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Re: [TowerTalk] Station Noise

To: Michael Ryan <>,
Subject: Re: [TowerTalk] Station Noise
Date: Sat, 17 Oct 2009 00:13:29 -0400 (EDT)
List-post: <">>

The earlier TT suggestion to call the A/C dealer/installer, to complain about 
EMI noise, is very good. Tell them the noise is unacceptable, and their 
equipment interferes with your electronics. The manufacturer may have a fix 
available. However, they may not, and you've probably paid for the thing by 

So, here are some ideas from a non-expert in modern A/C equipment, but one who 
has had very good luck in finding RF and microwave noise sources and 
suppressing the hell out of them.  These are based on trying to stop the noise 
at its source.  BTW, to prevent confusion, the term "AC" used here means 
alternating current, and "A/C" means air conditioner.

The continuous noise (hash) is likely generated by a variable-speed drive for 
the A/C air-mover motor. The noise is probably coming from the motor 
speed-control drive circuit (as the output transistors/triacs/whatever switch 
on and off). Variable speed motors are often used to reduce electrical power 
consumption to just what power is needed, at the expense of the switching 
noise.  If you change fan speed you can probably detect that change in your 
receiver (a pitch change).  A multiple-speed AC motor (synchronous) of course, 
would be much quieter, and may even be offered by the A/C manufacturer as an 
alternate to the VFD (Variable Frequency Drive).


Your motor noise could be CONDUCTED - backing up into the AC line (poor power 
line quality) and getting carried to your electronics by house wiring, or 
RADIATED - and then detected by your receiver. Most likely, a little of both.

SIMPLE TEST: Does a portable radio, running on batteries, get the same 
interference when the A/C is running? If the noise level on the portable radio 
is lower, the noise may be conducted instead of radiated. 

If the noise is conducted noise, you could use a big AC line filter at the A/C 
unit input terminals.  I've seen larger AC noise filters for sale at Surplus 
Sales of Nebraska (on the web) at reasonable prices [I have no affiliation with 
them]. You could probably suppress conducted noise a lot using a 60-Hz, 1:1 
input isolation transformer to the A/C (pretty expensive, but may also be avail 
at Surplus Sales). Ideally, it has one or more electrostatic shields between 
the primary and secondary windings - that you would have to tie to a good earth 
ground. This will heavily suppress noise trying to creep back upstream to the 
AC lines.

If the noise is radiated, you could start by bonding the motor body and all the 
metal panels of the A/C to the main A/C housing, and then connect directly the 
A/C housing to an isolated earth ground. A GOOD ground. I wouldn't rely on the 
air conditioner's AC plug ground path.  It's OK to connect a ground wire from 
the A/C housing to a good earth ground. Don't connect to a gas pipe thinking 
it's a ground. [Note: Check out your car hood sometime. You'll see bonding 
wires from the hood to the firewall, engine to the firewall, etc.  All for the 
same reason.] Last, try bonding the A/C ductwork, a piece at a time, so it's 
all electrically conductive (jumpers across ductwork joints, dampers, etc.)and 
at the same ground potential.

The AC line filter is probably least expensive. A synchronous motor next. A 
large isolation transformer the most expensive.

Good hunting.

Dennis Collins, K6IFB
Placitas, NM

-----Original Message-----
>From: Michael Ryan <>
>Sent: Oct 16, 2009 4:24 PM
>Subject: [TowerTalk] Station Noise
>My coax leads from my tower come through a soffit aside the tower, into the
>attic. I bring them then into the inside of a closet, and under the door of
>the closet to my rig.  Next to the closet is my a/c air handler. This is a
>new unit, just replaced a 20 yr old TRANE.  When the air handler is running
>the fan puts out some ungodly noise.  I have to shut it down to hear most
>anything that is not S-9plus in strength.  I sent an email off to ICE
>Products to see if they have some kind of a 220 volt line filter...never
>heard back from them, now after 2 weeks.  When the 220volt breaker is turned
>off, the noise disappears even if the fan is still running slowing to off.
>Anytime the fan is running and has current it is dirty. We have put bypass
>caps on the 220 volt supply line, and the fan leads.  No help.  Any ideas? -
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