9. Conclusions
Home-networking PLT devices called Power Line Adaptors (PLAs) communicate by injecting data
signals onto the mains wiring. Previous studies on PLAs that use HF for communication have
shown that the resulting unwanted emissions can disturb the reception of HF broadcasts. More recently
PLAs have become available that promise greater data-carrying capacity by exploiting the
VHF range. This raises the concern that they might interfere with reception of broadcast FM and
DAB radio services.
This paper describes experiments on a particular pair of PLAs, in the laboratory and in two homes
to try to establish their interference-causing potential. It was confirmed that they used frequencies in
the VHF range, roughly from 50 to 300 MHz. They could also default to operation at HF, with a
lower data rate, if the mains-path conditions at VHF were too ‘difficult’, although some VHF
transmissions would nevertheless continue. Indeed, whichever of HF or VHF was used for data
transmission, intermittent transmissions continued in the other, presumably for channel-sounding
purposes. Intermittent transmissions also took place in both bands when no data was being actively
transmitted, i.e. when the network was ‘idling’.
Although PLT networking using VHF was shown to be possible in the two homes, it was not always
readily established, especially in home B where many factors were found which could prevent it.
The presence of other mains-powered devices as well as the communicating PLAs, particularly
equipment with EMC filtering, was found to disrupt PLT operation at VHF. As a result, there were
many combinations of rooms in home B which could not be interconnected using VHF PLT — in
contrast to the universal coverage of the property already established using WiFi.
When VHF PLT networking was established and carrying data, the resulting emissions were found
to increase the reception noise floor by some 20 dB. In many cases this was found to disturb FM or
DAB reception indoors. Some disturbance to reception was also caused when the PLT network was
idling, even if only using HF because VHF PLT data transmission did not work over a particular
path. The precise impact on reception was found to depend on the RF signal-to-PLT-interference
ratio.
Spectrum analyser plots of the received signals in homes A and B show that the operation of the
PLAs elevated the apparent noise floor in VHF Band II by between 15 and 25 dB. A similar degradation
was also apparent in VHF Band III where DAB broadcast networks are deployed. The impact
on reception was found to depend on the reception margin of the victim radio service.
Home A suffered significant interference to portable indoor reception of national-network FM services
from Wrotham, but not to reception of those from Crystal Palace which had greater signal
strength (and hence greater margin). Note that home A would nevertheless be regarded as within the
coverage area of both.
Home B also suffered interference to portable indoor reception of national FM networks from Wrotham
(in this case the intended transmitter) — and very severe interference to reception of BBC
London, for which home B is predicted to be just at the edge of the coverage area.
Rooftop reception of FM services in home B was also disrupted, particularly when the PLA devices
were carrying significant traffic. The audible degradation was clearly noticeable on a ‘hi-fi’ receiver.
Reception of the service predicted to be just at edge of coverage was greatly disturbed, while interference
could also still be clearly heard when receiving the appreciably stronger national network
service.
The tentative conclusion is that the number of homes which could suffer interference to indoor FM
reception from VHF PLA devices could be quite significant, since interference was observed in both
homes even though the received signal levels exceeded the minimum field strength expected at the
edge of the coverage area. Homes enjoying signal levels that match or exceed that of the national
networks from Crystal Palace at home A would probably not be affected. Having said that, the reception
of weaker signals from distant transmitters would almost certainly be compromised by the
operation of PLAs, and for indoor portable reception, the percentage of locations where satisfactory
performance can be obtained will be reduced.
To quantify further the proportion of households that could be affected requires a detailed planning
study, perhaps using prediction techniques in novel ways. Additional variability will be introduced
by height loss, building penetration loss and location variation indoors. A preliminary estimate,
based on a 20 dB increase in man-made noise levels, suggests a substantial reduction of the coverage
area could result from widespread use of VHF PLA devices.
Similar reception problems were observed in the DAB band, with elevated noise floors clearly observed.
Home A, which receives very strong DAB signals, suffered no obvious effects from PLT
operation on indoor DAB reception, but the impact in home B (which normally has reliable DAB
reception) was quite severe. The ‘digital cliff’ effect of DAB means that only a very small change in
RF SIR is needed to go from unaffected reception to no reception at all. This could be directly confirmed
in Home B, since different DAB multiplexes were received with a range of signal strengths
and their performance in the presence of PLT emissions could be seen to range correspondingly.
Homes A and B are predicted to receive similar national-network DAB field strengths; since the reception
outcomes were so different it follows that variability in prediction, and in (height loss plus
penetration loss) is greater than the few-dB width of the ‘digital cliff’.
A similar conclusion must be drawn for indoor DAB reception: if they had similar PLAs in operation,
a non-negligible proportion of homes would be affected, but not all homes. Refinement of this
crude result looks to be more difficult than for FM, since prediction software only tells us about
outdoor field strength, and in this case the variability in (height loss plus penetration loss) has a
more drastic impact, being more than enough to go from no PLT-interference effects to total loss of
reception.
Nevertheless some pointers have been given how prediction tools could perhaps be used to try to
refine the estimate of the percentage of homes potentially affected, assuming a widespread deployment
of PLAs like those tested. Guidance has also been given regarding extra precautions to be
taken in any future experimental work.
It must be noted that any conclusions drawn here specifically relate to PLAs like those tested; other
PLAs also using VHF for data communication might well behave differently, although if they use
similar injection levels in the FM and DAB bands their impact on reception might be expected to be
generally similar.
