[RFI] UPS Radiated Interference

Bob K0NR (email list) list at k0nr.com
Sat Jan 19 20:31:37 EST 2008

Yes, I unplugged the equipment that the UPS is powering, so it really is
the UPS.

I said "switching regulator" but it might be more accurate to say
"switching inverter" (DC battery to 120 VAC).
These are el-cheapo off brand UPSs.


wa4tuk wrote:
> 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.
> Pat
> wa4tuk
> 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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