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Re: [TowerTalk] Station Noise

To: "Tower Talk List" <>
Subject: Re: [TowerTalk] Station Noise
From: "Jim Brown" <>
Date: Mon, 19 Oct 2009 13:32:32 -0700
List-post: <">>
On Fri, 16 Oct 2009 22:59:44 -0400, Roger (K8RI) wrote:

>Not being the patient type I'd expect the dealer or factory to fix it.  
>If not promptly addressed they'd find in on the loading doc some 

I strongly agree. Many modern appliances are being sold with 
electronically controlled variable speed motor drives. These units are 
NOTORIOUSLY BAD RFI generators. Tell your dealer to either fix it or 
replace it, and make it clear that it's on HIS nickel, not yours. This 
product violates Part 15 of the FCC rules. 

BTW -- I'm just working my way through Henry Ott's new book on EMC. It 
includes specific advice for these variable speed drives. 1) Run a 
dedicated equipment ground conductor between the controller and the 
motor, following PRECISELY the same route as the current-carrying 
conductors, and in VERY close proximity to them. This forms a 
transmission line for the noise, providing a return path that has a very 
small loop area. 2) Add a good power line filter between controller and 
the power line, carefully bonding the ground of the filter to the ground 
of the controller with a ZERO LENGTH wire. This is a two-step process, 
and you need both steps. #1 is probably the most critical, but both are 

A third step that also helps must be done by the motor/controller 
designer -- add a ferrite bead and small bypass capacitor to the gate of 
the switching elements to slow down the rise time a bit, which reduces 
the high order harmonics of the square waves that drive the motor. 


Jim Brown K9YC


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