[Lowfer] Rulemaking Ruminations - TWO: Supporting Data Needed

[N6IO]

Earlier I did come calculations about the required coupling factor to
insure that a ham 1W ERP signal would provide the required 25db s/n ration
for PLC systems.

Reading that 1985 report (http://www.its.bldrdoc.gov/publications/2204.aspx)
gives some very solid numbers.   I suggest that the data in that paper is
still valid when it comes to antenna/transmission line coupling factors.
Someone with more recent antenna modeling software might do better, but I
bet it's close.  The numbers on acceptable interference levels and s/n
rations seems to still be valid based on some more recent vendor
documentation I have seen.

Based on 1985 technology it confirmed that systems needed between 10 and
25db of s/n ratio.  Newer systems would probably be more immune to noise,
but I say we use the 25db number assuming the PLC industry is still using
30+ year old hardware.

Since a PLC system operates with a typical received signal level of +10dbm
and a noise level of -15dbm the coupled ham signal must be below -15dbm.
1W ERP would be +30 dbm so the coupling factor would need to be -45db.

The paper uses three methods to determine coupling factor - antenna
modeling software, signals radiated by the power lines and transmitted
signals from a nearby antenna received at a substation.  It looks like all
provide very similar results.

Going over all the data it looks like to get the -45db coupling factor an
antenna would have to be at least 90m from a transmission line.  I'd
suggest we shoot for a rule saying a transmitting antenna should not be
closer than 100m from a transmission line carrying PLC signals.

I'd further suggest that if operation is anticipated closer than 100m from
a power line, that the activity be restricted to "fixed locations" that
have been vetted by UTC to confirm the nearby system is not using that
radio spectrum.  For operation more than 100m from a transmission line no
fixed location or coordination would be needed.

I know some are against any sort of coordination or fixed location
requirements, but if we can limit those restrictions to just areas with the
potential of causing interference (less than 100M from a line) it may make
both sides happy.  The power industry can feel like they got some
restrictions, and very few hams would need (or want) their antennas to be
less than 100m from one of those very large transmission lines.  And we're
not talking about distribution lines you see around town or along roads -
but the large 100KV+ ones which are easy to spot based on the large
insulators they need.

The advantage of the 100M distance limitation is that it will be easy to
visually verify you are 100m from a large transmission line.  The 1KM limit
would be harder to confirm - especially in hilly areas with limited road
access.

Going to a 500m distance decreases the coupling factor to more than -50db,
and going to 1Km makes it -60db but those seem like overkill.  I think the
1Km limit quoted in earlier comments was assuming much higher radiated
power levels.  In this paper it was determining safe distances for GWEN
systems operating at 2000W ERP - a 33db higher level.

These are 1979 numbers, but provide some hint about how many PLC systems
were out there:

Freq KHz  Number of systems
9-23          300
23-60       2570
60-70        650
70-90       2320
90-110      2110
110-130    2140
130-150    2010
150-190    4480
190-200      860
200-282    1760
282-325      250
325-405      150
405-495        15
TOTAL     19615

Anyway - lots of interesting reading to help write a reply to the FCC
proposal.

Craig - N6IO




On Sun, May 31, 2015 at 5:40 PM, JD <listread at lwca.org> wrote:

> * TWO:  We need more technical showings this time, with as many hard
> numbers
> as possible.  I hope the Part 5 licensees are prepared to crunch numbers,
> but those of us who only monitored are also able to contribute.
>


> The utilities, IMO, provided no concrete technical information at all.
> ARRL
> cited the 1985 NTIA study on which the 1 W EIRP and 1 km separation idea is
> based, but the FCC is concerned whether that's still valid.
>