Baluns

Discussion in 'Homework Help' started by chesart1, Jan 23, 2006.

  1. chesart1

    Thread Starter Senior Member

    Jan 23, 2006
    269
    1
    Hi,

    If I am correct, baluns is used on a pair of balanced lines to unbalance them. A balanced pair of lines are two lines connected to a load whereby the waveform on the feed line cancels out the waveform on the return line. An example of balanced lines is your twin antenna leads for your TV set. The signals on the two lines cancel each other out to prevent transmission from the lines into the air.

    My question is: why would one want unbalanced lines?

    Thanks,
    John
     
  2. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    I'm not sure whether the US still uses 300 Ω balanced feeder for TV cable, but in the UK a 75 Ω coaxial cable is used which is an unbalanced cable.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Yeah, you can still get 300 ohm twin-lead, but RG-6 is so much better. You can hook the center conductor to one driven element and the shield to the other on a yagi antenna and avoid the balun, too.
     
  4. n9xv

    Senior Member

    Jan 18, 2005
    329
    1
    In RF/antenna work it is often necessary to use a balun when transitioning from balanced - "parallel" - feedline to unbalanced "coaxial" feedline. In Amatuer Radio we use 300, 450, & 600-ohm balanced lines but you can design your own balanced line to have any value of characteristic impedance. Whether or not to use balanced or unbalanced lines depends on several factors but primarily on the type of antenna your feeding. For instance, a Loop antenna is a inherently a balanced antenna. RF current flows out of one terminal of the balanced line, around the loop and back to the other terminal of the balanced line. Whereas, a 1/4-wave ground plane is an unbalanced antenna. RF current flows through the center conductor of the sheilded coax until it reaches the antenna where the 1/4-wave element radiates the signal against a "ground plane" of radials.

    The term balanced means that currents of equal magnitude are flowing in each parallel conductor but are flowing in opposite directions.

    The term unbalanced means that current is flowing in only one conductor. With coax, that is obviously the center conductor. No current flows through the sheild of the coax. A common matching arrangement for a yagi-yuda array is the gamma match which is feed directly with coax. The gamma match is an unbalanced match and provides a conveinient method of connecting the coax.

    The simplest antenna one can errect is 1/2-wave dipole. Two 1/4-wave lengths on each leg. This dipole antenna is a balanced antenna and is commonly fed directly with coax. It will radiate that way, but will do so much more efficiently if a balun is installed at the antenna. The balun helps prevent the feedline from doing the radiating. In a typical Amatuer Radio application, we are generally trying to go from unbalanced to balanced.

    Baluns can be very usefull in transforming impedance from one level to another. A balun is nothing more than a transformer operating at RF. A common application is transforming the 300-ohm impedance of a folded dipole to 75-ohms in order to be feed with 75-ohm coax. Hence, a 4:1 balun. Readily available baluns are in ratios of 1:1, 4:1, 6:1, & 12:1. However, a balun can be designed for vertually any impedance ratio. There is also such a device called a "balbal". This transforms impedance levels between two balanced lines. A "unun" transforms impedance levels between two unbalanced coaxial lines.

    Parallel feedlines offer much less attenuation and better matching capability than an equal run of coaxial feedline.
     
  5. chesart1

    Thread Starter Senior Member

    Jan 23, 2006
    269
    1
    Hi,

    Thanks for the replies.

    n9xv,
    Thank you for the detailed explanation of balanced and unbalanced circuits. Your time and effort is appreciated.

    John
     
  6. SoundGuyAndy

    New Member

    Jan 27, 2006
    6
    0
    You'll also find baluns in use fairly commonly now to allow video, which normally needs 75 ohm coax, down a balanced line, allowing it to go longer distances over more durable and less expensive cable. You can buy off-the-shelf baluns that convert from BNC coax to Cat-5 and back; in theatre they're often wired to allow video to be sent down standard balanced microphone cable, allowing video to be sent down the same multi-pair cable as video, saving on labor and expense.

    --A
     
  7. chesart1

    Thread Starter Senior Member

    Jan 23, 2006
    269
    1
    n9xv,

    You stated "The term unbalanced means that current is flowing in only one conductor. With coax, that is obviously the center conductor. No current flows through the sheild of the coax."

    This is my perspective: Lets consider two signals traveling down two separate lines to two loads. Line one has a sine wave traveling from a source through it's load to the common line. Line two also has a sine wave traveling from a source tthrough it's load to the same common line. The two loads have equal resistance. The loads are not reactive. Both loads are attached to the same common line.

    If the sine waves on the two lines are 180 degrees out of phase with each other, then these two sine waves algebraically add to zero and hence there is no current in the common line. This is considered a balanced pair.

    However, if the two sine waves are 40 degrees out of phase with each other, the common would have a signal traveling on it because the two sine waves algebraically do not add to zero. This is considered an unbalanced pair.

    To add further confusion, we have Kirchoff's current law which states that the current is the same in all sections of a series circuit. As we see in our electrical and electronic theory books, this applies to AC and DC circuits.

    I believe you intended to state that the power is completely dissipated by the load and hence there are no reflections. To have a current flowing to the load in the center conductor of a coax cable and no current in the shield makes sense to me only if the shield of the coax is not the return path.

    Could you or anyone who reads this please refer me to a book that explains the concepts being presented in this exchange of messages?

    Thank you,
    John Mario ...
     
  8. chesart1

    Thread Starter Senior Member

    Jan 23, 2006
    269
    1
    Hi n9xv,

    After re-reading the sections on transmission lines (matched and unmatched) and standing wave ratio in an Electronics Workbench manual, I realized where my analysis given in my last message was wrong.

    Kirchoff's current law does not apply to the instantaneous currents in any lines where the wavelength of the signal is smaller than the length of the line.

    Thanks for your help,
    John
     
  9. jerry-km3k

    New Member

    May 13, 2013
    1
    0
    1. This is wrong; very wrong!!! RF-current does flow on two conductors -- namely, the outside surface of the center conductor and on the inside surface of the shield.
    2. Futhermore, if the antenna-system is unbalanced (for whatever reason), there can be RF-current on the shield's outside surface as well. It is this current that can be a root cause of RFI. A balun will help to reduce that current.
    73 Jerry KM3K
     
  10. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    You are quite correct.

    A useful link (among many) is

    http://www.antenna-theory.com/definitions/balun.php
     
  11. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    ....perhaps the definition has changed since 2006? ;)