[Lf] Re: Definitely More QRM ! (or maybe not....)

Thu Apr 12 01:11:37 CDT 2001

> 
> > Dear all,
> > in order to convert just words into figures I feel it necessary to describe the
> > situation in Germany more in detail.
> >
> > The german NB30 standard has been rejected by all german radio users and
> > radio equipment manufacturers since it has been proposed. For those who know the
> > british MPT 1570 the problem is that NB30 allows interfering fieldstrengths 20
> > dB higher. But even the resistance of the german broadcasters could not prevent
> > the adoption of the NB30 standard. The declared aim of the german government
> > and the upper house of parliament has been to give "modern telecommunications
> > technologies" a chance. The CENELEC standard on CATV systems covering 5 MHz to
> > 3000 MHz has also been aligned to the NB30 limits earlier, at least in the HF
> > and VHF-UHF range. (NB is simply a german abbreviation for Nutzungsbestimmung =
> > operational condition or regulation. There are other NB numbers within the
> > german frequency allocation table.)
> >
> > A radio amateur living in a house in which cables are operated under the
> > conditions of the NB30 and having a half wave antenna 10 meters away from his
> > house will have to expect the following calculated S meter readings:
> >
> > Band, MHz       1,8             3,6             7       14      21      28
> > NB30
> > dB(muV/m)       37,8            35,1            32,6    29,9    28,4    27,3
> > S-Meter         S9 +12 dB       S9 +3 dB        S8      S6-7    S5-6    S5
> >
> > I hope the table will be delivered in the same order in which I have typed it.
> >
> > The second line gives the interfering fieldstrength permitted by NB30. The
> > situation is bad, especially for QRPers. But all efforts of DARC to prevent such
> > a decision were in vain.

<...snip...>

Hi to all,

just some comments to the bad news coming from DL-land, at least
according to what's happening here in Milan, Italy.

In 1999 the local electric power company (AEM - http://www.aem.it )
announced a joint venture with Nortel to promote internet connections
using PLC - see the press announcement at
http://www.publiweb.com/service/cavo.html (only in italian, sorry!). The
service was expected to hit the market by summer 2000; in the meanwhile
some test locations were quickly installed and internet-PLC experiments
began.

After few months PLC connections were abandoned and AEM started drilling
holes in the whole city to deploy a huge fiber ring, delivering 10 Mb/s
connections to each subscriber. Why ?!?
Well, I guess that these could be the reasons that could explain the
choice to withdraw internet-PLC here in Milan:

- sensitivity to radio interference: PLC connections can deliver
interference to the radio spectrum, but they can be interferred by radio
transmitters as well (and here in Milan there are plenty of them).

- power line noise: interferences generated by ballasts, halogen lamps,
engines, and so on that generate these familiar 100 hz spaced signals
(120 Hz on the other side of the pond) on our Spectran screens are
probably quite harmful to PLC connections as well.

- bandwidth: a PLC internet line has a speed that should be roughly in
the range of 1 MB/s. However all the users connected to PLC under the
same power mains station share the same bandwidth, i.e. during "internet
rush hours" the bandwidth is quite smaller. This does not happen (or
better shouldn't...) with ADSL connections.

- privacy: for the same reason all your neighbours can potentially
eavesdrop your IP packets, and possibly annoy you with DoS, IP spoofing,
and so on. I expect that some amount of scrambling will be superimposed
to the data stream, but the fact that your data can be so easily
intercepted is no good.

So I guess (and hope...) that PLC communications will have a very hard
life, when they will have to compete with ADSL or cable modem
connections.

Yes, there is also a lot of concern about interferences coming from xDSL
lines, but I wonder if anybody has ever tried to do some real
measurements on the amount of interference delivered by this kind of
communication system to the radio spectrum. 
Some months ago I did a very rough experiment with my portable receiver
Sangean ATS909. I placed the receiver at about 1 m from a working ADSL
modem installed in a friend's office, and looked for interferences and
or excess noise in the 0.5-1.5 MHz band (where I expected they could be
easily found, given the ADSL signaling rate). 

Well I don't know if the ADSL modem in use was a good one, however the
only noise I observed was coming from a neighbour stack of PC servers,
and I could find no evidence of noise coming from the modem (by the way,
my 56k modem I use at home produces funny signals on Spectran, if I
leave it on while listening to 136 kHz!). I wanted to make some extra
experiments looking for possible interferences along the telephone line
used for ADSL connection, but I run out of batteries: sorry! 

Don't know what does it happens with cable modems, since CATV is not
used here in Milan!

If somebody else ever tried this measurement on a *real* installation
(possibly using a spectrum analyzer, instead of a portable receiver!),
it could be quite interesting to know about.

vy 73 

Vittorio IK2CZL

-- 
*************************************************************************
Vittorio De Tomasi         ik2czl at amsat.org   
http://www.weaksignals.com

"Wir muessen wissen; wir werden wissen" (David Hilbert)