Bob, One of the cheap brushless fans that I found was RF-noisy would radiate EMI only while it was running, and the EMI would stop when I held the blades. I proved it was radiated EMI (RFI, actually) rather than conducted EMI by simply running it on a small 12V battery. I could see the noise on a spectrum analyzer, and it still put out a healthy signal even when I added a ferrite choke and some capacitors. The low-EMI Panasonic fan was noise-free.
The reason most cheap brushless fans have a noise radiation and conduction problem is because a Hall-effect device is used to sense the rotation of the armature. If you stall the fan, the EMI goes away. It's not the Hall-effect device itself that causes the problem; it's the lack of proper filtering and shielding within the fan's motor drive circuitry. I certainly agree with your point about mechanical vibration; some fans are poorly balanced and can cause microphonics. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Bob M. Sent: Wednesday, April 18, 2007 5:48 AM To: [email protected] Subject: Re: [Repeater-Builder] Re: Problem with fan noise in power line. I had DC fans on a Henry 100w repeater amp. The first time they came on there was a terrible buzz on the transmitted audio. It never bothered the receive audio. Henry supplied some AC fans and that took care of my problem. One way to test whether it's mechanical or EMI is to stop the fans from rotating. If the buzz stays there, it's EMI; if the buzz goes away, it's mechanical. I actually had a little mechanical noise, but I took care of that by using some rubber grommets between the cabinet and the MaxTrac exciter, around the mounting screws. ABsolutely quiet now. Bob M. ====== --- sgreact47 <[EMAIL PROTECTED] <mailto:echocomms%40hotmail.com> > wrote: > As Rod pointed out, the vibration from the fan > causing transmitted > noise in his GR300 Rptr. In the Motorola GM,Radius > M, and Maxtrac, > series mobiles the RX and TX VCO's are very > sensitive to vibration. > > Other radios may be sensitive to vibration also. > > [EMAIL PROTECTED] wrote: > > > > I too had a fan noise on my GR 300 repeater. As > Eric instructed, I > replaced > > the fan with a low EMI. Still had some but a lot > less. Then I took > the new low > > EMI fan and removed it from the repeater cabinet > and hung it from > the big > > cabinet, holding all the repeater items, so it > still blows on the > heatsink and it > > is almost not heard. I had two problems, 1) to > high of EMI rated > fan, 2) > > vibration noise. Thank you Eric L. for your help > with my problem. > > > > Rod KC7VQR
