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Interference sources - an update (and a puzzle)

Posted: 05 Apr 2018 09:51
by daveB
Curiouser and curiouser (as Alice said).

Previous post - streetlight interference?
One set of interference disappeared only to be replaced by another a few weeks ago - and this was horrendous on the aerial at the bottom of the garden. I mentioned it to my immediate neighbor and asked if he might have installed new solar lights - he told me hadn't a new ones for years and most of his solar lights had died. He said he'd switch off the others so I could check - result - a band clear of interference (almost - see later). So it isn't new lights - it's old failing ones.

Previous post - interference from a nearby parked car.
I am 100% certain the source is "the" car and this was reinforced when it ended up immediately opposite and the interference was much stronger. Then 3-4 days ago when I was actually watching the SDR the interference suddenly appeared and fluctuated a bit before settling down - indicating the car had driven past my aerial. So I rushed out to see if the owner was still with the vehicle. Had a brief chat and they assured me they knew of nothing that could cause a problem - no 'black box' and no device to play mp3 through the radio. The driver promised to have a word with their dad who was a mechanic. Guess what? I went back to the radio - and the interference was completely gone!!! and it hasn't returned so far even though the car is in it's usual place. Work that one out - I can't.

And now for the newest source of interference:
Right across the band at 400.000 kHz intervals interference 35 dB above the noise. This also appeared to start at dusk. However now the clocks have gone forward, and I checked more closely it comes on at a specific time and, after an overnight recording I've found it goes off 6 hours later. I will recheck - but it is exactly 6 hours. So 1800 - 0000 GMT. These spikes have a band width of around 25 kHz and radiate across a very wide frequency range - well beyond the FM band. Additionally when the interference starts, unlike the solar light stuff, there is no frequency drift - it is rock solid like a crystal controlled oscillator. So that it 52 frequencies, stating at 87.6 MHz that are knocked out, but 14 of them already have local or semi-local signals on them. Does anybody have any ideas what to look for - something operated by a timer?

And just for one more - Airband interference
All this playing round looking for constant interference sources has indicated another potential which might cause short term overload. I live under one of the main air routes to Europe and some of the short transmissions to the Air Traffic control are really strong around 121 MHz- although only brief. They might explain odd fluctuations in the noise floor that I see.

Re: Interference sources - an update (and a puzzle)

Posted: 05 Apr 2018 14:33
by John Faulkner, Skegness
Interesting and useful info Dave.

Modern cars can be shocking for causing interference.

Just to throw in something else completely random, which may be of use: My 2m band was often intermittently wiped out at my last Nottinghamshire QTH, depending on which way the antenna was beaming. I discovered that the interference across 2m AND FM, was due to mixing products from two paging systems about half a mile along the ridge where I used to live. I understood that they could be running 2kW each. Their operating frequencies were in the region of 153 MHz. and they were S T R O N G!

I hope somebody can suggest what your newest interference source might be.

Re: Interference sources - an update (and a puzzle)

Posted: 05 Apr 2018 18:08
by Andrew Webster,Wigan
Looks like you have been through the mill David with QRM problems and hope you can sort it.

Here's a banghead moment ever since I used RTL dongle for band 1 6 years ago I have suffered plenty lines on waterfall then the other week I used a different USB lead and then lines gone and could not believe it so went looking for a brick wall :lol:

I have faint bars 100 Khz apart so thought ah SMPSU so switched off and unplugged PSU on my CCTV PC bars gone these bars were only recent and I had to replace PSU in the PC a few months ago as the old one developed badcaps so will try others from my PSU pile.

Yesterday I bought a secondhand 24" LG monitor that uses a power brick but not the original one but a cheap chinese replacement one and that wipes out band 1 and puts noise on band 2 but as the supply voltage is 19 v I cobble up a quiet laptop supply.
I got this monitor cheap as the the front buttons didn't work but did when I got it home so happy about that.

Last year every Saturday for months I would have a blank carrier on 87.6 all day I traced this to the local pub nearby so probably an Itrip and not had it for a few months now.

I do consider myself being fortunate here as I have not had the problems you and others have had.


Re: Interference sources - an update (and a puzzle)

Posted: 06 Apr 2018 10:24
by daveB
Thanks for comments Andy. Maybe the 400 khz spikes are from a smpsu. My current guess is a motion sensor security light on a timer so it doesn't get triggered in the early hours of the morning by cats etc. I've confirmed it's turn on time is constant - so tonight I shall be hovering outside a certain neighbour's back gate a couple of minutes beforehand with my Tecsun PL380 and see if there is a sudden increase in noise on 87.6 at the appropriate time.