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Problem with massive RFI after changing router

I have owned a KiwiSDR for over a year and up until this morning, it has worked fantastically.

However, today I changed my Internet Service Provider (ISP) from Comcast/Xfinity to a local service here where I live. That means that the Xfinity router was replaced with a different router that the new ISP supplied.

Right after the setup was complete, I reconnected my KiwiSDR and immediately saw that there was massive RFI on the waterfall. This RFI was across all frequencies and essentially, has rendered the radio unusable.

I have tried everything that I know to troubleshoot the problem, including relocating the antenna farther away from the router. No luck. Ditto for relocating the KiwiSDR proper. No luck, the RFI is still there. I also tried snap-on ferrite beads on the Ethernet cable. Again, no luck.

To me, it seems obvious that the new router is causing the problem and I am considering purchasing a different router to see if that will remedy the problem.

I'd appreciate recommendations for the type of router that I should buy. I want to get one that is well-made, has a record of working the best, and most of all, one that is free from any and all spurious omissions that cause RFI.

Or, if you feel that I may be overlooking something else, please chime in with ideas. Thank you.


  • jksjks
    edited August 2020
    A couple of things. Post a picture of the 0-30 MHz waterfall with the spectrum display enabled and after pushing the "auto-scale" button. Or answer the question: Does the interference appear similar to the VDSL spectrums posted in other forum threads where it is (hopefully) "notched" at the ham-band edges and perhaps elsewhere? E.g.

    Try rebooting/re-powering the router. I have observed on my VDSL router that sometimes the startup line negotiation is different somehow and it results in much higher transmitter power levels being used by the router even though it is completely unnecessary. The result is a huge increase in the level of the common-mode VDSL interference emitted.

    If it's a cable modem, and not VDSL, then the issue might be similar.
  • I rebooted the router. I don't know how to post a picture here. But I have attached is a screenshot of the entire waterfall, with spectrum display enabled using the "Attach a file" option below. (I hope this is what you meant, and thank you for replying back).

  • Forgot to push the autoscale button on the last image. Here is is again.

  • I think it might be very profitable to investigate for the presence of common mode noise current from the new router, over the LAN cable, through the Kiwi and out the "other side" where "other side" is the SMA connector to antenna feedline and/or the power supply. Although the RJ45/transformer within the BeagleBones of the Kiwi does have some CM rejection, it often is not nearly enough to get one from large CM noise currents down to a level of current flowing through the Kiwi which is lower than the noise floor.

    Adding a 1:1 flux-coupled transformer with very low inter-winding capacitance right at the SMA connector will likely increase the Kiwi's rejection of these currents. Increasing the CM impedance on the LAN connection to the router with additional isolation, perhaps of the type sold by DXEngineering, and/or adding appropriate ferrite might help reduce the amount of injected noise current as well. These techniques may individually only buy you 20-30 dB so you may have to combine them.

    Since the significant change seems to have been LAN side, I'd look at increasing HF CM impedance with some or all of these methods.
  • n6gn: I'm not nearly tech savvy enough to understand or implement much of what you have suggested, but I thank you all the same.

    For me, 2+2=4 in this case, as in, before I changed out the router, everything worked fine. I am knowledgeable enough to just get a new router and replace the "new" one that was installed. Even if it doesn't solve the problem, buying a top of the line router can only be a good thing for the rest of my Internet doings.
  • Wow. A noise floor above -70 dBm (roughly) with those peaks at -60 is really, really terrible. Makes me not feel so bad about my noise problems.
  • edited August 2020
    I have seen several situations where the "old" router/switch was replaced with one that happened to support POE (Power Over Ethernet) - and the world of MW/HF/SW reception ended at that point.

    In all cases where I was involved, that new, noisy device had to be (for all practical purposes) scrapped as no amount of CM filtering seemed to be able to return the noise floor to its previous level - although placing a cheap, name-brand, non-POE switch immediately after it and running all cables through it, instead, may be able to break things up sufficiently: This was used as a stop-gap/diagnostic measure rather than a permanent solution. For those instances where a POE device had to be used (a wireless access point, a camera, etc.) a bit of flailing around had to be done to either find a "quieter" POE switch or to simply apply the POE supply at the far end, near as possible to the device being powered to avoid the LAN cable being an antenna.

    It may be that the (rather expensive) DX Engineering Ethernet isolators may do the trick - but they are damn expensive (about $50 each) for a pair of bog-standard Ethernet PHY magnetics simply wired back-to-back: An example of "What the market will bear", I guess...

    * * *

    While standard Ethernet is, by definition, a balanced transmission system with a good CM ratio, in most cases the "P" part of POE is not balanced at all - often connected to the butt-end of a noisy-@$$ switch-mode power supply via filtering that may, if the device is reputable, just barely meet Part 15 conducted noise standards on each individual cable. As we all know, however, Part 15 (even the more strict "Subpart B" for home devices) is nowhere clean enough to preserve the LF/HF or even VHF bands if a fairly low noise floor is to be maintained - and this is even less-so when multiple conductors emerge from the switch, acting as several, independent antennas, running around the house, spreading the joys of QRM far and wide.

    In the cases where the router/switch was involved, I have generally found that the use of a cheap, no-name Chinese device, in terms of HF reception - at least without taking extraordinary measures to quash noise - means that you will be doomed: I have seen "20 over" noise from these devices on multiple bands which, assuming a standard 6 dB/S unit calibration factor, implies that 40-50 dB increase from the original noise floor - and I only wish that this were an exaggeration.

    If you are *really* lucky, it may be the unit's power supply that is the main culprit - but it's often the case that most people aren't able to swap such things out to determine if it's the cause. The good news is that such a power supply will probably fail after 12-24 months, anyway!

    A better-known name (Belkin, SMC, D-Link, etc.) that does *NOT* support POE is more likely not to be a spread-spectrum transmitter in its own right.

    Good luck!
  • edited August 2020
    Just before swapping to something else, try a good few turns through a large snap-on (13mm) on the VDSL (incomming cable), unless it is a coax and you have something huge for that.
    Also the same on the supply cable to the router. I use both here, improvement instantly detectable.
    As others have mentioned it is often the noise source having access to enough wire to radiate, dump the common mode noise before it gets to a good radiator.
    Try another PSU on the router, if it does support POE try powering it that way, it might clamp those lines or at least flag up if that is the true culprit.
  • ka7oei: Thank you for your reply and for suggesting the DX Engineering Ethernet isolators. I may give those a try.

    Powernumpty: I have tried snap-ons already, with no luck. But thank you.

    Everyone: I forgot to mention that the router involved here is a SMART/RG 802.11av Gigabit Router.
  • edited August 2020
    You could try using another power supply at the router (maybe the one from the old router). Just make sure it has the same voltage and polarity and an appropriate amperage rating.

    Using a _non_ shielded ethernet cable may also help. (The BBG ethernet shield is connected to antenna ground. And some routers ethernet shields are connected to their PSUs minus pin)
  • As far as I can tell from a web search that might be a SR400AC (giving us the FCC ID: might help to check that).
    If it is that model, it's input is marked as 12V 3A which should give decent range of PSU's to test.
    If a clip on or two didn't do anything then larger FT-240 with multiple turns should show if it is coming from the leads.

    Personally I like a low power main router, something decent that sips electricity as it is harder to spray energy around if it is hardly using any, then get a separate WAP (or mesh?) and move those around as best suits your location and wireless coverage. One other option is to move that router away from the incoming data and bridge that gap with fibre to ethernet adapters. In the UK VDSL is ubiquitous and noisy, I have the VDSL-Ethernet modem then Eth-fibre-fibre from there, means the VDSL lead is kept short (with ferrite) and the rest of the stuff has no "long-copper" links to risk radiating (or injecting noise).

    (I'm going to have to put a personal web page up somewhere to avoid posting the same "fibre, ferrite.." comments on all threads, at least I didn't mention a brand of routers this time.)
  • First of all, make sure it's not the router's power supply. More often than not, a noisy router is not the router itself, but its PSU. If it is the same voltage, and the plug fits, try to power it up with the power supply from the router you replaced.
  • Bjarne: That will not be possible, as they took away all of the old router's stuff, including the power supply.
  • Too bad. Still worth a try if you have a power supply around with the correct voltage and connector. It's amazing how many RFI issues are caused by something as simple as a switching power supply.
  • I concur with the suggestion to try another power supply. They can vary enormously in noise generation.

    I've been switching to Cat 7 ethernet cables where possible. Each pair is shielded, and the entire cable has an end-to-end shield that connects the body of the connectors. This usually helps, sometimes a lot, but connecting the two device grounds can sometimes complete a ground loop that actually makes problems worse.

    I've also switched to fiber for all inter-switch connections. This gets rid of the longer copper runs and can be a major win. Unfortunately fiber is rarely if ever supported by end devices even though it's getting fairly cheap.
  • Changing the external SMPS is fine. But there is another issue that is more difficult to deal with. The use of internal DC-to-DC converters (that are also SMPS) directly on the PCB of the device.

    These days it seems a lot more devices, including PC motherboards, are powered with higher voltage from the primary supply. Then high-efficiency "point of load" DC-to-DC converters are used right at the consuming load. In the case of your router the external SMPS is now 12V instead of 5V and there is almost certainly a 12V-to-3.3V (or even less) converter internally (chips don't run on 5V these days and in many cases don't run on 3.3V either except maybe for I/O).
  • Yes, though the degree to which these internal DC-DC converters actually cause interference on HF depends on how much of that switching noise is conducted to the outside world. One of the biggest offenders in my shack is an external powered USB 3.0 hub that takes 12V. Lots of cables to the outside world.
  • A few suggestions, you may have already tried some of them, but here goes... (I assume you have one of those integrated routers/modems)

    Disconnect the coax cable and ethernet cables from the cable company router/modem, unplug the power supply from the AC, make sure the RFI completely goes away, this may sound redundant, but double checking this is really the problem.

    Plug the AC power supply into the wall, but leave it disconnected from the router/modem. Does the RFI re-appear? Is it the same as before? This checks just the PSU for RFI issues.

    Plug the PSU into the router/modem, leave other cables disconnected. What's the RFI like?

    Plug in the coax cable for the router/modem. Again... what's the RFI like?

    Start plugging in ethernet cables (not sure how many computers you have connected). Maybe plug in just one end at a time, first the router end, then the computer end. Note RFI levels. Repeat for any additional cables.

    The point of all this step by step stuff is to figure out what is producing the RFI and (maybe) how it is radiating.

    It is quite possible there is not one step that produces the RFI problems you see, but it is the summation of multiple sources/problems, this lets you figure out what is what.

    At some point in these steps, you may even want to try something as silly as wrapping the PSU and/or modem in a giant sheet of aluminum foil (possibly connected to ground say via a piece of wire with alligator clips). Not permanently of course, but as a test. If that solves the issue, the you can think of a creative way to do that permanently while still providing enough ventilation, etc.

    You can also try grounding various parts of the equipment, such as the router. And also try using shielded ethernet cables, but that only works if the stuff you're plugging the cables into actually has metallic ports. Lots of cheap stuff is all plastic, so there's nothing for the shield to connect to, it just floats, and is useless. If it's at least grounded on one end, that may help.

    Another suggestion... you (generally) do not need to use the cable company's garbage (there are lots of reasons why you SHOULD NOT, beyond RFI). I use my own modem and my own routers. You may want to do the same. It's probably cheaper to buy them yourself than trying to fix an RFI issue with lots of expensive ferrite and other stuff. Plus you won't be paying a monthly rental fee for their junky equipment that is full of security risks :)
  • ChrisSmolinski: Many thanks for the detailed reply and suggestions. I will certainly try what you have suggested.

    There have been some amazing replies to my post on this forum. Thank you again to everyone.

    Curt | W9SPY
  • Hello...You are destined for success. I think the divider mole is the guilty party. Would you be able to get a lot of ferrites, and envelop the force link by a couple. What's more, wrap the DSL approaching link.
    While you are grinding away, put ferrites on your PC mouse and console links.
    And all Ethernet links.
    A few switches can't be fixed thusly. Enclose it by foil or put it in the microwave. Does that have any kind of effect? There is your "flaw confinement"
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