RFI: actual signal levels.
 

I received an Email asking why I was unimpressed with FCC Class B
ratings.

Class A
0.15~0.50MHz 66 dBuV
0.50~30MHz 60 dBuV
Class B
0.15~0.45MHz 56 dBuV at 150KHz decreasing to 46 dBuV at 450KHz
0.45~0.50MHz 46dBuV
0.5~30MHz 50 dBuV

Most receivers use 30dBu or 34dBu as the S9 reference level.

So a legal Class B device can produce and couple RF well in excess of
S9 into
the house AC mains. We are lucky that the typical house AC wiring is a
very
inefficient antenna.

However any device which bridges two different conductor systems, such
as a modem
or telephone answering machine, is much more likely to be a very nasty
RFI source
because the radiation efficiency is greatly enhanced.

A few examples come to mind:
Network hubs bridge the AC mains and the Ethernet wiring.

Alarm and security systems bridge the AC mains and the alarm wiring.

Cable TV bridges the AC mains and the cable system coax. Hopefully the
grounding block will reduce the efficiency and minimize RFI.


I am using the term "bridge" to describe the fact that some devices
are connected to
more then one conductor class and these 2 or more conductor classes
can form very
efficient quasi or pseudo "dipoles".

While a dipole performs best when each element is an odd quarter
wavelength, significant
dipole action can be achieved with conductors as short as 1/20 of a
wavelength.

With a near by signal source of 50dBuV, we simply can not afford to
have "efficient" antennas
to radiate the RFI. This is the main reason why I push the idea of
adding additional filtering
to stop or at least reduce RFI at the source. This is why W1HIS's use
of 0.01 X class capacitors
in open AC power outlets through out the home can pay big dividends in
noise reduction.

DSL "filters" are cheap and fairly effective for decoupling in house
phone devices from the
telephone line.

DSL services are very tough RFI sources to tame!!!!

I am still working with a ham friend in this effort and I haven't
found an acceptable solution.
He powers down and disconnects the DSL modem when he operates on HF.

I hope my math is right, I left my notes at home and I have a
splitting sinus head ache today,
so my thinking might be muddier then normal..

Terry