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Broad Band Interference

Jan 1st 2012, 03:51

AC0XU

Joined: Oct 30th 2011, 02:35
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I was successful in getting Comcast to clean up a neighbor's RFI problem. It took them about 6 hours in several trips, the neighbor now apparently hates me, and truly the problem appears to have been that the cable wiring in the neighbor's house was so expansive and so tightly intertwined with power lines that the cable system was picking up noise from the power line and rebroadcasting it very efficiently.

Now, the neighbor's house does not appear to be broadcasting more than any of the others. However, I find that the power ground in my own house is radiating the same broadband noise (so the RFI is weaker than before but still present).

1) It does not appear to be generated by equipment in my house, since when I cut the main power breaker, there is no change in the RFI. The RFI is noticeable at the underground powerline feed, but not particularly strong. Again - no change with main breaker on or off.

2) My portable radio shows the strongest signal when in the proximity of house grounds.

3) The house ground is connected via short (3') heavy cable to a ground rod.

4) When I run the portable radio up the ground cable (about 10 feet long) from the ground rod under my shack (which is one the 2nd floor), the signal goes from weak at the ground rod to powerful about 3 feet above the ground rod when the antenna is placed 1" from the ground cable.

5) The RFI is not steady - it is a highly regular bursty signal and runs from about 2 MHz to 20 MHz.

My question is

1) Is the RFI coming from the powerline?

or

2) Is the RFI broadcast and my house wiring acting like an antenna?

When I try to use my portable radio with a loop antenna, the result is confusing because the noise appears to be coming from many sources.

I am puzzled about how I can figure out definitively. When I travel around the neighborhood, I am not getting strong signals, which would seem to indicate #1, but the signal is not particularly strong at the main breaker panel. The strongest signal anywhere is
Jan 1st 2012, 04:30

AC0XU

Joined: Oct 30th 2011, 02:35
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The strongest signals (as measured by my portable) are certain ground points (electrical boxes) around the house. Again, this is true whether or not the power to the house is on.

I would like to get my house wiring to stop radiating. Is it feasible to install an RFI filter at the power input to my house?

Thanks for your suggestions!

Jim
Jan 2nd 2012, 04:46

AA6E

Joined: Apr 4th 1998, 00:00
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Jim, does it help to cut the main breaker on your house power? If not, adding any kind of filter wouldn't help. Are you also using Comcast? You might try disconnecting your cable entrance. Another thing to check is the quality of your house grounding.

73 Martin AA6E
Jan 2nd 2012, 15:29

AC0XU

Joined: Oct 30th 2011, 02:35
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No - cutting my power makes no difference. I am not using Comcast but Centurylink. The Centurylink cable does have some of the noise at the entrance to the house - however, adding ferrite beads did not help. Furthermore, there is none of the bursty noise at the distribution box in the year, so I am guessing that Centurylink is actually picking up noise from my house electrical system!

Yesterday, I added two additional 10' ground rods and 3" copper strapping to the circuit breaker panel. That cuts the noise level around the circuit breaker panel, but makes little or no difference on the noise level in and around the house. At 20m, apparently every piece of wire sufficiently long acts like an antenna and picks up the signal.

I have tried a little dfing with a loop antenna and a portable radio. I am struggling with the 180 degree ambiguity, but it does provide a direction.
Jan 3rd 2012, 07:08

AC0XU

Joined: Oct 30th 2011, 02:35
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I walked around the neighborhood this evening with my portable radio and a loop antenna. Very confusing. The bursty noise appears to be strongest in the vicinity of the neighbor's house - the one that 3 Comcast techs spent 6 hours working on. However, it comes in strong for about a block all around that house.

It also comes in strong in my basement, in the vicinity of the cast iron drain pipe and all HVAC ducting. Repeat - this is true even with the power cut to my whole house. My impression is that everything metal in the house picks up the noise and re-radiates it. Makes it very hard to identify the source. However, it is possible that the the noise source could be related to the underground power lines?

Other strong noise sources are the mercury vapor street lamps. These are up on tall aluminum poles, and the whole pole appears to radiate continuous noise. Different noise entirely (continuous), but pretty strong.

Interestingly, even steel stop sign poles appear to pick up and strongly re-radiate 20m noise.

After this experience, it seems amazing that anyone in the city can communicate over 20m. I have the impression that 20m is just right for RFI- low enough frequency for lots of sources to contribute; high enough frequency that normal metallic structures act as both receiving and transmitting antennas.

Help!!





Jan 3rd 2012, 12:03

W1VT

Joined: Apr 4th 1998, 00:00
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How loud is the noise? How is it compared to your background noise level.

Beads are better suited for VHF RF suppression--to work at HF, you want 10 or 15 turns through the ferromagnetic material--this is more easily achieved with toroids, unless you are talking about suppressing RF on a very thin wire. Also, you want a high permeability mix, like type 43. Type 31 is better for 80M, but 43 works just as well on 20M.

Zack Lau W1VT
ARRL Senior Lab Engineer
Jan 3rd 2012, 14:55

AC0XU

Joined: Oct 30th 2011, 02:35
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The noise bursts are 20-30 dB over the noise floor in my station receiver on 20m. The "noise floor" is pretty high on 20m. That means that the noise bursts are stronger than most signals of interest. It isn't so bad to make 20m useless, but it is extremely annoying, because I have to work between the frequencies of the noise bursts.

For ferrite beans I use the type 31 split beads from DX Engineering and the (type 43?) baluns from Palomar Engineers.

I have what appears to be a sufficient number of beads on all of my antenna feeds.
Jan 3rd 2012, 16:03

W1VT

Joined: Apr 4th 1998, 00:00
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5) The RFI is not steady - it is a highly regular bursty signal and runs from about 2 MHz to 20 MHz.



In what way is it regular and it what way is it bursty? Is it regular in the time domain or is it regular in the frequency domain?

Switching power supplies are a common source of RFI--they often repeat every so many kHz all through the frequency spectrum--you tune up a few more kHz and you hear another burst of noise. They can be very bad generators of RFI if not properly filtered, as a large amount of power can be involved.

10 turns of wire is roughly equivalent to 100 beads, 15 turns is roughly equivalent to over 200 beads. I assume you are using lots and lots of beads.

http://lists.contesting.com/_rfi/2011-06/msg00006.html
Comcast has had problems with wall warts that were noisy switching power supplies.

Zack Lau W1VT
ARRL Senior Lab Engineer
Jan 3rd 2012, 19:43

AC0XU

Joined: Oct 30th 2011, 02:35
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Thanks for the assistance.

The RFI signal that is causing me the most grief consists of short bursts at about 1.4 second intervals. Of the material I have read from the ARRL website, this is most similar to the electric fence described there. I don't know why anyone in my neighborhood, consisting of small city lots, would have an electric fence, but I suppose it is possible.

I think that what this boils down to is tracking down the source of the RFI. I am pretty sure it does not come from my residence, because turning off the power and turning off all the UPS's makes NO difference. Beyond that, it is strongest in a one-block range around my house, or perhaps it may be centered around the neighbor's house. It is difficult for me to tell because I haven't figured out how to construct a portable antenna for 20m that has any useful front-to-back ratio. My loop does a good job - but

1) It has a 180 degree ambiguity.

2) Placing the radio and antenna in a large metal box doesn't seem to increase the front-to-back discrimination at all.

3) The direction of the signal source as measured with the loop does not appear to be entirely consistent with there being a single source. I think that multiple metallic structures, including the wiring and plumbing in my house, signposts, etc., are re-radiating the signal, making tracking down the actual source even more difficult.

At this point, I need some unambiguous DF mechanism so I can track it down. So... how can you do unambiguous portable DF at 14 MHz?




Jan 3rd 2012, 23:35

W1VT

Joined: Apr 4th 1998, 00:00
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One way to do unambiguous portable DF at 14MHz is to combine a loop or loopstick with a vertical sense antenna.

http://www.vk3vt.net/VK3MZ20DF%20receiver.pdf
Here is a design for 80 meters--ARDF is often done Internationally as a sport.

But, it may be more practical to just keep drawing headings on a map of the area and use trianglation, or the intersection of the headings, to find the source.

Zack Lau W1VT
ARRL Senior Lab Engineer

Jan 4th 2012, 16:01

AC0XU

Joined: Oct 30th 2011, 02:35
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Thanks for the suggestions. Specific challenges here are that 1) the signal appears to be reradiated from multiple points; 2) the signal most likely appears to be coming from a house whose residents won't cooperate by allowing me in their house or yard.

A couple of additional observations: 1) The RFI is on 24/7, so it has nothing to do with the sodium vapor street lights. 2) When I am out and about in the neighborhood (as opposed to in my house), the signal appears to be almost perfectly vertically polarized. I think that makes the electric fence idea very unlikely.

It also suggests that if I can deploy a horizontally polarized 20m antenna, vs. the vertical that is currently my only 20m antenna, that I might be in business. I have dreams of a high gain beam, but that will have to wait until I've got some money saved up.

I received an email from the FCC, making it clear that they expect someone complaining about an RFI problem, to correctly identify the source of the RFI before filing a complaint. So... a lot of detective work is required. I read all these stories posted to ARRL about detective work done by hams to track down their RFI sources. I am wondering how the hams managed to do it without getting shot or jailed. Those guys must be far more persuasive than I have been, or they reside in friendlier towns.

My plan at this point is to measure signal strengths and loop directions, plotting the data on a high res map. I hope to be able to confirm that the signal is coming from the suspect house or otherwise to pin it down more precisely.
Jan 5th 2012, 14:13

W1MG

Joined: Apr 4th 1998, 00:00
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Just as an FYI, if the noise is caused by an arcing source, a bad doorbell transformer can cycle on and off. You should, however, be able to hear it in the AM broadcast band and well into the VHF spectrum, especially at close range. Be sure to use the AM mode when listening at VHF. It will also not exhibit any discernable pattern as you tune across the spectrum, such as a regular and repeating pattern of peaks and nulls.
Feb 5th 2012, 06:28

AC0XU

Joined: Oct 30th 2011, 02:35
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Why would a doorbell transformer generate a regular burst at regular 1.3 second intervals 24x7? Thanks for the suggestion. If the neighbors ever let me back in their house I will check their doorbell system.
Feb 14th 2012, 16:39

WA0CBW

Joined: Apr 4th 1998, 00:00
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Just a thought............
Do any of your neighbors have an electronic dog fence? Some of these devices use a buried wire and produce regular pulses.

Have you tried this in your DF'ing........Use an attenuator between your antenna and the receiving device to keep the signal as weak as possible as you search. This might help in determining the direction of the signal.

I would also look at the possibility of wall worts. A neighbor's bad wall wort could radiate its noise back through the electrical wiring to anyone on the same distribution transformer phase. Tripping your own main breaker may not have any effect on the noise as it may enter through the grounding system.

Good luck on your hunt!

Bill
ARRL Technical Coordinator - Kansas Section

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