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An analysis of the ACA report on the Woomera Online BPL trial in Moruya, NSW between October and December 2004.

This report is an analysis of the ACA's report on the Woomera Online BPL trial in Moruya, NSW between October and December 2004.

Contents

Background

The ACA published a report on 5 April 2005 entitled "ACA Field Report Woomera Online BPL Trial Moruya NSW - Oct & Dec 2004". The report is an analysis of the ACA's measurements of electromagnetic field strengths of radiation from that BPL trial. The full ACA report is available online from the ACA's website at http://www.acma.gov.au/acmainterwr/lib284/woomera%20%20report%20public%20release%20v2_3.pdf

Analysis

This document is an analysis of the ACA's report and extrapolation of the reported measurements to predict the impact on a typical amateur radio station with an antenna located 15m from the nearest BPL activated power conductors, and should be read in conjunction with the ACA report.

The ACA report describes the trial configuration is some detail, their measurement configuration and results of their measurements on two separate occasions.

In brief, the BPL trial was conducted in the industrial area about 3Km north of Moruya township, just off to the western side of the Princes Highway. The ACA made field strength measurements using a R&S Field Strength Meter and loop antenna designed for EMC measurements.

The key data presented that allows interpretation and extrapolation of the impact on an amateur radio station is that presented in Appendix F (pp 8-10).

The first plot in Appendix F, entitled  "High Power (15) Test Site 3 ..." shows peak field strength measurements in dBµA/m in the S plane over the frequency range of 0.15MHz to 30MHz. The frequencies used for downstream traffic and that were observed at Test Site 3 covered the 80m amateur band and the 15m amateur band. Table 1 extrapolates field strength measurements scaled off that plot to model the impact on an amateur radio station in a residential environment. Note that the noise floor in this plot is the internal noise of the measuring instrument and not the ambient noise (clarified by email with the ACA 06/04/2005). The BPL system also radiated interference around the 30m amateur band (10.1MHz), but it was not possible to reliably read the measured field strength due to a minimum in the vicinity of 10.1MHz, and for that reason, this analysis ignores the 30m band.

The ACA's measurements were made at a distance of 7.2m from the nearest powerline using a field strength meter with an effective noise bandwidth of 10kHz. This extrapolation is to an antenna typical for each of the bands at a distance of 15m from the nearest BPL activated power conductors. The extrapolation factor used for modelling the distance is that power reduces as the inverse of the square of distance (which is an accurate model for a point source radiator in free space). The question of extrapolation of field strength with distance is a complex one, though power may fall more quickly in the near field, power will fall less quickly from a linear or area radiator. The error remaining is probably small compared to the excess of radiated interference over the expected ambient noise level. The industry is still working on agreement on a simple distance model.

Table 1: Extrapolation of ACA measurements to model impact on an amateur radio station in a residential environment.
Frequency Field strength Antenna gain (dBi) Antenna Factor (dB/m) Received signal level
Bandwidth 10kHz 3kHz 3kHz
Distance 7.2m 7.2m 15m 15m
Units dBµA/m dBµV/m dBm S meter dB wrt min expected ambient noise (1)
3.6 MHz 18 64 58 4 -23 -26 S9+47dB 75
21.0 MHz 16 62 56 8 -11 -40 S9+33dB 77
Notes to table:
1. Minimum expected ambient noise is taken from the minimum noise level expected line of Figure 2 of ITU-R P.372-8.  Ambient noise in a residential area could be up to 20dB higher than the minimum noise level expected from that figure.

The second plot in Appendix F is at reduced power, but is of limited value as the received BPL interference is below the instrument internal noise floor above 12MHz. This analysis has set the plot aside for that reason.

The third set of plots in Appendix F measures the effectiveness of frequency notching. Frequency notching is a technique to disable the use of a band or bands of frequencies for BPL transmission, for example to protect radiocommunications services from radiated interference from the BPL system. The notch in this case is about 0.5MHz in width, and appears to be about 22dB in depth. This measurement would indicate that notching could not be depended upon to reliably decrease BPL caused interference by more than about 20dB.

Conclusion

The ACA's measurements of the Woomera Online BPL trial at Moruya, NSW in October and December 2004 contains detailed information that allows extrapolation of the measured data to model the impact on an amateur radio station located in a residential area if such a BPL system were deployed in that area.

Though this analysis is restricted to the impact on an amateur radio station, the radiated interference affected much more than the bands used by amateur radio, and would have impact on other radio communications services operating in or near the BPL service area.

Amateur radio operations on the two bands affected in this case are most commonly limited by the ambient noise level (rather than the internal noise of the station receiver).

The modelled received signal level of the BPL interference would be -26dBm (S9+47dB) and -40dBm (S9+33dB), and  up to 75dB and 77dB above the lowest expected ambient noise in respect of the 3.6MHz and 21MHz frequencies.

An amateur radio station located in a residential environment where a similar BPL system was deployed would not be safely able to transmit at all on the 3.6MHz and 21MHz bands because of the high risk of interference to possibly active stations that would not be heard through the BPL interference, effectively curtailing all activity on the band by such a station.

V1.1 20/02/09 19:25:14 -0700

 


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