[RFI] Finding noise source

dgsvetan at rockwellcollins.com dgsvetan at rockwellcollins.com
Fri Aug 15 18:21:18 EDT 2008


Your noise problem sounds a lot like one that plagued me for most of the 
past several months.  In my case, here are the details:

1.  The noise affected 160 thru 40 meters the most, but was audible on all 
HF bands.  I do not have any HF beam, but I do have 2 HF wire antennas. 
The noise was worse on the antenna that runs E-W, meaning that the source 
was most likely north or south of me.

2.  I do have a 6 element yagi on 6m, and could peak the noise as being to 
the SE of me using my 6m rig in AM receive mode.  The nearest power lines 
in that direction are about 3/4 mile away.  I did not hear the noise on my 
2m SSB receiver, even with 8 elements aimed to the SE.  My 6m and 2m 
antennas are moe than 80 feet up, so they have a good "view" to the 

I am telling you this to illustrate the point that K9YC made about 
distance between the noise source and your QTH.  At HF, noisy power lines 
can cause me interference at distances greater than a mile, no problem. As 
frequency goes up, spectral density of AC power line noise drops.  Thus, 
the reason that you must be closer to the source when using VHF detectors 
than for HF.  As K9YC suggsted, take your 746 on the road, and set it for 
AM reception on 2m for best results.  For me, 3/4 mile is nearly the limit 
I could detect the noise on 6m, and that was too far for 2m detection.

The noise source near me seems to have quit for the time being.  If it 
stays gone, I'll be happy.  Otherwise, if it returns, I'll be out hunting 
the source just as you need to do. 

Here's a tip that confirmed for me that my noise source was 60 Hz related: 
 If you happen to have a monitor scope in use on your system to observe 
your transmitted waveform, especially when using a linear, consider 
connecting that scope to the IF strip in your receiver (or receiver 
portion of a transceiver) so that you can observe the INCOMING waveform to 
you.  Not only does that make it easy to tell why someone has crummy (or 
great) audio, it also can help ID the type of interference you are 
receiving.  My main HF station is a classic Collins S-Line, with the 
receiver being a 75S-3.  Years ago, I routed the 455 kHz IF output to a 
spare rear panel jack and feed that into the amplified and tuned vertical 
input of a Heath SB-610 monitor scope.  It took me all of about 5 seconds 
to confirm that the raspy buzz I was hearing was, indeed, 60 Hz driven. 
True, it still did not tell me where the noise source was, but it 
certainly helped to confirm the type of source I was after.  Note that a 
conventional oscilloscope connected to the receiving IF strip in your rig 
would also produce the same results, but beware of stray noise pickup in 
the scope probe due to its broadbanded nature.

Modern rigs with built-in band scopes might be able to provide some 
information.  Closely spaced "spikes" across the spectrum to which you are 
listening are indicative of power line noise if you can resolve the time 
base of the band scope.  (Spikes further apart could be indicative of 
switching power supply noise, for example.) 

Good luck.  Let us know what you find.

73, Dale


"Yuri" <ve3dz at rac.ca> 
Sent by: rfi-bounces at contesting.com
08/15/2008 07:48 AM
Please respond to
Yuri <ve3dz at rac.ca>

<rfi at contesting.com>

[RFI] Finding noise source

For a couple of years now, I am suffering from terrible noise at my place 
which covers pretty much all HF bands when it's on.
You may listen to it here: http://www.rigexpert.net/noise.wav
The noise seems like a power line one, and it stays on for days, but 
sometimes (very seldom though) when I listen to the bands in the morning 
before going to work, the noise is not there, which makes me think that 
might not be the power line noise.
I can easily determine the direction where it's peaking from - my 
neighbour's house, but this is also a direction to the street power lines.
Since I only have one tower with rotatable beam (TH6DXX), I decided to 
4 element 2 m yagi and try to determine the bearing from outside of my 
yard, using IC746 radio on 2 meters. But, either the sensitivity of the 
receiver on 2 meters is not enough, or the noise itself is not strong on 2 

meters, I can not tell by looking on S-meter, where the noise is coming 
from - S-meter stays on "0" all the time though the noise is still 
The question is, what would you recommend me to use as a "noise finding" 
device? I don't really have enough time to build something complicated. I 
have to find out either it's power line noise, or the one coming from my 
Thanks in advance for the tips.


Yuri  VE3DZ 

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