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Queanbeyan BPL trial revisited - 26 March 2005

This is a report on observation of a BPL trial in Queanbeyan on 26 March 2005 on aerial power lines in two separate precincts on the 7MHz amateur band.

Contents

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.

Measurement equipment

Antenna

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.

Figure 1: Loop antenna.

Measurement antenna system gain estimates are set out in Table 1.

Table 1: Measurement antenna system gain estimates
Item Value
0.5m square loop at 7.1MHz loaded with 50 ohms -50.3dBi
Balun transformer (RAK type BL) -0.2dB
Transmission line (8m of RG-58C/U) -0.3dB
Total -50.8dBi

Table 2 shows the Antenna Factor (E/V (electric field strength / receiver terminal voltage)) calibration estimates.

Table 2: Antenna Factor calibration estimates
Item Value
Antenna factor (E/V) 0.5m square loop at 7.1MHz loaded with 50 ohms 38.1dB/m
Balun transformer (RAK type BL) 0.2dB
Transmission line (8m of RG-58C/U) 0.3dB
Total 38.6dB/m

Receiver

The measurement receiver was a standard IC-706IIG with pre-amp on.

Software

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. 

Sites

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.

Table 3: FSM data for 48 Hinksman St.
ObsTime RxRms RxQp RxPk FsRms FsQp FsPk NormRms NormQp NormPk
20050326144014 -110.9 -107.9 -106.0 34.2 37.1 39.1 1.1 4.0 5.9
20050326144023 -110.8 -107.7 -105.6 34.3 37.4 39.5 1.1 4.2 6.4
20050326144031 -110.3 -107.5 -105.2 34.8 37.6 39.9 1.6 4.5 6.7
20050326144039 -109.7 -107.0 -104.8 35.4 38.1 40.3 2.2 5.0 7.1
20050326144048 -111.1 -108.2 -106.2 34.0 36.9 38.9 0.8 3.7 5.7
20050326144059 -109.4 -106.7 -104.7 35.7 38.4 40.4 2.5 5.2 7.2
20050326144108 -108.8 -106.1 -104.3 36.3 39.0 40.8 3.2 5.9 7.0
20050326144117 -109.3 -106.6 -104.9 35.8 38.5 40.2 2.7 5.4 7.0
20050326144125 -108.9 -106.0 -104.2 36.2 39.1 40.9 3.1 5.9 7.7
20050326144158 -109.3 -106.7 -104.0 35.8 38.4 41.1 2.6 5.2 7.9
Medians -109.6 -106.9 -104.9 35.6 38.3 40.3 2.4 5.1 7.1
Notes to table:
1. Units: RxRMS, RxQp and RxPk are in dBm, FsRms, FsQp, FsPk, NormRms, NormQp, and NormPk are in dBμV/m.
2. Rx* columns are received power at the antenna terminals in the receiver bandwidth, Fs* columns are Field Strength in the receiver bandwidth, Norm* columns are Field Strength adjusted to 1Hz bandwidth
3. Rms is true RMS, Qp is quasi peak using time constants similar to CISPR Publication 16, Pk is the instantaneous peak.

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.

Table 4: Extrapolation to a practical receiver scenario.
Frequency (MHz) 7.1
Antenna gain (dBi) 6
Distance from power line (m) 15
Ambient Noise Figure (dB) (per ITU-R P.372-8) 35
FS (dBμV/m) 37
Rx power (dBm) -51
Rx power (S units + dB) S9+22
Rx noise bandwidth (dB) 33.1
Rx Noise figure (dB) 6.9
Rx Noise Floor (dBm) -134
Interference wrt Rx Noise Floor (dB) 83
Ambient (per ITU-R P.372-8) (dBm) -105.9
Interference wrt ambient (per ITU-R P.372-8) (dB) 55
Notes to table:
1. Ambient noise figure is taken from ITU-R P.372-8, Figure 2 , Minimum Noise Level Expected line.

Location: 12 Bruce St

Observation 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.
Table 5: FSM data for 12 Bruce St.
ObsTime RxRms RxQp RxPk FsRms FsQp FsPk NormRms NormQp NormPk
20050326145701 -107.4 -103.6 -100.3 37.7 41.5 44.8 4.5 8.3 11.6
20050326145710 -106.9 -103.2 -100.7 38.2 41.9 44.4 5.1 8.7 11.3
20050326145717 -106.8 -103.5 -100.7 38.3 41.6 44.4 5.1 8.4 11.2
20050326145728 -105.8 -102.3 -99.7 39.2 42.8 45.4 6.1 9.6 12.2
20050326145736 -106.4 -102.8 -99.9 38.7 42.3 45.1 5.5 9.2 12.0
20050326145745 -108.2 -104.5 -101.8 36.8 40.6 43.3 3.7 7.4 10.1

Medians

-106.9 -103.4 -100.5 38.3 41.8 44.6 5.1 8.6 11.5
Notes to table:
1. Units: RxRMS, RxQp and RxPk are in dBm, FsRms, FsQp, FsPk, NormRms, NormQp, and NormPk are in dBμV/m.
2. Rx* columns are received power at the antenna terminals in the receiver bandwidth, Fs* columns are Field Strength in the receiver bandwidth, Norm* columns are Field Strength adjusted to 1Hz bandwidth
3. Rms is true RMS, Qp is quasi peak using time constants similar to CISPR Publication 16, Pk is the instantaneous peak.

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.

Table 6: Extrapolation to a practical receiver scenario.
Frequency (MHz) 7.1
Antenna gain (dBi) 6
Distance from power line (m) 15
Ambient Noise Figure (dB) (per ITU-R P.372-8) 35
FS (dBμV/m) 28
Rx power (dBm) -60
Rx power (S units + dB) S9+13
Rx noise bandwidth (dB) 33.1
Rx Noise figure (dB) 6.9
Rx Noise Floor (dBm) -134
Interference wrt Rx Noise Floor (dB) 74
Ambient (per ITU-R P.372-8) (dBm) -105.9
Interference wrt ambient (per ITU-R P.372-8) (dB) 46
Notes to table:
1. Ambient noise figure is taken from ITU-R P.372-8, Figure 2 , Minimum Noise Level Expected line.

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.

Table 7: FSM data for 16 Bruce St.
ObsTime RxRms RxQp RxPk FsRms FsQp FsPk NormRms NormQp NormPk
20050326150145 -118.0 -113.8 -111.2 27.1 31.3 33.9 -6.0 -1.9 0.8
20050326150153 -117.9 -114.5 -110.9 27.2 30.6 34.2 -5.9 -2.5 1.0
20050326150201 -117.7 -114.2 -111.4 27.4 30.9 33.7 -5.7 -2.3 0.5
20050326150245 -117.5 -113.6 -111.0 27.6 31.5 34.1 -5.5 -1.7 1.0
20050326150304 -117.5 -113.8 -110.9 27.6 31.3 34.2 -5.6 -1.9 1.0
20050326150312 -117.2 -113.6 -110.3 27.9 31.5 34.8 -5.3 -1.6 1.6
20050326150503 -117.6 -114.1 -111.9 27.5 31.0 33.2 -5.7 -2.2 0.0
20050326150513 -118.4 -114.4 -111.2 26.7 30.7 33.9 -6.4 -2.5 0.7
Medians -117.7 -114.0 -111.1 27.5 31.2 34.0 -5.7 -2.0 0.9
Notes to table:
1. Units: RxRMS, RxQp and RxPk are in dBm, FsRms, FsQp, FsPk, NormRms, NormQp, and NormPk are in dBμV/m.
2. Rx* columns are received power at the antenna terminals in the receiver bandwidth, Fs* columns are Field Strength in the receiver bandwidth, Norm* columns are Field Strength adjusted to 1Hz bandwidth
3. Rms is true RMS, Qp is quasi peak using time constants similar to CISPR Publication 16, Pk is the instantaneous peak.

 

Table 8: Extrapolation to a practical receiver scenario.
Frequency (MHz) 7.1
Antenna gain (dBi) 6
Distance from power line (m) 15
Ambient Noise Figure (dB) (per ITU-R P.372-8) 35
FS (dBμV/m) 26
Rx power (dBm) -63
Rx power (S units + dB) S9+10
Rx noise bandwidth (dB) 33.1
Rx Noise figure (dB) 6.9
Rx Noise Floor (dBm) -134
Interference wrt Rx Noise Floor (dB) 71
Ambient (per ITU-R P.372-8) (dBm) -105.9
Interference wrt ambient (per ITU-R P.372-8) (dB) 43
Notes to table:
1. Ambient Noise Figure is taken from ITU-R P.372-8, Figure 2 , Minimum Noise Level Expected line.

Summary

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.

Table 9: Summary of modelled impact of interference to a half wave dipole at 15m distance from powerlines.
  12 Bruce 16 Bruce 48 Hinksman
FS (dBμV/m) 28 26 37
Rx power (dBm) -60  -63 -51
Rx power (S units + dB) S9+13 S9+10 S9+22
Interference wrt Rx Noise Floor (dB) 74 71 83
Interference wrt ambient (per ITU-R P.372-8) (dB) 46 43 55

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.

Unit conversions

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.

Conclusion

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.

V1.03 20/02/09 19:26:07 -0700

 


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