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Re: [RFI] Fan Noise on Repeater

To: rfi@contesting.com
Subject: Re: [RFI] Fan Noise on Repeater
From: rrath@charter.net
Date: Tue, 09 May 2006 16:22:30 -0700
List-post: <mailto:rfi@contesting.com>
My comments:

> Rod,
> The replies from both Stu and Jim, K9YC, were right on target.  However, no
> one has addressed the issue of CONDUCTED emissions from the fan and the
> fact that those conducted emissions may very well propagate into the
> receiver via the power distribution system.  It is also possible that an
> exceptionally "bad" fan may, in fact, create enough noise with harmonics
> that extend into the 2 meter band that you really are getting the stuff
> through the antenna input.
I will be taking a new brushless 12v dc fan with me and the following 
items; three more ferrite loops (not the snap type), one 100 
microhenies 2 amp RF choke, two .01 cap and some shielded two 
conducter wire for the fan.
> Since the fan is a DC-powered type, it matters little whether that DC comes
> from a bank of solar panels or an AC-operated power supply, but the
> arrangement of the wiring or the characteristics of the power source may
> enter into the final analysis.  First, keep in mind that except for
> brush-type motors, most modern DC fans use electronic commutation to drive
> the fan impeller.  That work is done by a small DC to AC inverter located
> right in the hub area of the fan.  That inverter is a form of switching
> power supply.  Most of the time, those inverters have little or no RFI
> suppression.  So, where does that noise go?  Right back up the wires that
> power the fan.
The wire coming from the batteries is a pair of jumper cable wire, 
then to a distrabution strip (for the possitive wire only). The wire is 
fused at that point. Then 16 gauge wire up to the repeater housing 
and the fan wire is spliced into that wire (which is the wire for the 
Tx/Rx radios and the Motorola repeater controller. I have the two 
ferrite snap beads at the splice (not sure what mix they are - from 
Radio Shack)
> Suggestion:  I would plan to connect the AC-coupled input of an
> oscilloscope across the DC power feeders up at the repeater.  Be sure that
> you can turn the fan ON or OFF in some way.   Examine the noise across
> those DC feeders with and without the fan operating and record the
> measurements.  One thought I have is that when you ran the system at home
> from an AC-operated power supply, the output stage of the power supply may
> have been sufficient to attenuate the fan's electrical noise to a point of
> being a non-issue.  (Of course, there is also the dummy load for testing
> that someone else mentioned.) 
I do not have access to an oscilloscope. 
If your repeater site has more feet of wire
> and especially if there is a long distance between the repeater, the solar
> cells, and the battery bank, you may have a situation in which the fan
> nosie is less suppressed than at home. 
There is more wire on the hill top than at home site. Like you stated, 
the wire from the batteries and solar panels. 
Since the fan noise is usually not
> up around 2 meters, I will guess that the fan nosie is coming into the
> repeater on the common set of DC power feeders and somehow coupling into
> the audio or IF stages of the receiver.  While you might be able to lower
> the noise effects by adding more caps inside the receiver, the best thing
> to do with noise of any type is to kill it at the source.
> If you find a lot of noise on those DC feeders using the scope, plan to
> install a small filter on the DC power leads right near the fan.  However,
> the worst part of the filter may be in trying to find one with inductor
> values that provide attenuation in the audio range.  Most readily available
> filters work well for RF, but very well at all below 100 kHz.  Whatever
> filter you install, be certain that the by-pass capacitors inside have a
> short, low impedance path to chassis ground; failure to do so could render
> the filter nearly useless.    Another thought, especially if you do find
> that the fan noise is coming into the receiver via the antenna port, would
> be to use SHIELDED pair wiring for all of the 12VDC power distribution.  It
> will only help, however, if the shields are terminated to common chassis
> ground at each end of a wire run.  Again, better to kill the problem at the
> source.  Good luck.
> 73, Dale

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