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Stopping volts going down UHF cable ??

Discussion in 'Electronic Basics' started by UHFPD, Apr 16, 2005.

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  1. UHFPD

    UHFPD Guest

    Help !!

    I have a star type UHF Aerial DA feeding TV to rooms in my place.

    I also feed reverse direction from CCTV switcher via another discrete
    UHF cable, into the DA via splitter.

    There is no feed from DA into CCTV switcher, so no loop there.

    (apart from house mains that is)

    When I connect up this cable, there is a noticeable patterning, diagonal
    lines, fed into the DA, apparent throughout system

    upon checking, I find that there is a potential difference of around 48v
    AC, 8v DC, virtually no current, but enough to hurt a little, between
    UHF earth and equipment casing earth (UHF socket input).

    This is presumably due to slight domestic 240v mains differences between
    downstairs and upstairs, fed off same junction box, house is quite
    large, giving the potential difference.

    Is there a simple way to stop both DC and AC volts going down the UHF
    cable, without degradation of UHF signal.

    Don't want to get into mains isolation, but is there a similar device
    for use at UHF?

    I have tried another DA in circuit, this doesn't work.

    Presumable something like a suitable capacitor in series would work.

    Any suggestions ??

    Temporary solution is to reverse feed CCTV via 2ghz, but this is not
    perfect.

    TIA
     
  2. Chris Dugan

    Chris Dugan Guest

    A high pass filter tuned to suppress anything below 100Hz?

    The other thing you might want to check is that there are no mains cable
    running parallel to the cable anywhere along its length and/or maybe run a
    separate earth lead from the DA upstairs to the same grounding point as
    downstairs to eliminate any ground loop.

    Chris
     
  3. Andrew Holme

    Andrew Holme Guest

    Earthing methods are different at RF and AF. Whereas we avoid ground loops
    at audio frequencies, the correct method at RF is to earth the coaxial
    shield at both ends: ground loops are inevitable; however, UHF equipment is
    unlikely to be troubled by 50Hz pick-up.

    Furthermore, though you can measure voltages between the earths of different
    mains outlets, they should equalise when you plug-in the coax. You should
    find a very much smaller potential is developed if you connect (say) a 1k
    load resistor between them.

    Are you sure the coax outer-braid connections are sound? Could it be RF
    crosstalk that you're seeing? Can you try it with the cables connected, but
    with no signal on one of them?
     
  4. UHFPD

    UHFPD Guest

    Thanks for all your comments so far:


    Further information: forgot to mention that the cctv o/p(s) are (2) 1v
    p-p video, which is then fed via scart input to old vhs x 2 to RF
    modulate, before being sent downstairs via the RF cable.

    Upon further investigation, even altering rf o/p channel does not remove
    diagonal patterning from feed down UHF cable to downstairs.

    This is really obvious when the cctv rf feed is fed downstairs, into the
    aerial DA, without the aerial feed into the DA, seems to be reduced
    somewhat by combining at the aerial DA i/p, with the UHF normal earial

    Could this interference be similar to old radio IF's beating in the 2 x
    VHS recorders.??

    I've not had this problem before with 2 vhs in series, is it possibly
    because same mains used for 4 x cctv psu, 1 x cctv quad recorder, and 2
    x vhs machines, with seperate mains feed downstairs, though I don't see
    why it should.



    The volts, AC & DC, are co-incident, but probably not related to the
    interference, and they seem to do no harm to aerial DA i/p, but it would
    be nice, and safer to my hands, to remove these.

    This is presumably caused by floating or whatever RF earth o/p's on the
    vhs's

    I have come across a circuit on the internet, which suggest 1x 10nf 250v
    capacitor in series with both legs of the RF cable should do the trick
    in removing the volts, any comments on this

    TIA
     
  5. Andrew Holme

    Andrew Holme Guest

    "The" RF cable - singular - i.e. is everything modulated onto one coax?
    Just checking.
    This paragraph is unclear. A diagram might help.
    Sort of: it could be intermodulation distortion (IMD) in the DA or the final
    television receiver input.

    Is the DA meant to handle UHF modulator level signals? Does it have
    multiple inputs? You're not using an aerial splitter / combiner into its
    antenna input are you?
    I doubt it has anything to do with the mains.

    By "2 VHS in series" I take it you mean daisy-chained via UHF in/out. So
    you weren't using a DA then? Hmmm.... Can you post a link to spec / info
    on this DA.

    What is the angle of the diagonal pattern? How many fringes do you get
    across a TV line? What channels were the UHF modulators operating on at the
    time? Does altering the UHF channels change the pattern?
     
  6. UHFPD

    UHFPD Guest

    Both CCTV RF and Aerial connected gives less patterning than CCTV RF on
    it's own


    How many fringes do you get
    approx 30

    What channels were the UHF modulators operating on at the
    No
     
  7. UHFPD

    UHFPD Guest

    PS combining both signals now I have moved the daisy chained VHS's
    downstairs, feeding them from the 2.4ghz link, with the house aerial
    does NOT give any interference problems !!

    Must be something to do with the loop between downstairs and upstairs
    earths from mains / possible inductance into CCTV RF Cable from upstairs
    to downstairs, though I doubt this as the interference signal is so
    strong on it's own.

    Possibly something to do with CCTV HD recorder video o/p, VHS video
    i/p's, and RF cable earth ??
     
  8. Andrew Holme

    Andrew Holme Guest

    Have you tested the cable run (inner and outer) for continuity/shorts?
     
  9. UHFPD

    UHFPD Guest

    Yes..
     
  10. Make sure the signal levels of all channels are the same level, the aurial
    carriers are not too high , bandbass fitler the offending channel and then
    make sure the cable "tilt" is nearly flat before amplifying any of them.
     
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