[RFI] Comments on RFI from Ductless Air Conditioning Units?

[Dale]

Jeff,

The church I attend had 8 of these type of units installed 3 or 4 years ago.  (The building dates from 1904, so ductless was the only practical route.  Yes, our units resemble the Daiken ones to which you referred in another posting.)  I don't know the make of the indoor units, but the compressor units have a Fujitsu logo on them.  We do not have an HF radio station at the church, but we do have a sound system, and I am the engineer on that system.  

I moved the mixing console and related equipment to a position up in the balcony about the same time that the A/C units were installed.  The system move resulted in more than 120 feet of 20 line cable being routed up thru the attic area of the church to bring all of the mic lines up there.  (Note that all of these lines are balanced pair and individually shielded, with an outer braided shield over all 20 internal cables.)  At the time, I felt that we would have no problem.  So far, no problems.

Since our A/C equipment is "industrial grade", there is a lot of unshielded power wiring routed around to the 8 indoor units, much of it in the attic area.  The sound system cable passes near 3 of them.  The systems we have are inverter-type, and the inside air mover motors, as well as the outdoor coil cooling fan motors, are variable speed.  I do not know if the compressor is variable speed or not.  Obviously, all of the speed control is via one or more VFDs.  With the layout of these systems, the radiating loop area for emission of noise via the AC input and control wiring is huge.  I have monitored the output of the main mixing console with headphones while the systems were running and have heard no trace of hash or transients coupling into the sound system wiring.  Yes, the incoming AC power feeds the A/C units and the sound system (600 amp, 120/240 single phase service).

Of course, this is not the same as having HF antennas in close proximity to these A/C units.  However, I was impressed that we are able to carry many feet of cable with millivolt level signals close to these units and have clean sound.  It must also be remembered that unlike most HF antennas, use of balanced signal lines and shielding greatly reduces common mode coupling.  Thus, this is far from being an apple-to-orange comparison.  Still, keeping long runs of low level audio signals "clean" does take proper techniques.

I do not currently have an HF mobile setup.  If (when) I do, I will drive over to the church and see what I hear!  I would be able to park within 6 to 10 feet of all 8 compressors (4 on the east side and 4 on the west side). 

73, Dale
WA9ENA                     


-----Original Message-----
>From: Jeff Stevens <jeff at mossycup.com>
>Sent: May 8, 2013 3:24 PM
>To: RFI at contesting.com
>Subject: [RFI] Comments on RFI from Ductless Air Conditioning Units?
>
>Hello All,
>
>I may be in the market for a ductless AC unit.  Many of the newer
>models seem to come with variable speed compressors which are PWM
>driven.  I haven't seen any comments anywhere about these as potential
>RFI monsters.  Does anyone have any general experience?  Horror
>stories?  Happy non-interfering co-existence?
>
>73,  Jeff
>W7WWA
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