
Lawnmower robots – the end of VLF time signals? - icehawk
http://phk.freebsd.dk/time/20150509.html
======
Johnythree
The O.P. appears to live in Demark and is concerened about interference to the
German DCF77 signal on 77.5kHz from a buried-loop lawnmower control signal on
approx 77.5KHz.

His Spectrum Analyser pictures are not much help. About all they prove is that
his local DCF77 signal is very, very weak.

In order to investigate the problem, he needs to accurately measure the
modulation type, bandwidth and the frequency of the Loop TX, and then analyse
exactly how the interference is occuring.

Is the Lawnmower system genuinely overlapping in frequency with DCF77? Or does
the Clock receiver have insufficient Selectivity to separate the two? Or is
the Clock receiver being overloaded? Or is the local DCF77 signal just too
weak?

It is critical that he use a resonant and shielded magnetic antenna to measure
the DCF77 signal, so as to eliminate the electric-field rubbish his Spec-An is
picking up. If he has simply used a random wire antenna, his spectrum plots
will be pretty much meaningless.

Note-1. Being a buried loop, the Lawnmower antenna is designed to radiate only
a Magnetic field (eg the earth shielding and the loop balance should suppress
any EM waves). Either the loop is incorrectly installed, or the transmitter
and feed lines are physically close to his DCF77 Clock, and/or the DCF77
signal is very weak.

Note-2. In the USA, the equivalent time signal is WWVB on 60KHz. It is
possible that the Lawnmower system he has installed was intended for North
America only, and is not authorised for use in Europe.

Note-3. I don't know what the Service Area of DCF77 is, but it is likely that
it's signals will be very weak in Denmark. It is possible that he is simply
outside the DCF77 service area.

~~~
phkamp
The modulation type for the lawn-mower is digital current pulses with no
bandwidth limiting, as is obvious from the scope signal I conveniently
included for your and everybody elses information.

As for what exactly goes wrong with DCF77, that is pretty obvious from the two
spectrum-analyser traces provided by that signal: It gets overloaded with AM
noise which the simple AM-discriminator receivers used in clocks cannot deal
with.

~~~
Johnythree
The key is that the DCF77 signal is weak, and the lawnmower loop is very
close. It's a classic example of poor signal-to-noise ratio, made worse by the
DCF77 receiver being within the "near-field" of the lawnmower loop.

You could improve the S/N ratio by:

\- separating the wiring for the two systems as much as possible, and fitting
a mains noise filter to keep the loop signal out of the mains wiring.

\- putting a remote (ferrite rod) antenna on the DCF77 receiver and
experimenting to find a quiet location (possibly the interference can be
nulled out by rotating the antenna).

\- attending to the shielding, earthing and balance of the lawnmower loop
(especially shielding and twisting the feed wires).

You could document the problem and write to your local radio licensing people.
Unfortunately, I expect that they would consider that neither the DCF77 clock
or the Lawnmower Loop come under their jurisdiction.

You could also write to the manufacturer of the loop and ask for their
assistance. They may be surprisingly helpful.

Finally (if you haven't already) you could research the methods that Radio
Amateurs have developed for receiving VLF signals in noisy domestic
environments.

Bottom line: In my experience, correctly earthing the loop and shielding the
feed wires should do a lot to eliminate the interference.

But as before, you need to understand the modulation characteristics of the
lawnmower loop in detail. What bandwidth does the system actually need to
operate? Can the interference be reduced by fitting a better low-pass-filter
to the loop transmitter? Can you put a notch filter at 77.5 KHz?

~~~
phkamp
Don't worry about what _I_ can do to mitigate my problem, I have the
electronic and RF skills+kit to deal with this.

My point is that John F. Consumer will likely conclude that his "radio
controlled clock" is dying, throw it out, waste money on another one, and
maybe never make the connection to his (or his neighbors) lawnmower robot.

And more importantly, that if popular enough, such (badly designed) signals
will eventually make VLF timekeeping useless for everybody.

~~~
Johnythree
You're right of course, but this sort of thing happens whenever there is a
poor signal-to-noise ratio.

Like any Broadcast service, VLF timekeeping relies on a saturation signal so
it can override domestic electrical noise.

The level of Electrical Interference is steadily increasing world wide.
Already services such as AM Broadcast Radio are unusable for many people.

The problem is caused by (1) the laws of physics, and (2) by our compromised
EMC regulations.

At least your European EMC laws are much improved over the watered-down
American ones.

Bottom line: The issue is much bigger than just VLF Time keeping.

------
zik
This would be considered "harmful interference" in my country and our
communications authority would take a dim view of it. Amazingly they have way
more power than the police in these matters - if they detect harmful
interference they can enter property without a warrant, confiscate the
offending equipment and impose hefty fines.

~~~
nightpool
Which country, if you don't mind me asking?

(the idea of the FCC breaking someone's door down and taking their equipment
is pretty funny to me, although I can't say for sure it's never happened)

~~~
hlieberman
It's happened, actually. Look at the history of pirate radio - there were
armed raids done in cooperation and under the auspices of the FCC.

~~~
phkamp
That was mostly because the were (perceived to be) an economic threat to some
very big industries, it had very little to do with electro-magnetic radiation.

------
hlieberman
You should report this to the government agency in your region in charge of
telecommunications signals. If it is anything like the situation in the United
States, disrupting a licensed signal like this is probably not allowed.

------
rpcope1
I find it hard to believe that you can sell an electronic product that has
that poor a behavior (it's emitting that much RF noise); this is the sort of
hardware that really ought to be clamped down on, hard. RF Spectra is a shared
resource, and a rather limited one, and it only takes one careless neighbor
with poor electronic equipment to wreak havoc on other devices. If this
lawnmower is getting used in the U.S. I would really like to see the FCC pay
this manufacturer a visit.

------
claar
If the image in your article is from the manufacturer, then they're based in
Stockholm, Sweden.
([http://www.autobotics.co.uk/automower-265acx.html](http://www.autobotics.co.uk/automower-265acx.html))

If so, the Swedish equivalent to the FCC appears to be The National Post and
Telecom Agency (PTS) ([http://www.pts.se/](http://www.pts.se/)).

~~~
unwind
While Husqvarna Group have offices in Stockholm, they're actually based in the
small Swedish town of Huskvarna (see
[http://en.wikipedia.org/wiki/Husqvarna_Group](http://en.wikipedia.org/wiki/Husqvarna_Group)).

Also, your first link is to a retailer, which seems to be a UK-based company
called Autobotics. A bit confusing.

And yes, PTS is the proper agency to contact about radio interference issues.
They're a government agency, they can prosecute offenders.

------
trimble-alum
In California, at least, there are massive government credits for replacing
lawn with other alternative landscaping choices. Perhaps replacing water-
hungry landscaping with other lower-maintenance, water-efficient plants would
be less net work, cost and environmental damage (e.g., smarter) than to
engineer a solution to the wrong problem: continue enabling antiquated,
excessive water consumption habits?

~~~
msh
Consider that Denmark have so much rain that it's not a issue.

------
userbinator
Is there any portion of the spectrum left over for them?

It reminds me of this recent article: [http://www.wired.com/2015/04/irobot-
lawnbot/](http://www.wired.com/2015/04/irobot-lawnbot/)

Discussed here:
[https://news.ycombinator.com/item?id=9388115](https://news.ycombinator.com/item?id=9388115)

------
legulere
> I did what any sane person should do:

That actually would be to not get such a big lawn.

~~~
mauricemir
Or plant it as a wildflower meadow which encourages wildlife and plant species
and is much less high maintenance.

Ps

Shameless plug for a mate [http://www.daviddomoney.com/2015/03/13/how-to-grow-
a-wildflo...](http://www.daviddomoney.com/2015/03/13/how-to-grow-a-wildflower-
meadow-in-the-garden/)

