On Wed, 21 Nov 2007 08:17:45 -0500, VE3NE wrote:
>In this summer three old elevators were replaced with brand
>new ones in my building. My shack is adjacent to the elevator
>room on top of the building. When the elevators are running
>the noise is quite bad on 160 and 80 meters. One of elevator
>produces is around S7, the other two are not that strong but
>still bad and quite enough to wipe out even loud stations.
What you have encountered are variable speed drive motors. The
controllers for these motors take AC (lots of it, enough to run
the motors), rectify it, then pulse modulate it and vary the
width of the pulses to vary the speed. The noise you hear is
the result of two (at least) major design/installation
1) The pulse repitition rate is typically on the order of 10-20
kHz, and it has a fast rise time. The fast rise time produces
lots of high order harmonics.
2) The current for these pulses circulates between a 3-phase
transformer, the controller, and the motor. For the convenience
of the installation, those three components are often widely
separated, and the wiring forms a large loop. Large loop area
equals large magnetic field and good antenna.
The coupling is primarily magnetic, because you are in the near
field. The good news (damned little of it) is that field
strength dies off faster than inverse square law with distance
from a point source.
The solution to your problem is to reduce both the rise time
(and thus the higher order harmonics) and the loop area. That,
of course, must be done by the elevator mfr and installer. You
may get some resistance on rise time, because slower rise times
increase dissipation. You will definitely get resistance on
reducing loop area -- it means the transformer and controller
must be moved closer to the motors. You may need to involve (or
threaten to involve) the Canadian radio authorities to get
Also, because it is a magnetic field, you may be able to
organize your antennas and shack wiring to minimize the pickup
of that magnetic field. One of the first moves would be SERIOUS
ferrite chokes on all your antennas at the feedpoint.
Try to use dipole and beam antennas that have no active
elements near the elevators. Definitely avoid long wires that
run near the elevators -- they're going to pick up that field.
If you must use a long wire for transmit, use a magnetically
shielded loop for RX, or something that nulls in the direction
of the elevators and their wiring.
There are several documents on my website that describe
variable speed motors and their problems.
My RFI tutorial also includes a section on magnetic and
electric field concepts.
Sure hope you can fix this, Lali. We would miss you in the
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