John, Perhaps you should use DC fans that are designed to be low-EMI. Panasonic (among several other makers) offers a line of "low-EMI" DC fans that are specifically designed to minimize or eliminate any conducted or radiated EMI during operation. I have used such a fan on a solar-powered repeater, and have zero interference from it. Some cheap DC brushless fans are made to a low price target, and are prolific generators of hum and hash. I have found through several years of experience that a high-speed fan- which can generate acoustic noise- is often not needed. A small, 3-inch low-EMI fan that runs at a very slow speed is adequate for most applications. A fan that runs at less than 1000 rpm is usually adequate, and is almost noiseless to boot. Nobody likes a whiney fan!
73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of John Harrington Sent: Monday, April 13, 2009 7:59 PM To: [email protected] Subject: [Repeater-Builder] Funny noise on repeat audio Here's a problem that caused me much gnashing of teeth and tearing of hair before I nailed it. Some of you may find it old news, but here it is: Mastr II station with NHRC controller. Mobile PA, with fans added so the PA would live through nets and long transmissions. Problems with a "humming" noise on repeated audio, low deviation, but annoying. Station mike clean, but hum (sorta like PL tone) on any repeated audio. Lived with the problem for a while, but an overhaul on the bench on other problems (whiskers in the rx front end, etc.) gave me some time to play with it. Turns out it was the fans! I had used 4" 12V brushless muffin fans. The fans were running from the 12V station power supply. When I disconnected them in desperation, the noise went away! I built a little filter from a toroid core and a few caps in a pi-net, but the noise was still noticeable. I installed two 120VAC fans and the problem was solved. My theory is that the 3-phase oscillator in the fans produced square waves to run the little fan motor. Square waves have lots of harmonics and was radiating them into the rx audio line into the controller. Word to the wise-- be careful of 12V brushless fans around susceptible equipment. John W5EME
