[Elecraft] OT: K3 vs an RFI noisy furnace

[k2qi.nyc at gmail.com]

Hi Don,

I'm no expert on RFI so what I'm about to say, you've probably thought about already.

But, have you considered shielding the induction motor? Or if that isn't possible, using something to null out the noise?

James K2QI
Sent via BlackBerry from T-Mobile

-----Original Message-----
From: Don Nelson <n0ye at comcast.net>
Sender: elecraft-bounces at mailman.qth.net
Date: Wed, 10 Nov 2010 08:52:33 
To: <elecraft at mailman.qth.net>
Subject: [Elecraft] OT: K3 vs an RFI noisy furnace

Hi all,

I misspoke earlier this year when I said our new Lennox high efficiency 
furnace was not generating any noticeable RFI. Quite to the contrary, 
the new Lennox furnace is very rich in RFI energy. One observation is 
that the RFI has a very broad spectrum and it's presence is not 
noticeable by audio sound alone. However, my S meter noise floor goes up 
by 3 or more S units when the furnace is heating the house. The furnace 
we have is the Lennox G71MMP, said to be a 95% efficient furnace.

The “reference” antenna that I use is a Cushcraft R7 vertical mounted on 
the side of the house. The magnitude of the RFI decreases as I listen on 
each higher band. On 40m, the RFI from the furnace is over S9 compared 
to baseline noise floor of S6 when the furnace is off. The receiver is a 
K3 with a pretty good S meter calibration. The difference from 
background noise is on the order of 20 dB on 40m and on 20m. The noise 
decreases as I listen on higher bands. What the furnace RFI is on 80m 
and 160m are unknown because I do not have a “reference” antenna for 
those bands.

There are two variable speed motors in this furnace. One motor blows air 
throughout the house. The second motor is the “induction” motor and 
delivers outside air into the burn chamber. This induction motor and the 
drive circuit are the source of the RFI. The blower motor, when running 
alone does not generate noticeable RFI. I have used a clamp-on toroid 
with a multi-turn secondary connected to an oscilloscope to sample RF on 
various leads emanating from the control board inside the furnace. The 
four leads going to the induction motor have significant noise on each 
as does a ground wire going to the furnace chassis. The other leads 
going elsewhere are quiet. The noise I observe on the induction motor 
leads is characterized as fast rise time pulses.

I have tried to engage Lennox in discussions on this RFI problem with 
limited success to date. The furnace installer has basically given up 
without trying pleading ignorance which is understandable.

I am trying things to mitigate the RFI with limited success so far. I 
would very much like to hear what others have done to mitigate their 
furnace RFI problems.

Don, N0YE

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