Queanbeyan BPL trial revisited - 26 March 2005
A report of observations of the Queanbeyan BPL trial site was published in November 2004, and this report should be read as an addendum to that report.
The receiving antenna was a square loop with sides of 0.5m, and fed with a 1:1 transformer balun (RAK type BL) and 8m of RG-58C/U coax.
Choices for a measurement antenna fell mainly into two categories, antennas responding to the E field and those responding to the H field. Short dipoles respond to E field and have a very small equivalent series resistance and very high capacitive reactance. Small loops have a very small equivalent series resistance and moderate inductive reactance. It was felt that the impedance of a small loop would not be unsuited to the 1:1 balun that was available, whereas a short dipole would not be as suited to the balun. So, a square loop with 0.5 metre sides fed in one corner was constructed, the antenna is pictured in Figure 2. The loop is readily rotated and can be used to identify the source of radiation.
Measurement antenna system gain estimates are set out in Table 1.
Table 2 shows the Antenna Factor (E/V (electric field strength / receiver terminal voltage)) calibration estimates.
The measurement receiver was a standard IC-706IIG with pre-amp on.
FSM (for Field Strength Meter) is a Windows application that extends a conventional SSB receiver to allow measurement and calculation of field strength of radio signals or interference.
Location: 48 Hinksman St
Observation was made from the public footpath outside 48 Hinksman St. The locality is a business district. The nearest power line is a flat four wire three phase distribution at about 6m height and 13m distant from the measurement point. The BPL injection equipment was about 80m from the measurement point. The BPL interference was apparent at similar levels across the entire band.
An S-meter reading was taken and calibrated from a signal generator as a cross check. The S-meter reading corresponded to -108dBm +/-5dBm which reconciles well with the FSM median RxRms of -109.6dBm.
Location: 12 Bruce StObservation was made from the public footpath outside 12 Bruce St. The locality is a residential district. Nearest power line is a ABC service line about 3m distant from the measurement point. This location was about half way between two equipment enclosures, a head end to the east and repeater to the west. The source of the 7MHz interference was the head end equipment about 60m to the east. The BPL interference was apparent at similar levels across the entire band.
An S-meter reading was taken and calibrated from a signal generator as a cross check. The S-meter reading corresponded to -102dBm +/- 5dBm which reconciles with the FSM median RxRms of -106.9dBm.
Location: 16 Bruce St
Observation was made from the public footpath outside 16 Bruce St. The locality is a residential district. Nearest power line is a flat four wire three phase distribution at about 6m height and 8m distant from the measurement point. This location was about half way between two equipment enclosures that were repeaters. The source of the 7MHz interference was a head end modem about 100m to the east. The BPL interference was apparent at similar levels across the entire band.
That report suggested that interference from the Trial BPL system was evident in both trial precincts, and that the electric field strength measured in a SSB typical receiver bandwidth would be around 48dBμV/m at a distance of 15m from the powerline.
It is four months since those measurements, and the field strength in Bruce St appears to be around 15dB lower than previously measured nearby. Nevertheless it is than 40dB above the ambient noise level expected in that type of environment.
The observed field strength in Hinksman Street was also lower than last November, by about 10dB. It is 30dB to 50dB higher than the ambient noise that might exist in that district, depending on the level of manmade noise.
A question that is often asked is "how far can the interference be heard?" The extrapolation models for Bruce St indicate interference around 44dB above ambient at 15m. Though not a point source radiator, reducing the field strength as 20*log(d1/d2) gives a fair indication where the trial site is relatively small. The interference should be reduced to the point where it doubles receiver noise output by 15*10^(44/20)m or 2.4Km from the trial site. If the actual ambient noise is higher than used, the distance would be less, 1.2Km for 6dB higher ambient noise level. This projection agrees with anecdotal evidence that the interference can be heard for about one Km by mobile stations (higher ambient noise environment due to vehicle noise), and explains why it cannot be heard at the naval communications station nearly 4Km distant.
The key metric used for comparison in this report is the field strength extrapolated to 3kHz measurements bandwidth. Extrapolation of measurements in narrow band receivers to the CISPR specified bandwidth is prone to errors depending on the type of noise or interference. RMS measurements can be reliably extrapolated by adding 4.77dB to the 3kHz figures. See FSM documentation for discussion of the issues related to extrapolation of QP and Peak values.
The field strength measurements in this document are expressed in dBμV/m. Under free space conditions, the field strength in dBμA/m can be calculated by subtracting 20*log(120*π) or 51.5dBΩ from the dBμV/m figure. Free space conditions do not necessarily apply in the near field zone, so some error may be introduced in such a conversion.
Although the interference radiated by the BPL system appears to have been reduced in level somewhat from that observed in November 2004, the interference appears to be 40 to 60dB above the lowest expected ambient noise level on the 7MHz amateur band, and would be stronger than most signals that would otherwise be heard.
An amateur station located in the trial precinct would not be safely able to transmit at all on the 7MHz band 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.
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