[Elecraft] Origin of Signal-Fine Structures in SSB Pass Band Waterfall, mainly 80/40m

Sat Jan 6 19:05:51 EST 2024

Hi Chris,

I have some articles on this subject at:

https://www.nk7z.net/category/info/rfi-mitigation/

I would recommend the one on using an SDR, as an RFI site survey tool as 
a starting point for our friend.

73, and thanks,
Dave (NK7Z)
https://www.nk7z.net
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources

On 1/6/24 09:54, Chris wrote:
> Hello Andy:
> 
> I will offer some experience and opinion....
> 
> Your note implies that the interference is coming from the ionosphere, 
> and you may be correct, but it is also worth considering that you may be 
> looking at digital-based interference from local devices (in addition to 
> possible distant signals coming from afar).
> 
> Digital signals (square waves, waveforms with sharp edges) have very 
> high spectral content (_many_ harmonics), so you may see many lines at 
> constant frequency spacing all over the HF spectrum and beyond. 
> Switching power supplies (inside electronic devices, wall-warts, etc), 
> and digital signals from monitors, computers, routers, coffee makers, 
> washing machines, (seems everything that plugs in these days has 
> microprocessors), LED lighting, etc. can all cause interference like this.
> 
> These sources may also shift or drift (slowly shift) due to changes in 
> temperature or operating (power supply/load) conditions in the digital 
> devices.
> 
> In my experience the root causes of this kind of RFI are usually the 
> digital devices, and possibly poor design for attenuation of the 
> harmonics resulting from the oscillators generating the broadband 
> interference lines. Not all oscillators are crystal based BTW, so they 
> may have significant change in frequency over short or longer time 
> spans. L-C and R-C circuits are just drifty.
> 
> BTW, I would also try to see if the lines are from within the radio. Put 
> the radio on a dummy load and look at the spectrum. If it is clean, then 
> you can probably eliminate the problem as being internal (either 
> hardware or signal processing software). Knowing Elecraft, I suspect 
> that the problem will NOT be inside the radio, but it is a place to start.
> 
> The solution to external RFI _should _be in the design phase of these 
> devices. Design should include "right at the source"  shielding of high 
> frequency oscillators, power supplies and digital circuits which should 
> use circuit layout which shields clock and data circuits with ground 
> lines right in the PC board. If you let the signals escape the board, 
> you now have to add ferrites and shielding on the wires leading to 
> peripheral devices. It is far better to prevent the emissions at the 
> source. But it seems to be cheaper to some manufacturers to ignore the 
> problems and trust that your RFI generating device will escape scrutiny 
> of regulators.
> 
> Devices using high frequency signals are required by the FCC to be 
> inspected and to have limits on their emissions. In practice it seems 
> many devices, perhaps especially from overseas, have somehow eluded 
> these certifications and can become real "firestorms" of RFI. Here in 
> Colorado we have experience with grow-light power supplies that have 
> strong emissions for miles around some "greenhouses" nearly wiping out 
> the HF spectrum. In my own home, when I had ADSL/VDSL internet-phone 
> service, I was able to show service technician the spectrum emitted by 
> his cable coming into the house. He confirmed that those are the 
> frequencies his equipment used, and when I pointed out that those 
> signals become interference if they do not _stay_ inside his cable, he 
> replaced the feed cable from the pedestal to my house.
> 
> This should be of special interest to hams, who are still attempting to 
> communicate using the HF spectrum, though the emissions can still be 
> strong way up into VHF and even UHF bands. A local parking lot with 
> solar-powered lighting generates signals every 15 KHz right up to high 
> VHF. I suspect that the solar panels use MPPT switching chargers to 
> power their batteries.
> 
> Perhaps clubs should consider forming RFI committees to investigate this 
> interference and try to identify sources, maybe even by 
> manufacturer/model number. It is difficult to know when a device exceeds 
> the FCC limits for emission without a measurement lab. Maybe the effort 
> would include getting the ARRL to either build such a lab or fund tests 
> on apparently noisy consumer devices (?).
> 
> This has been a slowly growing problem for decades now, and in suburban 
> areas HF spectrum can become almost unusable, with so many houses with 
> computers, TVs, video cameras, and appliances emitting at low levels and 
> adding up to make noise well over S9 at locations of which I am aware. 
> No wonder so many hams are using remote stations - perhaps their home 
> stations are almost unusable due to RFI.
> 
> To troubleshoot, I would recommend powering your radio on a battery and 
> then turning off all house power and looking again at the spectrum. If 
> it has significantly reduced interference, that source is in the house, 
> and if _not_ reduced, it is external (neighborhood or atmospheric). 
> Troubleshoot the house by turning on rooms one at a time to see where it 
> is coming from. Then plug/unplug devices one at a time to see if you can 
> find a single device as a major contributor. If you do this, be aware 
> that some appliances, notably refrigerators and  A/C units may need to 
> be off for a minute or two to allow the pressurized system to relax 
> before power is again applied. Also note that some digital devices like 
> internet routers may take 2 minutes or so to become fully operational 
> after power is again applied.
> 
> RFI is a difficult problem to diagnose, and it seems to me that it is 
> receiving very little attention from regulators. Cheap appliances are 
> popular and convenient but some may have bad effects on our spectrum. 
> Cutting corners in design affects spectrum users, and having dozens of 
> devices in every home adds up...
> 
> 73,
> 
> Chris K0PF
> 
> 
> 
> On 1/6/2024 6:33 AM, Andy via Elecraft wrote:
>> I have a general question to better understand analysis of SSB ( RX Pass
>> Band) signals displayed as waterfall, mainly on RX of night 
>> time/evening low
>> bands (80m/40m) propagation.
>>
>>
>> (Sorry up to now no waterfall screenshots available yet. Anomality is 
>> only
>> sometimes occurring, but not very short lived.
>>
>>
>> Waterfall:
>>
>> The horizontal axis is frequency , the vertical axis is time and the
>> brightness corresponds to signal amplitude received via any 
>> propagation mode
>> (Skywave/NVIS/Groundwave, or mixtures)
>>
>> Modulation/ splatter become visible.
>>
>>
>> The propagation effect I try to understand does not always occur.
>>
>>
>> It is a "specially (modulated) signal pattern" , a vertical waterfall 
>> signal
>> fine structure with periodic signal amplitude (brightness) minima and 
>> maxima
>> lines.
>>
>>
>> These lines are sometimes in SSB just straight down vertically with 
>> ca. 300
>> to ca. 400 Hz intervals (6 periods) .
>>
>> Digital signals seem to have more lines in RX Pass Band on the waterfall.
>>
>>
>> Sometimes in the RX  Pass Band these minima/maxima (QSB) lines are 
>> shifted
>> right ( QRG >)
>>
>> Or at later observation times also may be left shifted (QRG<).
>>
>>
>> I can rule out EMI of broad/narrow band nature. It must be due to
>> propagation of the very observed signal
>>
>> Sometimes signature appears as " distributed cloud islands" in the 
>> vertical
>> SSB PB waterfall band
>>
>>
>>
>> I observe for TX SSB Mode (e.g., 2.7kHz BW) in the RX waterfall display :
>>
>>
>> 1.    About 6 Periodic, straight vertical lines (periodic signal 
>> amplitude
>> minima, which are darker), --normal propagation
>>
>>
>> 2.    About 6 Periodic, right shifted ( staring low f to higher f)
>> vertical lines (signal amplitude minima) , --special propagation mode?
>>
>>
>> 3.    About 6 Periodic right shifted ( staring high f to lower f) 
>> vertical
>> lines (signal amplitude minima) , -- special propagation mode?
>>
>>
>> 4.    The angle to the vertical axis seems to change often over time.
>> Probably  according to HF Condx/Propagation?
>>
>>
>> 5.    It seems to be independent of TX LSB or USB mode.
>> 6.    Digital signals have often more vertical lines
>> 7.    The angle to the vertical axis seems to change often over time,
>> according to HF Condx/Propagation.
>>
>>
>> 8.    I have observed this effect also on day on 80m night time on a KIWI
>> SDR only 12km away in hilly HB9 ( combination of NVIS/ groundwave?)
>>
>>
>> Questions:
>>
>> 1.    Is this an indication of Multi-Path Reception? May be with 
>> different
>> propagation modes at that time over certain distances possible?
>> 2.    Is this therefore, e.g., based on delay times (phase/arrival time
>> differences) of reflections from different ionospheric layers ( e.g. 
>> f1, f2
>> etc.) ?
>> 3.    Is this based on quasi simultaneous occurrences of skywave, NVIS 
>> and
>> may be even groundwave mode ?
>>
>>
>> Any comments or reporting of similar observations are greatly 
>> appreciated.
>>
>>
>> PS:
>>
>> I observed this " shift effect" also sometimes for OTH-Radar (real or 
>> faked,
>> yes some are faked, see my qrz.com under DQRM) signals recently on 
>> 40m, in
>> EU late afternoon , night time here.
>>
>>
>>
>> Tnx, Cu, vy 73 de Andy
>>
>> HB9CVQ, DK2VQ, AK4IG
>>
>>
>>   <https://www.qrz.com/db/HB9CVQ>  https://www.qrz.com/db/HB9CVQ
>>
>>
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