Topband: lift station noise
jim at audiosystemsgroup.com
Tue Apr 7 16:06:06 PDT 2009
On Tue, 7 Apr 2009 15:24:57 -0400, Larry Pasman wrote:
>I am having a problem with a noisy sewage lift station near my QTH.
>When trying to work 160 DX early mornings I will hear a "pop" then
>nothing but hash until the lift station shuts off with another "pop".
>Of course during the morning hours people are getting ready for work,
>school and such so the lift station comes on frequently. Any ideas
>how to approach this.
Not an easy solution, Larry. On YOUR end, you might be able to kill it
by putting it in the null of a directional RX antenna, but that won't
help for DX coming from the same direction. :)
The root cause of the problem is probably a noisy controller for a
motor associated with that system. These controllers work by first
rectifying the power line to create DC, then chopping it and using the
chopped waveform to drive the motor. Speed is controlled by varying
pulse widths. That chopping creates strong harmonic content in the
current waveform. The stepdown transformer, the motor controller, and
the motor are often widely separated from each other (to make life
easier for the electrical contractor), and there are bypass capacitors
on the controller and leakage capacitance to ground at both the
transformer and the motor. All of this combines to form a current loop
for that HF noise, which in turn creates a big magnetic field.
Problems like this are most effectively solved at the source, but that
means you must get the noise polluter motivated to do so. That's
probably not easy. If I were the engineer responsible, I'd first
contact the mfr of the controller and try to get a noise-reduction
modification for it. That would most likely involve slowing down the
rise time of the control pulses -- the strength of harmonics is
approximately proportional to the rise time.
Jim Brown K9YC
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