OwenDuffy.net 


 

Observations of the Queanbeyan BPL Trial Nov 2004

This is a report on observation of a BPL trial in Queanbeyan on 21 November 2004 on aerial power lines in two separate precincts.

Contents

BPL overview

Broadband over Power Lines is the latest incarnation of attempts to utilise power transmission and distribution infrastructure for electronic communications. Earlier efforts included PLC which was principally used for protection, supervision, telemetry and control, and PLT which included extension to more generalised telecommunications (eg voice channels).

Although BPL offers the opportunity for power industry telemetry and control (eg remote meter reading, dynamic load management etc), it depends on delivering commercial Internet access to subscribers for its economics.

Tuning across the band

Though the receiver test setup was relatively insensitive (using a -50dBi antenna), interference from the BPL systems was readily observable to medium strengths on the receiver, more than 35dB above the receiver internal noise.

Here is a short mp3 sound clip (10s / 40KB) of the SSB receiver being tuned across about 20kHz of the 40m band over 10 seconds. The beat notes are from the BPL carriers spaced about 1.1kHz apart, and the noise that is sometimes heard is the effects of data modulation of the carriers.

These carriers were measured at about 3.5µV at both precincts with a -50.8dBi gain small loop antenna system. With a half wave dipole mounted at about 10m and feeder (gain about 6.5dBi), one would expect about 2573µV, or about 34dB above S9 (assuming s9 equivalent to 50µV).

The electric field strength in 9kHz measurement bandwidth would be 10.8dBµV (3.5µV) + 38.6dB/m (Antenna Factor) + 4.77dB (bandwidth factor) or 54.3dBµV/m measured at a distance of 15 metres from the nearest power line. The electric field strength normalised to 1Hz bandwidth is 14.8dBµV/m measured at a distance of 15 metres from the nearest power line

At the time of observation, the system in Hinksman Avenue seemed to radiate frequencies from 5.5MHz to 8.0MHz, and the system in Campbell Street seemed to radiate frequencies from 5.5MHz to 8.0MHz, and 9.0MHz to 12.0MHz. It is not known whether they use other frequencies, eg in response to demand.

This study measured interference over a narrow band of frequencies around 7MHz, although the equipment radiated interference of similar intensity over a much wider range. The impact will vary with frequency.

The observed interference levels at 7MHz would be extremely disruptive of radio communications for hundreds of metres from the deployment, and would exceed the background noise level at distances over 1Km from the deployment. If this infrastructure was deployed on all LV distribution in a geographic area, it would render radio communications unusable at this frequency and probably all frequencies where it radiated such interference.

The infrastructure

The infrastructure used in the Queanbeyan trial includes injection equipment that is mounted in plain view on power poles in the street. The LV distribution is 240/415V 50Hz three phase with neutral. Figure 1 shows the injector equipment mounted on poles at two locations in Queanbeyan.

Figure 1: Injector equipment deployment.

The injector equipment in Hinksman Avenue. The fibre feed enters from left in a thin plastic duct bound to the neutral conductor (far conductor) and down  to the large grey equipment enclosure, entering it from the underneath. The large equipment enclosure feeds a smaller grey box attached to the cross arm. This smaller box has thin wires to line taps on each of the three active conductors just to the left of the upper cross arm.

This precinct is a light and medium industrial area, most premises taking supply at LV, although the HV lines at the top of the pole (barely visible in the picture) supply a customer at HV just a few hundred metres down the road.

The injector equipment in Campbell Street. The fibre feed enters from left in a thin plastic duct bound to the neutral conductor (far right conductor in the LV tier) and down  to the large grey equipment enclosure, entering it from the underneath. The large equipment enclosure feeds the two smaller grey box attached to the pole above the cross arm. The smaller box has thin wires to line taps on each of the three active conductors  in front of the cross arm.

The precinct is generally residential with a large park and commercial swimming pool complex opposite.

Note the aluminium power line conductors at the Hinksman Avenue site. A recent report commissioned by government stated:

Impedance is an expression to describe the opposition that an electronic component, circuit, or system offers to electric current.

The metal from which electricity cables are made, possesses a natural physical resistance, referred to as impedance. This resistance is measured in Ohms. The impedance is largely dependent on the type of metal and the diameter of the cable. Copper or steel cables possess comparatively low impedances whereas aluminium has higher impedance. The higher the impedance the shorter the distance the signal will travel along the medium.

(Parsons Brinckerhoff Associates "TECHNOLOGY REVIEW OF POWERLINE COMMUNICATIONS (PLC) TECHNOLOGIES AND THEIR USE IN AUSTRALIA" 7 October 2003)

Clearly this contains errors of regarding the resistivity and permeability characteristics of the materials (copper, steel and aluminium), skin effect, inductance, capacitance, reactance, and understanding of theory and practice of transmission lines at radio frequencies.

Measurement

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 2: 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.

The receiver S-meter calibration was measured about two hours after the field observations, and there is no reason to expect that they would have been different during the observations. Note that the reported receive signal strength in µV does not depend on any assumption of relationship between S meter reading and terminal voltage, the relationship was measured for the test receiver and the results are set out in Table 3.

Table 3: Receiver S meter calibration
S-meter indication Receiver terminal voltage (µV) 
S5 1.3
S6 2.0
S7 3.5
S8 7.5
S9 14
S9+10dB 26
S9+20dB 50
S9+30dB 90
S9+40dB 200

Location

The plan was to make measurements in the vicinity of the injector equipment, but not so close to the injector equipment to be subject to leakage if any from the boxes themselves. The criteria for a measurement location was a place on a public street 50 to 100 metres from the injector, and as far as possible from aerial power lines (either the lines down the street or branches to customer premises).  This strategy was intended to avoid hot spots that would misrepresent the interference level that would be generally experienced. At both locations small changes in the antenna position did not make measurable changes in received signal strength. The loop antenna was always rotated for maximum signal as the antenna gain calibration was calculated on the major lobe of the loop.

Comparison with BPL Interference Evaluation Tool results

The BPL Interference Evaluation Tool calculates the impact of radiation at the maximum level permitted under the draft CENELEC standard.

I am not sure of the equipment in use in Queanbeyan, either brand or model. I have searched the web site of the company that is rumoured to supply the equipment, but they are unusually shy about their BPL product range. I have no knowledge of its compliance with any standards.

Nevertheless, it is interesting to compare the measured interference level with the predicted interference from the maximum permitted radiation under the draft CENELEC standard, as predicted by the BPL Interference Evaluation Tool.

The measured value is +12dB +/- 3dB compared to the maximum impact predicted by the BPL Interference Evaluation Tool.

Conclusion

A Broadband over Power Lines (BPL) installation is operating in at least two separate precincts in Queanbeyan.

The BPL installation does radiate electromagnetic energy on a wide range of radio frequencies and the observed radiated signal levels are much higher in intensity than the background noise level.

The measured interference levels at 7MHz would be extremely disruptive of radio communications for hundreds of metres from the deployment, and would exceed the background noise level at distances over 1Km from the deployment. If this infrastructure was deployed on all LV distribution in a geographic area, it would render radio communications unusable at this frequency and probably all frequencies where it radiated such interference.

Links

Glossary

Term Meaning
BPL Broadband over Power Lines, a technique for high speed data transmission over power infrastructure
HV High Voltage
Interference From the Radiocommunications Act 1992 (Cth), interference means:
(a) in relation to radiocommunications - interference to, or with, radiocommunications that is attributable, whether wholly or partly and whether directly or indirectly, to an emission of electromagnetic energy by a device; or
(b) in relation to the uses or functions of devices-interference to, or with, those uses or functions that is attributable, whether wholly or partly and whether directly or indirectly, to an emission of electromagnetic energy by a device. 
LV Low Voltage
PLC Power Line Communications
PLT Power Line Telecommunications

Disclaimer

Use at your own risk, not warranted for any purpose. Do not depend on any results without independent verification.

V1.06. Last changed: 20 February 2009 19:25

 


© Copyright: Owen Duffy 1995, 2017. All rights reserved. Disclaimer.