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Re: [RFI] Elevator noise

To: "rfi@contesting.com" <rfi@contesting.com>
Subject: Re: [RFI] Elevator noise
From: "Jim Brown" <jim@audiosystemsgroup.com>
Date: Wed, 21 Nov 2007 07:56:21 -0800
List-post: <mailto:rfi@contesting.com>
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 


Jim K9YC

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