Topband: RFI

Paul Christensen w9ac at
Tue Nov 1 07:22:26 PDT 2011

Typical switch-mode noise.  Over the years, I have had to identify several 
of these noise sources from within my own home as well as neighbors.  Keep 
in mind this is a different type of noise to localize than that produced by 
power lines.

The first step involves powering down your hone, one circuit at a time, and 
defeating battery-back systems like security.  It's most helpful to power 
the rig by battery during testing.  If the switching noise stops as you 
deactivate breakers, then you know the noise is on your property and you'll 
need to investigate appliances on the affected branch circuit.

If the noise remains, you'll need to identify the direction, likely coming 
from a neighbor who has recently added a switch-mode appliance.  I use a 
directional loop antenna, SDR-IQ receiver, and notebook PC to document the 
noise with screen captures while on foot.  Most of the time, I can 
triangulate the source with the loop.  This can save a lot of time.

The usual question arises:  What could it be?  It can be many things and you 
need to investigate rather than ponder the question.  For example, here's a 
list I'm recalling from memory of devices belonging to neighbors that I've 
had to either replace or add RFI/EMI chokes to cabling: Neighbor's 
micro-refrigerator, Neighbor's PC power supply, Neighbor's scooter charger. 
In my own home:  Many wall warts, Sony Playstation II, Microwave oven, 
Toshiba DVD player.  Each one of these devices produced a typical switching 
noise signature.  Noise "bumps" that show up on the panadpater every 40-80 
kHz or so depending on the device's switching frequency.

In short, anything with a switch-mode supply is a potential unintentional 
radiator.  If the neighbor's device is relatively inexpensive, I'll just 
replace it on my dime.  If it's unusual or expensive, then I'll do my best 
to work with the neighbor to add common mode choking.  Most of my neighbors 
are willing to work with me.  Not all, but most.

Advice: try to work with your neighbors before pulling the FCC or ARRL 
trigger.  They may view it as threatening action even through they are most 
certainly under a duty to mitigate the interference.  If you can't resolve 
the issue to your satisfaction, read the "next steps" material on the ARRL's 

Paul, W9AC

----- Original Message ----- 
From: "Mike Greenway" <K4PI at BELLSOUTH.NET>
Sent: Tuesday, November 01, 2011 8:22 AM
Subject: Topband: RFI

> This year I am encountering a hash noise that repeats about every 66 to 70 
> Khz.  It is maybe 20 or 25 khz wide with a peak at the center.  Sound like 
> the background noise is just increasing as you tune across it.  Has anyone 
> identified something like this?  73 Mike K4PI
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK 

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