[RFI] Fan Noise on Repeater

Stu Benner w3stu at myactv.net
Tue May 9 17:02:15 EDT 2006

Dale, you're right on point. Actually, a number of my questions (#4, #5, #8)
were digging at understanding what the CE coupling paths may be and what the
power source impedances may have looked like in each configuration. I
suspect that is the most likely culprit here, but didn't want to steer Rod
down one path before all of the data was in.

Tom, I'm also in agreement with your comments. #6 and #7 were in part
probing for the possible "microphonics" type problems. #10 was there partly
to rule-out the possibility that the antenna was within feet of the fan.

Since this is a commercial radio, it's less likely that some conducted noise
is getting into a varactor circuit somewhere. That was part of the reason
for #2. I've noise on the DC power bus of a spacecraft get into even a
rather well designed modulator - it only takes microvolts in the right place
and if there's enough current flowing where it's not supposed to, it's easy
to develop microvolts!

Enough of this. Let's see how many more clues that Rod can bring forth.

Stu Benner

-----Original Message-----
From: rfi-bounces at contesting.com [mailto:rfi-bounces at contesting.com] On
Behalf Of dgsvetan at rockwellcollins.com
Sent: Tuesday, May 09, 2006 2:54 PM
To: rrath at charter.net
Cc: w0rfx at netins.net; rfi at contesting.com
Subject: Re: [RFI] Fan Noise on Repeater


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

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.

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.)  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.  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

             rrath at charter.net                                             
             Sent by:                                                      
             rfi-bounces at conte                                          To 
             sting.com                 rfi at contesting.com                  
             05/09/2006 07:31                                      Subject 
             AM                        Re: [RFI] Fan Noise on Repeater     

1.2 meter repeater
2.Motorola GR300 Repeater and GM 300 radios
3.12volt DC
5.the same 12v line the radios do - batteries charged by solar/wind gen.
6.mounted on the back of housing 7.I do not know 8.I bypassed the power
supply 9.I do not know

Need to get to work. Will answer more after work.
Thank you.



A few questions for you to help better understand the problem:
1) On what band does the repeater operate?
2) What make and model are the repeater radios?
3) What is the operating voltage of the fan?
4) How is the fan controlled (e.g., thermostat, continuous, proportional,
5) From where does the fan obtain its operating voltage?
6) Where on the repeater is the fan mounted and how?
7) Is there noise from the fan on the repeater receiver (with a weak signal
applied) when the repeater transmitter is off?
8) Explain how the repeater was connected to operate from 120 VAC at one
location and battery at another.
9) What is the make and model of the fan?
10) Describe the antenna height and distance from the fan for both sites
11) Describe any other differences in setup between the two sites such as
transmission lines, etc.
12) For what purpose is the fan used?

Stu Benner
www.emcwizard.com _______________________________________________
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