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Does a HF vertical antenna need a balun?

Discussion in 'Electronic Design' started by Nick, Jun 24, 2008.

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  1. Nick

    Nick Guest

    Hi All,

    I keep reading on various websites and in various documents that HF
    verticals should have a "line isolator" (a choke/current balun)
    located near the antenna's feedpoint to prevent coax feedline re-
    radiation, disruption of the antenna's radiation pattern, sub-optimal
    vertical performance, and RFI in the shack. My question is this: just
    how important is a choke balun when using a ground-mounted HF (for
    40M) quarter-wave vertical antenna with 32 quarter-wave buried
    radials? And if it is important, why wouldn't the choke be placed at
    the shack's input, so that the coax's outer shield could act as
    another radial? (I had always assumed that a vertical didn't need a
    balun at all, and that only dipoles used them. Am I wrong about


  2. Guest

    I think line isolator is slightly different from balun. Here line
    isolator would be necessary for lightening protection etc and hence
    better kept at antenna base. Balun is for converting a balanced
    structure like dipole antenna to unbalanced structure like coax. Since
    monopole antenna is unbalanced structure it does not need balun at its

  3. Jeff

    Jeff Guest

    A "line isolator" (a choke/current balun) will provide no lightening

    What is being talked about is a choke to prevent currents from flowing on
    the outer of the coax.
    It is not a balun in the purest sense, but it does stop unbalanced currents
    from flowing on the coax.

    The problem with an HF vertical is that the ground plane will never be
    anywhere near prefect, so it will
    not be possible to feed with coax without some current flowing on the outer.

    I guess the only problem with putting the choke at the antenna base will be
    pickup onto the outer as is
    crosses the groundpalne. so I can see the idea of putting the choke at the
    edge of the GP might be better.
    I guess it either suck it and see, or try to simulate it with NEC.

  4. Leon

    Leon Guest

    A balun isn't required.

  5. Ian Jackson

    Ian Jackson Guest

    It's not a balun in ANY sense of the word. Some of the manufacturers of
    the 9:1 isolation/semi-matching transformers (essentially for listening
    only) persist in calling these 'baluns'. But baluns they ain't.

    Unfortunately, 'balun' has become a generic word for many types of RF
    transformer (especially those wound on ferrite). It's a bit like calling
    all vacuum cleaners 'Hoovers' - which is what we do, at least in the UK.
  6. Ian Jackson

    Ian Jackson Guest

    In message
    With this type of antenna, it certainly is not common practice to
    choke/isolate the coax, especially the coax braid can be grounded at the
    antenna feedpoint and in the shack, and possibly at some intermediate
    points along its length. It wouldn't do any harm to use one, but it
    probably wouldn't do very much.
  7. Nick

    Nick Guest

    Thanks for all the great responses guys! That is exactly what I
    had thought, that since the outer coax braid is grounded at the
    antenna, and at the shack, and (in my case) at the center with a
    lightening protector, then I just couldn't figure out why some antenna
    companies, such as reliable DX engineering with their DXE-VFCC H05-A
    Vertical Feedline Current Choke, strongly recommend such a line
    isolator for verticals. A direct quote from DX Engineering's Website:

    "2/5 kW Vertical Feedline Current Choke
    If your antenna SWR is already low and you wish to reduce feedline
    radiation and improve reception, a Feedline Current Choke is
    recommended. Adding a DX Engineering Feedline Current Choke at the
    base of a vertical antenna will substantially reduce unwanted feedline
    radiation (RFI), reducing the need for improved station grounding.

    When quarter-wave antennas are constructed over a good radial system,
    they have a feedpoint impedance of about 36 ohms. When they are
    constructed over less than a good radial-system there is a loss
    introduced into the feed system that adds to the 36-ohm figure. This
    improves the SWR but there is a loss in the efficiency of the antenna,
    signals transmitted and received have a higher take-off angle and
    often there is current introduced on to the feedline.

    With a ground-mounted quarter-wave vertical, regardless of the radial
    situation, but especially with poor radial systems, the feedline can
    become part of the radial system, causing RFI and poor antenna
    performance. By using a VFCC at the feedpoint, the feedline is
    effectively de-coupled from the antenna system, preventing interaction
    with the radial system, improving antenna efficiency. You may notice
    improved bandwidth as well.

    The Advantages of Using a VFCC:
    Prevents unwanted RFI by eliminating feedline current and radiation
    All power goes to the antenna, improving efficiency
    Reduces noise or unwanted signals picked-up by the feedline
    Overcome a less than optimal ground system
    Bracket isolates the VFCC case from ground for best de-coupling"

  8. Cecil Moore

    Cecil Moore Guest

    Get the best of both worlds. Put one choke at the antenna
    base and one more 1/4WL down the feedline which will be
    close to your "edge of the GP".
  9. Cecil Moore

    Cecil Moore Guest

    Nick, think about it. If you ground the feedline at exactly a
    common-mode standing wave voltage node, the ground accomplishes
    absolutely nothing. You certainly don't want to use "plumber's
    delight" techniques in your ground system.
  10. Nick

    Nick Guest

    Thanks Richard and Cecil.
    Cecil, I understand what you are saying, but still why not use the
    outer shield of the coax as one of the radials, and simply choke off
    the coax just before it enters the shack? I also don't want to add
    what may be a useless choke at the antenna's feedpoint, since that
    would decrease the antenna system's efficiency due to the choke's
    resistive losses...


  11. Nick

    Nick Guest

    Thanks for the further info Richard! I guess I don't really fully
    understand why the pure resistive losses in the coax choke wouldn't
    affect the efficiency of the monopole. If I understand you correctly
    (and I may not!), then even if we increase the impedance of the coax
    choke to insane levels, such as by making it 1000 feet long and
    winding it on a six inch diameter PVC form, then the huge losses
    normally inherent in such a long run of coax would not be seen at all
    by the vertical because it is wound on the PVC form? (I'm not arguing
    with you at all, I just really want to know).


  12. Jeff

    Jeff Guest

    A simple way to think of it is: Ferrite beads over the outer of a coax has
    no effect on the signals flowing 'normally'
    up and down 'inside' it. The beads only effect signals that are flowing on
    the outer (treating it like a piece of wire rather
    than a coax transmission line). I know that this is not quite the full story
    but it is a simple explanation of the situation
    under discussion.

  13. Cecil Moore

    Cecil Moore Guest

    Unless the coax is up and in the clear, common-mode power is
    probably wasted whether you choke or not.

    If your choke at the shack happens to be placed at a common-
    mode voltage-maximum/current-minimum, it will be ineffective.
    One reason for placing a choke at the antenna feedpoint is
    that is probably the maximum current point where the choke
    has a good chance of being most effective.
  14. Wad is dis? Is that something to protect your coax from becoming too light?

    Jes kiddin'!

    then I just couldn't figure out why some antenna
    Since they make them, they're happy to sell them to you.

    A direct quote from DX Engineering's Website:
    I am really at a loss as to how this isolator improves reception.

    Am I a hopeless noob, or are there a lot of things wrong here?
    Feedline radiation = RFI. and almost a lure toward poor station grounding?
    Are there tests showing this somewhere?

    Anyway, you are probably best served by placing as many radials as you
    can handle - but more than 4. I just went till my knees and back said
    "no more", then came back the next day and laid more. Have as good a
    grounding system as you can get, and good lightning protection. IMO, if
    that isolator is helping any, it is just trying to mitigate a problem
    that should be taken care of elsewhere.

    - 73 de Mike N3LI -
  15. Nick

    Nick Guest

    Thanks again guys. I'll have to study common versus diff mode
    currents on coax cable again to gain a better understanding of the
    situation! And as was stated by many, a good radial field should
    mitigate the situation anyway.


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