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"Sharing" ground between satellite and OTA/cable?

Discussion in 'DIRECTV Installation/MDU Discussion' started by slice1900, Sep 17, 2013.

  1. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    Dec 9, 2006
    "nope" on the open shield connector idea.
    It's hard to get specs on $3 DC blocks, so I'll leave you to search.
    DC blocks come in three varieties:
    1. open outer
    2. open inner
    3. open inner and outer
    "Open" means capacitive coupled.
     
  2. slice1900

    slice1900 Well-Known Member

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    Feb 14, 2013
    Iowa
    Why would the open shield not work? Is there a reason why you'd want the shield to be capacitively coupled? The signal travels on the inner conductor only, right? From what you're saying it sounds like I may have a fundamental misunderstanding about how the signal travels if an open shield would be a problem.

    I thought the shield was only for grounding and to protect the signal traveling on the center conductor from interference from other sources nearby as well as interfering with other sources itself. An open shield that was completely covered by a connector would seem to still protect from interference, and you'd have separate grounds on either end of the open shield but the open shield would avoid this fact inducing current across the cable like it otherwise would. From what you're hinting it perhaps the shield needs to be continuous to avoiding causing problems with the signal traveling down the center conductor, or needs to be capacitively coupled.

    Can you suggest any links I visit to learn more, or some search terms to throw at Google? I think I'll try to build one to throw inline on an existing connection, so I might learn something through its failure :)

    I'll guess I'll also see if I can find out anything on the DC blockers I have. I assumed they were all the same - blocking DC on the center conductor (though looking on summitsource.com right now I see some that say they block 48v DC and 60v AC, which may indicate they're different than the others that don't mention AC at all)
     
  3. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    Dec 9, 2006
    If you want to try it, go for it.

    "Experience" says it will have problems, as I've had to repair a broken shield on a connector way too many times.

    RF through a transmission line is fairly complex, so thinking that the shield is merely for interference isn't the whole story.
    The shield is a "ground plane", so its distance and the dielectric between it and the center conductor defines the impedance of the coax.
    For a given frequency, there may be a point in the wavelength, where an open shield has no affect.

    "for all practical purposes" a center DC block and a ground block are "good enough"

    For $150+ you can get a 75 ohm inter and outer DC block.

    http://www.pasternack.com/75-ohm-f-inner-outer-dc-block-0.1-2-ghz-pe8242-p.aspx?gclid=CI_Ombuv2bkCFZRj7AoduAgAMw
     
  4. slice1900

    slice1900 Well-Known Member

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    Feb 14, 2013
    Iowa
    Yikes $150! I had seen cable isolators for $30 or so and thought that was already a bit much for something I wasn't sure I really need (and certainly shouldn't need if the cable company does its job right, just not sure if I trust them)

    Last night I did a little googling and found some closer to $10, so if I decide I want to add one I'll buy it instead of trying to make something someone who knows way more about RF than I ever will thinks is not a very good idea :)

    If the distance from the shield to the conductor is how the impedance is defined, essentially making it a 75 ohm cable versus 50 ohm or whatever, then obviously messing with that is a very bad idea unless you really know what you're doing, which I most certainly do not.

    Thanks for the help, as always I learned something new from you!
     

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