VDSL Learning?

Discussion in 'Interference Problems' started by G3RIR, Jan 24, 2019.

  1. G3RIR

    G3RIR New Member

    3 hdmi. 1 for Humax, 1 for PC (normally not connected) 1 for SKY (not often used). There is also a scart connector for the DVD. I think I have tried disconnecting all of these whilst testing for the TV drop out. But I realise that the Sky dish is very near the one leg of the 80m dipole. I will do further experiments disconnecting all the other cables to the TV. Thursday will be busy.

    Neil G3RIR
  2. Ken G3SDW

    Ken G3SDW Moderator

    Try using each HDMI inputs to see if they alter things Neil, they sometimes can had one similar recently that by using a different input changed everything.
  3. G3RIR

    G3RIR New Member

    I am certainly willing to try putting a ferrite clamp on the hdmi from the Humax but the problem is surely RF getting into the router not directly into the TV. The clue must surely be that something "learns" about the emc and after a short while the interference disappears but reappears when I change frequency even by a small amount.

    Neil G3RIR
  4. Ken G3SDW

    Ken G3SDW Moderator

    At this stage we must try anything and everything, got nothing to lose.
  5. Martin G0HDB

    Martin G0HDB New Member

    Hi again Neil (and all), just some further thoughts...

    How long is your inverted-V antenna and how are you feeding it - what type of feeder are you using and do you have any form of balun anywhere in the antenna system? Also, have you tried changing the lengths of the legs of the inverted-V, to see what (if any) effect it might have on your RFI problem? Are the legs of the inverted-V both exactly the same length?

    If there's any imbalance anywhere in the antenna system, which includes the feeder, then you might be suffering from common-mode currents on the feeder that putting a balun up at the antenna's feedpoint and/or at the bottom of the feeder might help to reduce, and changing the length of the antenna slightly might move a high-voltage point a bit further away from your vulnerable equipment, thus alleviating the RFI problem.

    Are you using any form of matching unit between your rig and the antenna feeder? If you are, is it a truly balanced type such as a KW104/107 or an SEM TranZmatch or is it an unbalanced design with a ferrite toroid on its output for the connection to a balanced feeder? There can be saturation issues with such toroids if the antenna presents an impedance the matching unit doesn't like and this can cause some odd effects. For feeding a doublet type of antenna (in any configuration), the ideal solution is open-wire ladderline feeder fed from a properly balanced matching unit and although this might not always be possible it's worth considering if you might be able to make any changes to the overall antenna system to see if it helps reduce or eliminate the RFI problem.

    --
    Martin G0HDB
  6. Ken G3SDW

    Ken G3SDW Moderator

    Martin, this is Neil`s first post which will answer some of your questions, its always worth reading it before you comment, sorry to be so blunt.


    "I have identified an emc problem with my use of BT Broadband and would welcome pointers to understanding what is going on.

    I have an inverted vee on 80m one leg of which goes over the house and is terminated in my front garden. If I transmit 100W CW on 80m the picture on the tv breaks up completely if it is connected to my youview box which is in turn connected to my router. My broadband is BT and speedtests show that I getting 60Mbps download speed.

    If I transmit a test CW message on say 3538 then the tv picture breaks up. If I then pause a short time and resend the test message on the same frequency then the tv picture initially breaks up but then clears. If I pause again and then send the same test message again on the same frequency the tv picture is clear.

    Now if I move frequency to say 3601 and send the same test message then the tv picture breaks up completely. staying on 3601 and sending a second and third test message the tv picture completely clears.

    It is if something is learning about my transmission and somehow stopping the interference.

    I have tried a mountain of ferrite on the incoming broadband cable, on the ethernet out of the router and on the ethernet on the input of the youview box. The ferrite doesn't seem to do anything. My transmitter mains is fed by filters as per GM3SEK. The antenna has a balun at the centre and the coax has lots of ferrite as it leaves the transmitter.

    The broadband into the house comes underground (only about half a metre underground) and is underneath one leg of my 80m inverted vee.

    What is going on? Does ADSL or whatever have some learning capability? Is that in my router or at the exchange or somewhere between?

    I do not know how long I have had this problem. It was only identified a couple of weeks ago when I was operating in AFS on 80m and my XYL (Janette G8TKQ) was wanting to watch Leicester Tigers on BT sports. She reported the break up of the picture. I had a conflict as I would like to have watched the Tigers but AFS came first, just.

    We have had the very high speed BT broadband for about a year but it is very rare that only one of us wants to watch BT sports whilst I am on 80m"

    Neil G3RIR
  7. Martin G0HDB

    Martin G0HDB New Member

    Ken:

    I did read Neil's original post, several times, before posing my set of questions and whilst I acknowledge that I missed Neil's statement about there being a balun (description not given) at the centre of the antenna there's little further information about the nature of his antenna system, hence the majority of my questions are perfectly valid and are aimed at getting a better understanding of possible causes of, and potential cures for, Neil's RFI problem. Sorry to be so blunt.

    --
    Martin G0HDB
  8. Ken G3SDW

    Ken G3SDW Moderator

    Carry on Martin i have thick skin, hope you have too.
  9. G3RIR

    G3RIR New Member

    Rather restricted testing possible today as I cannot raise my tower because of the very strong winds.

    However, The 80m inverted vee provides about 1.5:1 swr at 3.75 MHz even in this lowered condition. This enables me to put 100W out. I made the mistake of removing all the cables from the TV except the one HDMI from the Youview box before testing. But the TV picture is completely clear on ch 408 which BT Sports main channel. I will try adding the cables back one at a time but the real tests will have to wait until I can raise my tower.

    My 80m inverted vee is fed by Aircell 7 double screened coax and has a balun at the feed point. I do not use any tuner in the shack for this antenna. I have trimmed the antenna so that I get very close to 1.0:1 SWR at about 3.7 MHz. In order to transmit at 3.55 MHz I add on extensions to each leg so that I get again close 1.0:1 SWR.

    I try to avoid ATUs (they should be called AMUs) as they are inherently lossy and certainly don't tune the antenna but enable the transmitter to match to the antenna.

    I am appreciative of any of the suggestions to find the cause of the emc problem.

    It will be several days I think before I can report any further tests.

    Neil G3RIR
  10. Ken G3SDW

    Ken G3SDW Moderator

    Well i do not think increasing the height of the tower will make much difference at all if any.

    You say that today when testing with all but the HDMI cable connected from the Humax box that you do not get any picture break up, this is a good sign.

    Now what you need to do is to plug the other two HDMI cables back in one in at a time and fire up your rig to see when it returns, when that happens the you know were it is getting into the TV.

    Look forward to the results Neil.

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