[Fists] BPL

[Andrew Roos]

Hi Ralph

The big problem with BPL is the frequencies at which it operates. According
to Shannon's theorum, the amount of information that can be transmitted on a
communications channel depends on the error rate of the channel and the
bandwidth available. BPL is an attempt to offer broadband data services over
power lines, where "broadband" means something in the order of 10 MBPS and
above, compared with the 56 Kbps or so from a standard phone line modem.
These high data rates require considerable bandwidth, typically around 50
MHz or so, which means that power lines will be carrying broadband signals
that occupy the entire LF, MF and HF specrum, plus the lower part of the VHF
spectrum.

Since the power lines are unscreened, the only thing that prevents them from
radiating is if they can operate as a balanced feeder with equal but
opposite currents flowing in adjacent parallel conductors. In order for
balanced feeders not to radiate, the separation between conductors should be
less than 1% of wavelength or so. For the other signals you mention -
telephone, AC mains, and the narrowband control signals used by power
companies - the wavelengths are of the order of 100 Km or so (that is the
wavelength of a 3 KHz signal), so the separation between cables in a typical
overhead line, which may be several metres, clearly meets the requirements.
Hence the line does not radiate appreciably. Also, propagation at
frequencies in the VLF range (up to 30 KHz) is relatively poor, and these
bands are not widely used by amateurs (or anyone else who doesn't own a
submarine fleet for that matter). Hence even if these signals were to
radiate, they would not get very far, and there would be few users to
interfere with.

However in the HF spectrum, wavelengths are of the order of 10-80m or so, so
a separation of say 5 m between the conductors may be as much as half a
wavelength, which makes the power line into a nice efficient antenna. Now of
course HF frequencies do propagate efficiently, so you could end up with
interference from allaround your city, country and even across the world.
And of course these frequencies are in regular use by amateurs and other
services, so interference is a very real concern. Add to this the sheer size
of the power line antennas, and the number of people who could be using PLC
modems, and the potential for massive interference becomes clear. PLC is not
a hyped-up concern, it is the most important issue to have faced amateur
radio in several decades. (Except the Morse issue. I am NOT going there!)

73,
Andrew ZS1AN
FISTS 8921


> -----Original Message-----
> From: fists-admin@mailman.qth.net [mailto:fists-admin@mailman.qth.net]On
> Behalf Of Ralph Sutton
> Sent: 05 February 2004 12:21
> To: Fists North America
> Subject: [Fists] BPL
>
>
> Re the BPL controversy.
>
> At present data transmitted through phone lines or "so called" wireless
> connections doesn't seem to impinge on amateur HF transmissions.  I am a
> true amateur as far as technical matters are concerned, so I consulted a
> friend who had a career in radio.  He explained that the problem
> with BPL is
> that the cables are not screened, so that data transmitted through that
> medium will radiate.  To me this did not seem a logical
> explanation as even
> though nowadays with optical fibre transmission and underground cabling
> there is a high proportion of unscreened copper wire above ground and for
> house connections.
>
> Perhaps one of our more learned FISTS members can tell me if I am
> wrong and
> if so what is the problem with BPL? Or is this another example of the
> scaremongers art like cellphone towers supposedly causing awful illnesses.
>
> Vy 73 de
>
> Ralph Sutton ZL2AOH
> 12c Herbert Gardens
> 186 The Terrace
> Wellington 6001
> New Zealand
>
> Tel:+64-4-473-0847
> Fax:+64-4-473-0848
>
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