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

Sat Jan 6 12:54:24 EST 2024

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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