BNC 50 Ohm coaxial replace with 75 Ohm

Discussion in 'Wireless & RF Design' started by Walter_S, Mar 16, 2018.

  1. Walter_S

    Thread Starter New Member

    Mar 9, 2018
    13
    0
    Hello All,

    I have an equipment where 50 Ohm RG58 coaxial cable is used for transferring up to -2 kV from one side to another one (4 m cables). Actually this is shv bnc cable. It seems that cable is broken and I have spare but RG59 (75 Ohm), 50 Ohm I'll have to buy which is quite expensive as it is SHV not normal BNC.
    My question is - can I just replace 50 Ohm with 75 Ohm cable?? It's just DC signal, high potential but almost no current so I guess I can, but want to get your opinon. According to wiki https://en.wikipedia.org/wiki/BNC_connector connectors 75 and 50 Ohm should fit to each other.
    Right now I'm not close to equipment, so want to be sure that I have everything when I go onsite.

    Thank you very much for feedback.
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    If the above is correct, the remaining concern is whether the cable and connector can withstand the 2 kV
     
  3. bertus

    Administrator

    Apr 5, 2008
    18,336
    3,514
    Hello,

    The maximum usable voltage for RG59 is 2300 Volts RMS, wich is close to the mentioned 2000 Volts you want to use.
    Better use a better type of coax to avoid break through.
    The following page will give you a table with the properties of a great variety of coax cables:
    http://www.rfcafe.com/references/electrical/coax-chart.htm

    The normal BNC is only rated for 500 Volts:
    https://www.amphenolrf.com/connectors/bnc.html

    Better look at the MHV series:
    https://www.amphenolrf.com/connectors/mhv.html

    Bertus
     
  4. Walter_S

    Thread Starter New Member

    Mar 9, 2018
    13
    0
    Hello Bertus,

    This spare 75 Ohm cable which I have is SHV, so no problem. Summarazing, I can replace 50 Ohm with 75 Ohm if it's just DC signal using, right?? When would be a problem to do that in case of frequencies??

    BR
    Walter
     
  5. MrChips

    Moderator

    Oct 2, 2009
    16,722
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    Before you go any further, is the signal on the BNC cables DC or AC?
    If AC, what is the max frequency?

    Secondly, check that the existing BNC cables and connectors are rated for high voltages. Not all RG58 cables are specified for 2kV.
     
  6. Walter_S

    Thread Starter New Member

    Mar 9, 2018
    13
    0
    This is only DC. I asked about frequencies for the future reference - I supposse when frequency increases then could be a problem with replacing 50 Ohm with 75 Ohm, but I'm not sure. They are rated for 5kV I think - https://www.pasternack.com/shv-plug-standard-rg58-connector-pe4194-p.aspx
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    You have SHV connectors.
    BNC, SHV, and MHV are not compatible.
    For DC, the impedance of the cable will make no difference.

    You really need coax cables with SHV connectors on the ends.
    RG58 or RG59 will make no difference as long as the dielectric is solid polyethylene and not foam.
     
  8. RichardO

    Senior Member

    May 4, 2013
    2,270
    878
    As MrChips says, never try to plug a cable with a BNC connector onto a high voltage connector. They look the same but they are not. If you manage to get the cable connected you may never get it off without damaging something. (This is a mechanical problem having nothing to do with the voltage).
     
  9. Walter_S

    Thread Starter New Member

    Mar 9, 2018
    13
    0
    Thank you All for info - don't worry, cause I have SHV connectors on both sides on my spare cable. The only difference between original cable and spare one is impedance - original is 50 Ohm and spare 75 Ohm.
    Assuming that signal is not DC, but has few kHz - then still impedance doesn't matter??
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    Now you have mismatch of impedance and the signal will be degraded with reflections in the cable.
     
  11. crutschow

    Expert

    Mar 14, 2008
    19,516
    5,443
    That's not a problem until the cable gets long enough to generate a standing wave (which at a few kHz is a very long cable). ;)
     
    DickCappels and nsaspook like this.
  12. Walter_S

    Thread Starter New Member

    Mar 9, 2018
    13
    0
    OK - thank you all. I think topic can be closed.
     
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