15 Mhz noise problem created by outside AC unit. Blocking 20 Meters BPSK31

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W6RRS, Jun 6, 2021.

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  1. W6RRS

    W6RRS Premium Subscriber QRZ Page

    I think my header says it all. So sometime back, before I had two operations I had no problem working BPSK31, with or without the outside AC running. Was able to get back to the shack about a month ago to see Picket Fence noise all over the data end of 20 Meters. Then as if by magic, fence drops and I make a couple DX contacts until, Fence noise starts up. I scan the noise to see the fundamental frequency is 14.620 Mhz. I have scanned the Web and read a lot of technical articles, so now I have a headache. Looking for some easy fundamental fixes. Help please, my age may be clouding my judgement, thank you
    Ron Speer W6RRS
  2. N4CVX

    N4CVX Premium Subscriber QRZ Page

    The fact that the "fence noise" starts and stops and it is presumably coming from your HVAC unit, means that there is something inside the unit producing wideband RF energy. First thing I would look for is the motor and compressor starting relay system, but also and more likely - since it is off and on -- at the rotating devices inside the unit. An electric motor can generate a lot of wideband RF noise, usually from the carbon brushes which are fitted to the armature of the motor. If the springs are weak, when rotation is happening, there will be sparking between the pads of the motor armature (commutator) and the carbon brush surface as the brushes "jump" up and down in their holders. That jump only has to be a millimeter or so to cause sparking, especially with 220/240 Volts AC.

    Once upon a time, we (in the SIGINT world where I worked) we had a wide-band RF jamming device that worked just like that and was used to mask TEMPEST emanations from non-TEMPEST treated sensitive devices.

    Solution? I would first get your HVAC guy out to take a look at the brushes and tell him about the RF interference problem you are having (stop-start, etc). Second, have the HVAC guy look at all of the rotating equipment inside the unit -- which is usually the shaft from the motor to the fan system -- where there may be a low-level electrical coupling or a friction-generated static electric charge. Third, I would make sure that the HVAC unit is properly grounded.

    GL and come back here to let us know how the problem solving worked out.

    Dave N4CVX
    N0TZU likes this.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Does the noise definitely coincide with the A/C starting and stopping? Easy test is to just shut it off to see if the noise all disappears, then turn it back on to see if it reappears.

    Maybe you've already done that, and if so then the previous suggestions might help. But I'd definitely confirm the source before putting any more effort into trying to cure it.

    Here we've hardly had to use A/C yet this season. It's been in the low 90s a lot, but very dry and is down to 65 by 7 PM and about 60 by 9 PM so the house doesn't heat up much (yet)...but I see you're in Florida and that's a different scenario with the humidity.

    Hope you find the source for sure and can fix it! Few things are more aggravating than having the bands wiped out by RFI.
  4. W9KEY

    W9KEY XML Subscriber QRZ Page

    When running your tests - just remember ....

    If you shut off a running A/C (or any refrigeration system), most of them will not immediately restart when power is re-applied. Compressors generally require some "off time" to allow the internal pressure to equalize, before they can restart - several minutes. That off-time may be electronically controlled or (in the case of a refrigerator) simply a function of the overcurrent protection relay tripping several times due to a locked rotor motor condition (while the gas pressure equalizes).
  5. W6RRS

    W6RRS Premium Subscriber QRZ Page

    The outdoor unit is outside my shack window. Unit starts, Picket Fence starts. Unit stops, noise drops. It’s the offender.
    I think it’s noise clamped to power line supply, or maybe RFI being transmitted from AC unit. Checked out AC power connection. It’s shielded cable tied into metal disconnect box. So, I’m leaning on clamped noise running down power supply line. So how many and size of ferrite snap on’s to suppress 14.620 MHz.? Plus I guess I can buy a load of ferries for my RG8x antenna coax.
  6. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    What’s different in the HVAC system from before the noise started?

    If something was changed, make sure the HVAC guy knows about it. If nothing was changed, then as suggested above it’s likely a failure or maintenance issue of some sort.
  7. W6RRS

    W6RRS Premium Subscriber QRZ Page

    Thank you for your comments. Unit gets serviced every six months. Going to give them a all tomorrow. 73
    N0TZU likes this.
  8. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    I just looked at your QRZ biography and I’m sure you’re well versed in troubleshooting! Best of luck and please let us know what you find and what the solution was.
  9. W6RRS

    W6RRS Premium Subscriber QRZ Page

    Your right. So I contacted Bob at Palomar Engineers and came up with an action plan.
  10. W2AAT

    W2AAT XML Subscriber QRZ Page

    My heat pump sits outside next to my shack. I had some noise from it. I installed "ferrite snap on’s" on on most every cable in my shack (see my shack picture in my QRZ page) . I also put ferrites on all the wiring coming and going to the heat pump. My heat exchanger is above my shack. The motor appears to generate no noise.

    At the same time, I also had an issue with the heat pump shutting down for no apparent reason. I also installed these ferrites to insure that my radio wasn't causing the shutdown. Discovered that there was a bad pressure switch.

    I have no noise coming from these systems...
    Last edited: Jul 2, 2021

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