New interference - any ideas?

Ideas, technical topics, help and discussion for ATV enthusiasts
Forum rules
This forum is run by the BATC (British Amateur Television Club), it is service made freely available to all interested parties, please do not abuse this privilege.

Thank you
Post Reply
radiogareth
Posts: 585
Joined: Wed Jan 06, 2016 9:46 am

New interference - any ideas?

Post by radiogareth » Mon Oct 19, 2020 7:22 am

Started up my Langstone yesterday (as the build progresses) to be greeted with very strong broadband interference. It starts around 65MHz and raises the noise floor 10-20dB all the way up to 1GHz. Photo shows a 8MHz wide clip. Nothing new in my house/shack. What on earth could cause this?
New problem.PNG
New problem.PNG (343.02 KiB) Viewed 768 times
So strong it wipes out GB3VHF which would usually be S5 with me :-(
Gareth

g0mjw
Posts: 1365
Joined: Sat Sep 20, 2014 9:15 am

Re: New interference - any ideas?

Post by g0mjw » Mon Oct 19, 2020 7:58 am

What is the bandwidth of your bandpass filter on the front end? Could be DVB-T.

g8gtz
Posts: 1332
Joined: Wed Apr 07, 2010 6:26 pm

Re: New interference - any ideas?

Post by g8gtz » Mon Oct 19, 2020 8:21 am

More likely DAB at ~ 2MHz bandwidth - have a look around between 200 and 220 MHz and see if it is very strong there....

https://www.electronics-notes.com/artic ... encies.php

Noel

radiogareth
Posts: 585
Joined: Wed Jan 06, 2016 9:46 am

Re: New interference - any ideas?

Post by radiogareth » Mon Oct 19, 2020 8:40 am

Thanks for the suggestions: This is a NEW interference, clobbering the Pluto (previously fine without any filtering on the beacon), my FT897 too so I don't think its an image issue. Last time I had something similar it had a huge carrier on 123MHz with rubbish up to about 175MHz. Turned out to be my neighbours SWANN video surveillance recorder. Went when he replaced it. I'll have a walk around with my yagi and FT817.
Gareth

g0mjw
Posts: 1365
Joined: Sat Sep 20, 2014 9:15 am

Re: New interference - any ideas?

Post by g0mjw » Mon Oct 19, 2020 11:16 am

A front end filter is absolutely essential for any SDR. I am surprised you did so well without one up to now.

Just because it is new does not mean it is not an existing source you are seeing. It could be a mixing product from a change somewhere making DAB or Data or 4G or 5G signals appear. I originally thought you said it was 8 MHz bandwidth, which would be DVB-T, but if it is 2 MHz chances are it is DAB. That's at 225 MHz so easy to filter out, but it might not be the cause. I had pagers around here just above and just below that used to be really strong making any SDR unusable without filtering. The way to find the strongest signals is to put attenuators on the front end until the interference goes away and then scan the spectrum for strong signals. An FT817 will only find things if you know where to look. It's very unlikely this interference is really on 2m. If there is a masthead pre-amp in line, they can suffer too. That would explain it appearing on other radios.

Mike

radiogareth
Posts: 585
Joined: Wed Jan 06, 2016 9:46 am

Re: New interference - any ideas?

Post by radiogareth » Sun Nov 08, 2020 4:27 pm

Just been talking to my neighbour and he 'recalled' that he's recently fitted a HDMI to CAT6 distribution system to the loft chimney breast, (4 tellies in the house and one down the shed 120 foot away) the other side of the party wall to my loft radio room. That matches with the 'packets of data' I was seeing.
I'd hope that it would not radiate much if its a decent spec device, but a few feet away is a bit of a pain. My triband colinear is on the chimney directly above it too.
Resolution??
Suggestions welcome....
Gareth

radiogareth
Posts: 585
Joined: Wed Jan 06, 2016 9:46 am

Re: New interference - any ideas?

Post by radiogareth » Mon Nov 09, 2020 8:15 am

I'm hoping some clip-on ferrites might choke it off at source, or at least curtail it a bit.
Something like these: https://www.ebay.co.uk/itm/4pcs-5-0mm-C ... 2288602455

Fingers crossed....

Gareth

radiogareth
Posts: 585
Joined: Wed Jan 06, 2016 9:46 am

Re: New interference - any ideas?

Post by radiogareth » Fri Nov 20, 2020 12:36 pm

SOme clip-on Fair-rites arrived today so I had another look/listen.
Picture shows the worst part. There's a strong 'raspy' carrier (possibly carrying some form of mod) on 79.925 and these data blocks, noteworth is the third one across which seems to have 3 sub-bands that drift to the left.
Hoping the ferrites cure it....
Gareth
Drifting....PNG
Drifting....PNG (869.76 KiB) Viewed 262 times

Post Reply

Return to “General ATV Discussion”