how a coaxial cable works with RF

Discussion in 'Wireless & RF Design' started by antennaboy, Apr 9, 2011.

  1. antennaboy

    Thread Starter Active Member

    Jan 31, 2008
    Hello forum.

    In elementary DC circuit theory I have learned that current travels from a voltage battery, through wires to a load and back to the battery via the return wires. This is a simple model that breaks down when we move to AC.
    Interesting things happen with transmission lines and RF.

    If we connect a voltage source+coax+load, there are going to be 3 currents on the coax: one current i1 of the surface of the inner conductor 1, a current i2 on the inner surface of conductor 2 (shield) and a last current i3 on the external surface of conductor 2 (shield) (skin effect). i3 is called the common mode current and it is ideally 0 when there is not radiation emitted from the coax.

    I read that "...the common return current as the 3rd current, the one on the outer surface of the shield and say that it is the unbalanced current not returned within the coaxial cable..." What does it mean "not returned"? Not returned to the source/transmitter?

    When we move the RF regime, how do we apply the "going wire and return wire" concept so used in DC theory? Does the concept of return wire still make any sense?

    It is said that it is better that there is no current going back to the transmitter (no common mode current) because it could cause "RF burn". In DC theory it seems that there must always be a return current on a return conductor for the circuit to function properly. In RF it seems that we don't want that....

  2. beenthere

    Retired Moderator

    Apr 20, 2004