[RFI] 8 KHz spurs across much of amateur spectrum

[Hare, Ed W1RFI]

These are characteristics that typically identify a switch-mode power supply.

1.  The noise is not uniform across an amateur band, but has peaks that occur every N kHz, with N typically being between 5 and 100 kHz.  These are harmonics of the free-running frequency of the supply.
2.  This noise can range from an almost pure carrier to just a broad hump of noise if seen in a panadapter display or waterfall.
4.  The carriers tend to drift with temperature by a few kHz ad the power supply warms up.
5.  When there is a voltage surge or dip on the ac mains, the power supply free-running frequency changes slightly, causing the harmonics to be heard to vary frequency with the voltage change.

PWM motors also have harmonics like this, but they do tend to come on and off as the motor is turned on and off and, in many cases, the nature of the noise will change as the motor is sped up or slowed down for various reasons.

Ed Hare, W1RFI



-----Original Message-----
From: RFI [mailto:rfi-bounces at contesting.com] On Behalf Of Lee STRAHAN
Sent: Tuesday, March 13, 2018 1:18 PM
To: N4ZR; RFI List
Subject: Re: [RFI] 8 KHz spurs across much of amateur spectrum

Pete,
    As I recall you may be a Hi-Z user. If you are hearing these spurs on a Hi-Z array you may want to check all the elements. If one gets shorted or partially shorted to ground we have had cases where an array will pick up spurs similar to this. On almost all of the later versions of the Hi-Z amps there is 4 to 5 VDC present on the element. A simple test for grounding is to measure this DC on the elements. They should all match within 0.2 VDC of each other.
   If the noise is not on a Hi-Z array then look for a VFD running a pump or any other synchronous motor. They use switching frequencies this low.
   Good luck,
Lee    K7TJR   OR

-----Original Message-----
From: RFI [mailto:rfi-bounces at contesting.com] On Behalf Of N4ZR
Sent: Tuesday, March 13, 2018 4:42 AM
To: RFI List <rfi at contesting.com>
Subject: [RFI] 8 KHz spurs across much of amateur spectrum

This may be a fool's errand, but just in case - I have discovered relatively pure carriers every 8 KHz 2-28 MHz (not heard on 160), tapering in strength as the frequencies get higher.  I assume some sort of switching power supply, but from what I've read most run at higher frequencies than that.  Is this by chance a signature of a particular source?

-- 

73, Pete N4ZR
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