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.
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.
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
5.the same 12v line the radios do - batteries charged by solar/wind
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.
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?
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