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Do dipoles require baluns?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K9PSM, Jan 25, 2010.

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  1. ZL3GSL

    ZL3GSL Ham Member QRZ Page

    Professionally engineered antenna systems often work at one frequency.

    Baluns can work very well when they are working with resonant antennas. that is, with no reactive part of the feed point impedance. They work less well when the frequency is varied from the resonant

    Amateur antennas often work well, with no bad effects without a balun. If there are unwanted effects, a balun might fix them. Bad effects seem to happen most often when high power is being used.

    A simple dipole is a good enough match to 50 ohms, so doesn't need impedance transformation. A balun will give interesting transformations when the antenna is off resonance.

    The outer of the coax coming down affects the radiation pattern, when the antenna is modeled. In the real world, if the antenna is not particularly high, that doesn't matter: there are usually other real world objects which have more influence.

    Coiling the coax into a "choke balun" might be all that's needed to get rid of RF in the shack if that happens.
     
  2. G3TXQ

    G3TXQ Ham Member QRZ Page

    Paul,

    For single-band use an air-cored coiled-coax choke (balun) is satisfactory and cheap. But, because they are relatively narrow-band, you need to get the number of turns, and their diameter, correct for the band you are using.

    A more effective, and broader-band, balun can be made by winding 10-12 turns of coax on a ferrite toroidal core. Here's one that Cecil uses:

    http://www.w5dxp.com/121Choke.JPG

    Here's another example:

    http://www.karinya.net/g3txq/temp/current_balun.png

    Don't get confused by the "cross over" in the middle of the winding - that's just a convenient way of arranging for the ends of the winding to be at opposite sides of the core.

    73,
    Steve G3TXQ
     
  3. G3TXQ

    G3TXQ Ham Member QRZ Page

    A 1:1 balun constructed by winding 2ft of 50 ohm coax on a ferrite core will produce no worse a transformation than 2ft of extra 50 ohm feedline.

    Steve G3TXQ
     
  4. N8WWM

    N8WWM Ham Member QRZ Page

    To balun or not to balun....

    I just roll up 6-8 turns of coax at the feedpoint, about 6 in. diameter. A field strength meter shows a drastic reduction in shield radiation.

    73 Doug
     
  5. W5DXP

    W5DXP Ham Member QRZ Page

    Trouble is, that only works well for an approximately 3 to 1 frequency spread.

    http://www.k1ttt.net/technote/airbalun.html
     
  6. KD0CAC

    KD0CAC Ham Member QRZ Page

    If I'm fallowing this correctly , an multi-band antenna , may have issue's with a balun not fitting all frequencies that the antenna is built for .
    Is there something like a antenna tuner , adjustable/tuning balun ?
    I see all kinds of baluns out there , and if like it seems most Hams are using many bands , and many power levels , QRP to legal limit .
    Having a tuning balun on the radio side of the antenna switch , would start to make things a little closer to having your cake & eating it also .
     
  7. G3TXQ

    G3TXQ Ham Member QRZ Page

    If you use an antenna which provides a good match to 50 ohms on several bands - such as a trap dipole - you would need to use a 1:1 balun at the feedpoint which has reasonable choking impedance on each band, and which does not materially change the impedance of the antenna. A "coiled coax" balun works well for one band, but not so well over several bands that are widely spaced in frequency. The better option is then a design using 50 ohm coax wound on a ferrite core, such as Cecil's.

    However, if the antenna was a multiband doublet fed via ladderline and used with a tuner, you need a 1:1 balun between the tuner and the ladderline. Again, a wide frequency span argues for a ferrite balun; but now, the characteristic impedance of the balun winding is not critical because the tuner will adjust for any impedance transformation. Because a balun in this position can be subject to quite high voltages, and because the winding Zo is not critical, these so-called "tuner baluns" are often constructed with bifilar windings using wires with a high breakdown-voltage capability - typically Thermaleze wire inside Teflon tubing.

    73,
    Steve G3TXQ
     
  8. G7LSR

    G7LSR Ham Member QRZ Page

    Below say 5w no need for a balun. but don't turn your shack into a microwave cooker. if you intend to run 2kw, keep it outside!
    G7LSR Ken
     
  9. G3TXQ

    G3TXQ Ham Member QRZ Page

    Don't ignore the potential for a balun to reduce noise pick-up on Rx - it might make the difference between hearing that 5W signal and not hearing it!

    Steve G3TXQ
     
  10. K6JJH

    K6JJH Ham Member QRZ Page

    I just installed a DX-EE dipole on my house, fed with coax, and I'm not sure it's producing as it should in RX. (I don't have my General ticket yet, so TXing on HF isn't an option). I only hear a few stations loud and clear, most others are almost buried in the static. I have an FT-950, and as it sits right now, the coax isn't grounded to anything. I am sinking a grounding rod right outside the window of my shack, and I'll ground the coax shield to that before it enters the shack. That grounding rod will run directly to the main grounding bus for the shack.

    I know since my antenna is oriented east-to-west that it will work better north-to-south.

    I plan on raising the center point higher in the future....right now it's about 25' above the ground, and about 10' above the peak of the roof.

    If I'm reading everything correctly, it might be of benefit to put a 1:1 balun up at the antenna to help with noise and RF? I don't want to nickel and dime my system with useless junk, but I also want the best performance for what I can put up. :p

    Also, for anyone that knows....why are all the grounding straps supposed to be braided line? Does braided line conduct any better than stranded wire?? It seems to me there isn't much of a difference. :confused:

    Thanks!!

    Jim (KJ6EQO)
     
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