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I am new to this, having just set up my station. I have "very bad" QRN on 40m: S9. Here is the setup:

  • High density suburb with flats and houses
  • Yaesu FT450D with fan dipole at 6m-9m above ground (sloping), this is lower than a building with flats to the North East. Wires for 40/20/10m, nicely resonant, checked with NanoVNA.
  • 1:1 Balun (ie choke) at feedpoint. 15 turns on FT240-52. No evidence of RF in shack.
  • 22m RG58 run back to station which is on first floor
  • Station earth dropping about 3m from 1st floor to 1m driven ground stake (I realise this may not be effective, as run is too long)

These are the things I have tried:

  • 40m QRN is consistently > S8 mostly S9 during the day. Grainy white noise.
  • 20m and 80m are only S3.
  • Noise drops slightly at night but only 1S unit.
  • Turning off all power in house with radio on battery reduces it to ~S7.5 so about 1 S-unit. Still a long way from level on 20m and 80m.
  • I have tested calibration of S-meter on all bands with signal a generator and a 40db Attenuator. It's within spec acc to the service manual.
  • There are some "VDSL" (UK broadband) training tones giving me a "birdie" every 4kHz. But this is only on some days and appears not related to the background S9 "grainy white noise floor".
  • Our own broadband line runs almost parallel to the dipole (I have put as much angle as I can but it's < 20 degrees). But turning our router off (as above) doesn't remove the "main" source.
  • I cannot detect the "VDSL guard bands" at 5.2, 8.5, 12Mhz
  • Noise appears solid across all of 40m, ~6Mhz to about 8Mhz
  • I can make contacts on 40m but only with "booming" stations.

Edit: additional info from discussion in comments below To test for radiated vs conducted noise I also performed these tests:

  • I have a second FT240-52 toroid with 10 turns of RG58 at the radio end about 10 inches before the PL259. It was spare so I thought I would add it. Made no difference.
  • The radio running on battery without antenna is totally quiet on all bands.
  • With linear 12V power supply it is also totally fine without antenna.
  • As soon as I plug in the antenna I get S9 noise on 40m.
  • The same even with a "random 5m wire thrown out the window and shoved into the centre of antenna connector on the back of the radio."

Also: I was taking HTs for a walk with my son today, and we walked past one of those green (UK colours) Telecom boxes about 100m (?) from my dipole. The interference coming off that thing was enough to break through the squelch of our Baofeng HTs tuned to 2m. On FM! Similar cabinets around the neighbourhood showed no sign of interfering with the radios, even with the squelch fully off. Could be a lead?

What is my logical next step?

Run around the neighbourhood with a portable HF radio and a directional loop antenna? Requires some investment? How about a NanoSA, which might be a useful tool anyway?

If I find source, what are options: 1. work with neighbours 2. use a phasing noise canceller?

Any tips much appreciated

M7BTU

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I'd suggest trying a different time of day, a different radio, a different antenna, and a different location; not necessarily all at the same time though. There's probably nothing wrong with your antenna, but you might get different results at different heights, or if you raise the height of the ends to the same height as the middle.

Have you tested the noise level at different times of the day? Maybe there is a lot less noise at 4 AM. That would be a clue.

You could ask other local hams who are on 40m. What are their noise levels? Are they using vertically-polarized or horizontally-polarized antennas? Your high noise level might be particular to your neighborhood, and asking several different hams might help answer that question. If a local ham reports that his or her noise level is lower, you might ask if you could test your radio in his or her shack. The problem isn't likely to be the radio, but it's possible, and testing your radio against another radio with the same antenna could help determine whether the radio is the problem or not.

You could try a portable HF radio and a loop antenna, but those can be expensive; you could also try your existing radio powered by a battery connected to a 40m dipole flung up into the trees in a nearby park or wilderness area, if you have such places in your area that allow such things. If your problem is local noise, which it likely is, then your radio and a dipole in a rural or wilderness area would likely be quite an improvement (assuming that you live in an urban area).

Once you have a better idea of the source of the problem, then you can think about potential ways to narrow the problem down further, or ameliorate or solve the it. If your problem is local noise, then there are lots of questions and answers on this site about tracking down and dealing with local RFI. Feel free to ask a new question once you know more.

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  • $\begingroup$ thanks for response. I have tried at night, and it is slightly better but only about 1S unit. I guess I need to remove the radio as a possibility and prove it is the location. So radio with a battery and a random wire to the local wilderness (we have that). I am 90% convinced that it will be MUCH improved, but it needs proving as you say. $\endgroup$ – Oliver Schönrock Apr 6 at 20:06
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If the RF noise is that strong, then you should be able to pick it up with a much smaller movable antenna. Even scrap wire or conductive tape on a large cardboard box. That may allow you to build small directional antennas (tiny loop and stub dipole) to determine if the noise is stronger from some specific directions, or a polarized antenna to determine the noise polarization, if any.

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You might consider an RF choke at the interface between your radio and feed line.

The choke at the feed point will help prevent common mode current reflecting from the antenna... but the outside of the shield can still be a source of noise on receive if it's not also choked at the receiver.

In fact, you might test what happens when you put a dummy load at the feed point, and then with/without a choke at the transceiver/feedline.

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    $\begingroup$ I actually have one of those, because I had a spare ferrite toroid (the second one wouldn't fit in my balun box). I will update the question. I like the dummy load idea, good way of narrowing down shield noise. I haven't tried that, and will now. $\endgroup$ – Oliver Schönrock Apr 8 at 8:27
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    $\begingroup$ @OliverSchönrock let us know what, if anything, the dummy load test reveals... I'm very curious! $\endgroup$ – webmarc Apr 15 at 20:08
  • $\begingroup$ Just ran this experiment. With antenna unplugged at feed point, replaced with a dummy load, and hoisted back up ( to keep the feedline in the same place), the radio was totally silent. S0. That is both with and without the toroid at the radio. Not so interesting? I guess it rules out screen noise being the cause? $\endgroup$ – Oliver Schönrock yesterday
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Start by trying to determine if the noise is "conducted" or "radiated" into your receiver. Basically, "conducted" noise gets into your receiver from the power supply or a ground wire or some other cable. "Radiated" noise gets in via the antenna.

Start by powering your radio with a battery instead of the mains-powered supply. Or borrow a battery-powered shortwave radio, preferably one that accepts an external antenna. (If the alternate radio is quiet, you've just learned something very useful!)

Remove the antenna cable and all other connections to the radio. All bands should be very quiet. (If they aren't, you have a more serious problem than I would know how to deal with!) Begin adding cables to the radio, starting with the mains-powered supply and ending with a short length of coax that is open at the far end. Finally, connect the coax that goes to the antenna.

You may learn from this process that you have one or more major and/or minor culprits. What you learn will help determine the remedial steps to take. Please amend your question, or start a new question, when you have gone through this procedure.

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  • $\begingroup$ Sorry, maybe I was not clear in my question. I have done these kinds of tests. The radio on battery without antenna is totally quiet on all bands. With power supply it is also totally fine. As soon as I plug in the antenna I get S9 noise on 40m. The same with even a random "5m wire thrown out the window" and shoved into the centre of antenna connector on the back of the radio. $\endgroup$ – Oliver Schönrock Apr 6 at 20:29
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    $\begingroup$ @OliverSchönrock This is helpful information, so please edit your question to include this. Comments are not searchable, and most of all others often don't read everything on the page. :-) $\endgroup$ – Mike Waters Apr 6 at 21:25
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    $\begingroup$ @MikeWaters Thanks, done! $\endgroup$ – Oliver Schönrock Apr 6 at 22:03
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You've received several good answers here. To Brian's answer suggesting that you should try powering it from a battery, I would add start with your own house before running around the neighborhood with a radio by turning off your main circuit breaker. This will determine whether or not the RFI is coming from something in your house, which is quite likely.

If your noise significantly decreases, then turn on one branch circuit at a time until it appears again. You may have more than one RFI source, so turn on just one breaker at a time.

A nearby amateur mostly solved his noise by unplugging a laptop charger. Any unfiltered switching power supply will do this, even if the device is plugged in and not turned on.

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    $\begingroup$ Thank you. Please see question, I already ran on battery having turned off our house completely. Only a small change, worth addressing once the major source has been reduced/eliminated. I will drive the radio to the countryside today and update question with the results. (I expect it will be a very quiet radio, but lets see) $\endgroup$ – Oliver Schönrock Apr 8 at 3:23

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