On of the main causes of "common mode" [CM] noise is "differential
mode" [DM] noise that gets
converted to into common mode noise.

There are two main means of conversion.

The least common but most powerful occurs when a device like a laptop
is powered by the AC
mains and is connected to a phone line or Ethernet cable. The noise
from the power supply is
coupled back on the AC mains and into the telephone or Ethernet line,
and this forms a very
effective unintentional antenna. Any device that is connected to the
AC mains and any other
conductor is very likely to be a source of significant common mode
noise. This other conductor
can be telephone, Ethernet, Coax Cable for DBS or CATV, or any other
conductor. While we
are used to thinking of antennas as needing to be an odd quarter wave
length, effective antennas
can in fact be much shorter. Ground leads approaching 1/20 f a wave
length are effective
radiators.

The most common conversion of DM to CM occurs due to the imbalance in
the AC mains. I have
looked at over 20 different brands of 3 conductor cable used for
domestic residential wiring. Some
of this is new and some dates back to cable that is at least 30 years
old.


There are 3 main types:
Type 1: H-G-N arranged in a triangle with the ground between the hot
and neutral.
Type 2: H-N-G arranged with the hot on one side, the neutral in the
middle and the ground
on the other outer side next to the neutral.
Type 3: The H and N run, more or less in parallel, with the ground
weaving back and forth from
one to the other.

At the start of this investigation I assumed the primary reason for DM
to CM was caused by the
interruption in what I had perceived "the natural balanced
transmission characteristics" of AC
power cords. I do accept that when the conductors that are nominally
on 3/8" centers are spread
for outlets, switches, light fixtures, and the mess inside a breaker
panel, don't help the situation.
However it became apparent in some tests at a new home with all of the
AC and other cables
installed, but with no connection to the real world via power,
telephone or CATV, that my original
idea was flawed.

I now believe the primary cause is the extremely unbalanced
"transmission line" effects caused
by the ground conductor's relation ship to the purported balanced 2
conductor hot and neutral.

In a simple test last night an Electrician friend brought over a ~100'
piece of Romax of the type 1
construction. We(he) laid it out on the surface of ground and I used a
Balun to couple the Romax
to my R2000 operating on battery power. I terminated the far end with
a non inductive 120 Ohm resistor. When this experiment is performed
using 2 conductor zip or speaker cable there is
minimal reception of all but my local 770KHz MW pest. However with the
Romex reception of even
distant MW and HF signals was significant. Grounding the Romex
"ground' conductor did not
have a significant effect.

I suspect that if we could wire our homes with 2 conductor cable and
run a separate grounding
conductor we could reduce the conversion of DM to CM. Since the NEC
would frown on such
a scheme and because the costs would be extraordinary the only
effective way to stop the
conversion of DM into CM is to eliminate or at least reduce DM noise
at the source.

I hope to put a more detailed version of this experiment along with
the new house test results
on Will's stopRFI page "some day real soon".

Terry

ding the "Ground" conductor changed the