In article ,
"Robert11" wrote:

Hi,

Any idea how quickly in distance these household noise generated fields
diminish to values unlikely to be picked up ?

Snip

This is a complex question. Firstly to consider is that there is two
types of noise sources or ways for noise to find its way to your radio:

1. RFI, radio frequency interference, which is radiated noise. Radiated
noise can travel long distance and is an example of the signals you want
to receive.

2. EMI, electromotive interference, which is conducted noise. As
separate local electric and magnetic fields the energy falls off rapidly
over distance. A few feet can make a big difference in noise level.

If the problem electronic device is radiating the noise (RFI) then how
much it bothers your reception will be determined by how much noise it
generates and how efficiently it radiates that noise. Generally the
dimensions of the device mean that any radiators in the device will be
electrically small and the efficiency the device to radiate noise
energy will be poor. The result is most of the noise energy will become
localized electric and magnetic fields known as inductive fields as
opposed to what is know as a far field EM wave. The radiated EM wave
has a special relationship between the electric and magnetic component
fields that allow the long distance propagation to occur. The localized
electric fields tend to spread out more than the magnetic fields in the
low dielectric air and the majority of non metallic house building
materials slow them down by tending to bend them but don't stop them.
The result is that the inside of most homes are much noisier inside
then just a few feet outside most structures.

So you can draw several conclusions from this such as that the local
noise field energy will fall off rapidly over distance from the noise
source. This means that you can reduce noise by moving a portable radio
or antenna farther away from the source. Antennas that are sensitive to
the magnetic component and not the electric component on the EM noise
fields will fare better than an antenna that is an electric field type
in the area of local noise sources. A common mode electric field antenna
will the worst performing antenna around local noise sources.

If the main noise generation is EMI meaning that the noise is conducted
down the power cord into the AC mains supply then several things can
happen:

A. If you plug your radio into the AC mains supply then the noise goes
up the radios power cord and is conducted into the radio.

B. In addition to this direct conduction if you have an external antenna
connect by coax cable this noise can be further conducted on the outer
shield and then transfer to the antenna as common mode energy.

C. Noise that starts out as EMI in the mains AC supply now has found
larger radiators, which is the wiring in your house and then can radiate
more efficiently becoming RFI.

"C" can be an especially insidious problem to solve.

Strategy to follow to reduce local noise:

1. Put as much distance as you can between the radio, antenna and the
local noise sources.

2. Use an antenna type sensitive to the Magnetic component of a EM wave.

3. Use RF chokes on noise source device power cord and other cables.

4. Turn off and actually disconnect electronic noise sources.

5. Identify troublesome noise sources by unplugging or disconnecting
everything in the house and turn them on one at a time after a scan on
the radio.

6. Use chokes on the coax to the antenna or choose a lossy RF path for
the coax such as putting it in the ground on its way to the antenna from
the house.

7. Locate external antennas away from power lines, the house and
neighbors homes or other noise sources.

--
Telamon
Ventura, California