain hot spots that could cause exposure
  above current ICNIRP limits and, by implication,
  EMC problems.
• We have undertaken preliminary measurements in a
  simulated cabin environment.

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The cabin environment!
The simulated cabin is a Screened Room 4.7m long
with a cross-section of 2.37m by 3m. It holds
nine passengers with business class spacing!
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The cabin environment!
Windows are simulated by blocks of AN79 absorber.
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A flying screened room!! (Shorts 330
G-BEEO)Photo by Richard Hunt,UK
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Frequency Response and Statistics of the Empty
Cabin
Measured coupling between two roof mounted 50mm
monopoles in the frequency range 900MHz to 920MHz
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Frequency Response and Statistics with nine
Passengers
Nine passengers seated (green) and standing
(red). Statistics are for seated passengers.
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Simulated Results I
Results of simulation of response of room, for
various values of Q. Blue Q10000 green
Q1000 red Q100.  
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Simulated Results II

• The simulation was done by combining the
  frequency responses of all modes with resonant
  frequencies in the range 880-940MHz, with random
  phase differences between the modes, and random
  coupling coefficients. Compare these curves with
  the measured responses of the room. I estimate
  that the empty room has a Q of about 10000.
  Putting in the windows brings the Q down to
  about 1000. Filling the room with passengers
  brings the Q down to about 100.

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Simulated Results III

• We have also done some rough calculations of
  these Q-factors, using a paper by D Hill for the
  windows and resonant perturbation theory for the
  passengers. These give contributions of Q32400
  for the windows, and Q2 of somewhere between 3.7
  and 720 for the passengers, depending on their
  orientation. These figures are of the right
  order of magnitude compared to the measurements.
•  The statistics are also similar to those of the
  measurements. The blue curve (Q10000) has
  Rayleigh-like statistics, the red and green
  curves dont.

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Simulated Coupling to Transmission Line using TLM
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Equivalent Intermediate Level Circuit Model
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Near end coupling
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Statistics of near end coupling (1.25-3GHz)
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Concluding Remarks I

• Empty cabin with no windows responses follow the
  expected highly resonant behaviour of an
  over-moded cavity with Rayleigh like statistics.
• The inclusion of representative apertures and
  passengers gives substantial damping of the
  resonant behaviour and completely alters the
  field statistics.

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Concluding Remarks II

• The average coupling levels between antenna pairs
  is reduced in the presence of loss mechanisms.
• The assertion that vehicle bodies will act as
  resonant cavities resulting in hot spots for
  fields from multiple sources is cast into doubt.

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Concluding Remarks III

• We have rather more to do in this study!