why the shield of coaxial cable should be grounded?!

Discussion in 'Wireless & RF Design' started by buffon2009, Sep 10, 2014.

  1. buffon2009

    Thread Starter New Member

    Dec 7, 2011
    28
    0
    hello everybody
    i need to use antenna for tv so i picked up a coaxial cable then i striped it from two sides
    one side i put it in the roof of the house as i keep inner away from shield on the that side
    on the other side i put the inner on the connector in tv
    and when i connect shield on the body of connector the picture get more perfect

    i wanna know how it comes?!

    as inner and shield are conductors and the electric fields of channels introduce current in both inner and shield then why connect shield to ground get more perfect signal

    could i use inner only with out shield?

    could anyone explain more detials from how coaxial pick signal to how connect it to circuit
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    If connecting coax to a antenna with a symmetric, balanced 300 Ohm feed point, then use a balun (looks like this).

    If the TV end has 300 Ohm balanced input, then use one there too. If it has a 75 Ohm unbalanced input, then connect the cable directly to the F-fitting.

    Coax cable is intrinsically "shielded", so doesn't pick up interference inside your house. It also is intrinsically higher loss than 300 Ohm balanced twin-lead, which can become significant since TV broadcasts have moved to UHF with the advent of Digital TV.
     
  3. BR-549

    Well-Known Member

    Sep 22, 2013
    1,981
    388
    Grounding the shield gives the signal something to work against. For instance.....let's say that you like to jump up and down. Stand up at your desk and jump up and down. That is a signal without a ground. Now stand on a trampoline and jump up and down. That trampoline acts like a ground to your jumping process. See how much easier it is to jump, and see how much higher you go? That's what a ground does for a signal.
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    the shield contains the rf energy inside, and interferance outside. the rf energy is guided between the inner and shield to prevent leakage into and out of the coax. if yo0u do not connect the shield to ground on at least one end, itis capacitively coupled and will radiate into and out of the coax.
     
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