[TowerTalk] Station Noise

[yetiguy at earthlink.net]

Mike,

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 now. 

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).

CONDUCTED NOISE vs: RADIATED NOISE.

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 <mryan001 at tampabay.rr.com>
>Sent: Oct 16, 2009 4:24 PM
>To: towertalk at contesting.com
>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? -
>Mike
>    
> 
>
>__________ Information from ESET NOD32 Antivirus, version of virus signature
>database 4515 (20091016) __________
>
>The message was checked by ESET NOD32 Antivirus.
>
>http://www.eset.com
> 
>
>_______________________________________________
>
>
>
>_______________________________________________
>TowerTalk mailing list
>TowerTalk at contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk