RFI Investigation At My QTH
09.01.18


It was not until my good radio friend Geoff, G0LUJ, began experimenting with remote reception that I became fully aware of just how bad HF RX degradation had become here. I live in the suburbs so I do not expect to be able to hear as well as Geoff's Kiwi station (See here, but be prepared to be depressed when you return to your own receiver unless you too live in the wilds). However I had always been able to receive stations on 80m, though it seemed to me they had once been much clearer. After much procrastination I decided to find out what, if anything, I can do to improve things here. The results are mixed news.

Method (V Madness)

I decided to begin with what I could control. That would probably be also where it would have to end. We have to face it, radio amateurs are becoming an anachronism - 'lame', my kids call the hobby, HI. Harmless eccentrics at best, or easy scapegoats for technical ignorance 'that bloke with the CB aerials who makes my phone not work!' at worst. I also wanted to keep the costs down. Fortunately I already have an Icom 7300 with its wide view of the spectrum. I had an old camcorder. I made short videos at 1mhz intervals from 0-30mhz of the noise. After fitting a 5ah gel battery to the IC7300 and then, knowing it would screw up half the appliances in the house, I turned off the mains. I have tried isolating circuits and seeing if it made any difference to no avail. I suspected there would always be something I had missed - and it did turn out that way - so a drastic Zero Power test seemed in order.

I then repeated the videos at 1mhz intervals. The antenna is just a 25m inverted L against a ground system, hardly the 'quietest' antenna but one suitable for relative comparisons.

Measurements

I have not as yet analysed each video, on each mhz, against each other. I do not really need to. What I noticed was pretty clear and it is a mixed message shouting back at me. On the lower HF bands, below 10mhz, much of the RFI is generated when our house is powered up - S9+ noise dropped to about S5 when I tripped the mains. Doubtless some of it is our VDSL modem, but it was NOT the main culprit. One source, at lower HF, was immediately isolated and resolved. The RFI from 8-30mhz is widespread, pulsing and very hard to ignore - even my Red Pitaya doesn't pull much out on 10mhz any more. That noise was present during power down. It would appear I shall have to put up with it.

Subsequent Action

I was almost immediately able to identify the (lower HF RFI) source as a single switch mode power supply. I had changed this before, and I was certain it was quiet. This test revealed it was not. A single ferrite core sorted it out. I now have reasonable reception below 10mhz once more - average S5+ noise as opposed to S9+ before suppression: chalk v cheese in terms of listening). Nothing like Geoff's remote station, but useable. For this alone, all that resetting of appliances I am about to begin is worthwhile.

Without v With Choke


Simple choke


These images are from the noisier part of the day, nevertheless reception is much improved.

It is not such good news above 10mhz. Whatever it is that wipes out vast swathes of the upper HF bands does not originate from G0MJI.

Conclusion

Before blaming others look to the mote in thine own eye! I am really pleased to have the lower HF bands once more. Then I guess it is a case of being realistic. If you cannot afford to relocate to a deserted island perhaps make the most of what you have. The IC7300, any SDR in fact, is very good for identifying 'windows' in the noise, as well as for recording and monitoring the RFI environment. For instance perhaps is is no coincidence that both (harmonically related) 10mhz and 20mhz are wiped right out here. It could be one piece of dodgy equipment located nearby, perhaps one day it will be removed and the problem along with it. It is just as likely to be replaced/added to by another source. I suppose frequent scans of the bands could be useful in this regard. As I listen to 80m even as I type I can hear a qso about RFI. I am not alone. I concur with the sentiment expressed on there (synchronicitously): where it is noisy use the internet stations for RX. And those people who spend many hours and pounds setting up/running remote stations, for the benefit of those of us with noisy QTHs, should be thanked. So, thank you G0LUJ.

Note

It was apparent that the noise above 10mhz is quite wideband and varied from S9+20db hash to almost ignition pulse type. At least so it appeared between amateur segments ** (See Extra Note below). The IC7300 is of course an amateur band transceiver with wideband receive. When tuned to the amateur bands the extra selectivity of the filtering deployed here was clear, and not just in relay clicks - the noise dropped markedly. So that although it might be exceedingly noisy either side of a particular amateur band, it was sometimes quieter within, say, the 15m band. (Hmm, see below)

Look at the 15m Amateur Filter Window
This led me to consider that at least some of the 'interference' is strong, localised and breaking through (from other frequencies) the lesser defences of the IC7300 in what might be called comms receiver mode rather than actually on that 'apparent' frequency. The one amateur band which retained its interference was 10mhz. Which leads me to think that on 10mhz (at the very least) the signal is genuine RFI. Of course the filtering allows for amateur band windows, if not on 10mhz presently. This may explain the absolute dearth of Wspr spots on 30m at G0MJI over the past couple of years i.e. at least some of this RFI is spot on the 10mhz band, no filtering can defend against a signal that is actually there. As to what the source of the RFI may be, who knows? VDSL at the neighbours? Something to ponder...

Extra (Extra, Read All About It) Note

Geoff has been in touch and correctly identified the interference in the final picture as caused by powerline ethernet adapters. He tells me these have 'windows' for the amateur bands designed into them (thus dismissing my filter theory) and the type of noise is consistent with their variations in transfer of data. I suspect that when I do get round to analysing the videos this component of RFI will be missing in the powered down videos - More progress! He further suggests, as I also suspect, that the 8-13mhz noise is VDSL. As this is present even when my own router is completely powered down (the whole house in fact), then I suspect there is little to be done, although I did contribute to the RSGB VDSL Survey last year. I shall ponder and look more closely at the videos soon...

Meantime I am off to have a listen around on the 'restored' 80/60/40m bands.

Happy listening!

PS The Plot Thickens

Well I had considered rewriting this whole thing. New findings change the picture each time. However these are my notes and so I will leave the saga as it is. Perhaps the twists are all part of such an investigation. It turns out the Mains Ethernet Adapter (PLT) interference, which is quite distinctive on an SDR screen does NOT originate at my QTH. Here is an image of the 21mhz band with no mains electricity at my QTH - hence the dark and blurry screenshot. (Note time is 9.02 when the mains was cut off here). This 'Smart Notching' as described by Ofcom in the document below is clearly visible in this image as well as the one above.

Mains Off, PLT Noise
Here is an Ofcom document. Although published in 2009 it remains both prophetic and relevant. It states somewhere inside of it that these PLT devices can radiate up to 300m. So it looks like the PLT noise experienced here originates from a neighbouring property. It seems unlikely I can do anything about this, even if I knew which property.

Typical PLT Device

Final Conclusion (For Now...)

24 hours ago I powered down the house. There are many devices which do not like this, gone are the days of 70s power cuts - long gone now I think about it. It is a bit of a nuisance to restore everything, but well worth it if you are willing to take the risk - your responsibility. It seems to me this is the only way to establish a definitive baseline of the RFI from the surrounding environment. I discovered quite quickly that the main source of irritation was a single 1amp switch mode power supply I had thought clean. I used a choke to sort that out, and today I replaced it with a linear psu - I am listening to 80m as I type. So that was a good result from the point of view of lower HF reception. It is good to have those bands back.

There are two further sources of local RFI (at least): VDSL from 8-13+mhz and pulsed wideband noise from PLT devices up to 30mhz (either via 'infected' powerline, or house wire radiation). These overlap and appear right the way from 8-30mhz at least. Thanks to Geoff's experience I have been able to untangle these sources one from another. Unfortunately it seems these two sources cannot be tackled. Furthermore as new people move into the area then it is likely this situation may deteriorate further. At least I have a baseline for comparison, as well as an improved knowledge of what to look for.

As always this 'final' conclusion is subject to revision - repeat these tests at your own risk.

Further Thoughts:

Since identifying some of the noise as PLT based further research revealed that the ignition type component of PLT is symptomatic of BT Vision television distribution. There is no such system in my QTH. I strongly suspect one of my neighbours has installed this. See this document for more information. It also suggests an explanation for the RFI noticed on MW broadcast bands at this location, which precipitated a move to DAB some years ago. I do not want to go door-to-door until the 'culprit' is identified. I doubt the innocent party would understand.

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