[RFI] UPS Radiated Interference

wa4tuk wa4tuk-rf at comcast.net
Fri Jan 18 22:11:54 EST 2008

They certainly could be switching supplies; however, the ones I've 
disassembled after failing have had iron core laminated transformers. 
These were APC and Tripp-Lite brands. In fact, I've contemplated using 
them in reverse as filament transformers. It could be that newer models 
have all moved to switching supplies.

When these UPS's are asked to provide emergency power they do make an 
strong RF buzzing howl.

I would guess that you pulled the PC's and monitors AC cord to be sure 
it wasn't the PC/monitor that was making the noise? Many newer PC's have 
a small switching supply that is always on to support "wake-on-lan" and 
other features. And the monitors just go to a standby low power mode. 
That's part of the reason my PC's are fed from a switched outlet strip.


Bob K0NR (email list) wrote:
> I tracked down some RFI problems on the 2M band to several small
> Interruptible Power Supplies in my house. These are the half-a-shoe-box
> size units intended for use with PCs. I assume the switching regulator
> inside is the source of the noise.
> I am thinking my best strategy is to replace them with higher quality
> units (rather than try to suppress the noise).
> Any suggestions on models of UPSs that don't radiate?
> Any other ideas?
> Bob K0NR
> Bob Witte K0NR
> bob at k0nr.com
> Technical Coordinator
> ARRL Colorado Section
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> RFI at contesting.com
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