[TenTec] strange QRM/QRN/RFI source?

[Reed O. Krenn]

Very Interesting! During the 80's I worked at Sangamo Weston in the Advanced Technologies 
Group where we were exploring PLC with Direct Sequence Spread Spectrum (DSS). Our 
research was for remote metering and distribution control. We found the power line to be a 
pretty hostile environment for data communications of that sort and, as you indicated, high 
power levels were required to get any type of useful range.

One interesting thing we found was that distribution transformers have a mechanical resonance 
in the 6 KHz range, and this produced an audible (and annoying!) singing in the transformers. 
Worse, in the area where we were testing, office workers could hear the signals in their 
computers, copiers, and fax machines. 

The most embarrassing situation occurred when the DSSS signal set off a building alarm in a 
warehouse district late one night. The alarm monitoring company had the ability to dial in and 
listen for sounds of activity in the facility, and all they heard was this eerie collection of audio 
tones from our test equipment. As you might guess, the next morning we were invited to 
remove our test equipment from the premises. The owner was none too happy about being 
called out after hours to meet with the police.

Anyway, after 10 years, many $$$, and an unpredictable market, Sangamo abandoned the 
project. Fortunately, I bailed out about a year before the axe fell.

And now.......BACK TO MY ORION!

Reed / WW3A

On 15 May 2003 at 10:59, Dr. Gerald N. Johnson, electr wrote:

> <snip>
> I did my graduate work on digital communications over the power line
> (thinking very low speeds for control and meter reading and using the
> spectrum near 60 Hz). I looked at noise levels in the spectrum I was
> interested in. I concluded that its going to take a lot of power to make
> anything fast work and then if its above ordinary power frequencies its
> going to radiate as if connected to a dipole antenna. And that was
> before the days of switching supplies and variable frequency drives. SCR
> controlled induction heaters in one industrial neighborhood pegged my
> noise meter.
> <snip>
> Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
> Reproduction by permission only.
> _______________________________________________
> TenTec mailing list
> TenTec at contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec