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Continuation of investigations into the possible effect of DSL related systems on Radio Services |
ERA Technology Ltd - Report
2001- 0333
RA Reference AY 3949
Executive Summary
Recent studies by York University for the Radiocommunications Agency (RA) concluded that mass deployment of broadband data access technology through telecommunication subscriber lines has the potential to cause unintentional radio frequency emissions which may adversely impact on existing radio services in the low frequency to high frequency (HF) bands.
ERA Technology Ltd were subsequently commissioned by the RA to specifically investigate the potential for interference from Asymmetric Digital Subscriber Line (ADSL) systems on aeronautical radiobeacons operating in the frequency range 255 kHz – 526.5 kHz. Preliminary consideration has also been given to the potential for interference to HF radio services due to unintentional emissions radiated by Very High Speed Digital Subscriber Line (VDSL) systems in the frequency range 1 MHz to 30 MHz.
The main stages of the validation and development of the York model are listed as follows;
The radiation efficiency of a typical subscriber drop wire was measured, accurately
and unambiguously, in order to provide confidence in values predicted using
Numerical Electromagnetics Code (NEC). It was found that good correlation is
achievable between the model and the ‘real world’ case if mismatch loss in the
common mode circuit is accounted for.
The vertical gain of the subscriber line radiating sources was calculated using
NEC in the frequency range 100 kHz to 30 MHz to cover the ADSL and VDSL band.
In the ADSL band, the space wave gain for all the subscriber line models is
a maximum at an elevation angle between 15° and 20°. The gain perpendicular
to the earth is relatively low in the ADSL band. As the frequency is increased
to the VDSL band, it is shown that more power is directed vertically. At frequencies
above 10 MHz the drop wires begin to show resonant behaviour and multi-lobe
radiation patterns.
The city model was developed to account for the distributed and non-homogeneous
nature of the interference sources. This has been achieved for Greater London
by specifying different subscriber line statistics for each of the 33 Greater
London Boroughs. Population and housing data from the Office of National Statistics
have been used to estimate relative percentages of different types of subscriber
line drop wire for each borough. Simple geometry is used to predict the electric
field at the aircraft position from each borough across Greater London.
Active Excel spreadsheets have been produced which enable the cumulative emission level to be predicted as a function of height above London and distance from London across the frequency band. A macro has been written which enables the production of surface plots of the field distribution above London.
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May 2001 |
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