Do I need a balun and what kind?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AB1YN, Mar 29, 2021.

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  1. AB1YN

    AB1YN Ham Member QRZ Page

    Hello,
    My friend is setting up an end fed 1/4 wave on his sailboat focus on 40m. This will not be a traditional back stay, but the wire will be run in a similar fashion (not touching the backstay) Radio: Yeasu 991A, Tuner: is Yeasu FC40 - this tuner is designed to be mounted outside. Here is the question: The FC40 is essentially set up for a bare wire connection, the tuner has a wing nut for the antenna connection. Could you recommend a balun for this application?
    Thank you,
    Andrew
    AB1YN
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Why use a balun?*

    More important, I'd think, is "what will the other half of the antenna be?" A quarter-wave end-fed is "current fed," so the current is maximum and the Z is low, and the tuner will require an excellent ground connection; any conductor between the tuner chassis and whatever "ground" is used will radiate, like the antenna -- which isn't necessarily a bad thing, it's just a thing.

    *I'd use a 1:1 current (choke) balun on the "coax" side of the tuner, between the FT-991A and the tuner, close to the tuner, to prevent the coax from radiating and also becoming part of the antenna system.
     
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  3. AB1YN

    AB1YN Ham Member QRZ Page

    Steven,
    Thank you for this great advise.
    Andrew
     
  4. AB1YN

    AB1YN Ham Member QRZ Page

  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    The notion that it is ok to hang an insulated wire parallel, and very close to, a metallic, conductive backstay (usually stainless steel) that is electrically bonded to a vertical, aluminum, grounded-to-sea-water at the bottom mast, and that produces a useful "antenna" is bogus from the get-go.

    In this scenario, you are hoping that by taping a random insulated wire to the backstay, it somehow "induces" current into the backstay, the mast, and even into the forestay and other conductive rigging, and then you "hope-for-the-best"! That is likely to a be a marginal antenna, and you will not have a clue as to what impedance you are asking the tuner to match.
     
    AB1YN likes this.
  6. AB1YN

    AB1YN Ham Member QRZ Page

    WA7ARK - You are correct that taping a wire to the back stay is not a good idea. It is interesting that you did not notice or mention how I said " (not touching the backstay)" there are standoffs on this rig that will keep the end fed away from the backstay -I think it will work well - we will see, Ill post back some photos of propagation to confirm. Thanks for the input, BTW could you recommend a balun for an end fed based on the scenario? This was, after all the question.
     
  7. AB1YN

    AB1YN Ham Member QRZ Page

    @WA7ARK Lets set up a qso once we get this rigged setup, I see your profile says you are active on 40m, we could try some SSB voice.
     
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    I cannot suggest a balun until I know the feedpoint impedance of the "antenna" at the various frequencies where you intend to use it. To determine the feedpoint impedance, I would have to know a lot of details:

    Height from hull grounding plate to mast tip?
    Length of backstay?
    Length of all other stays that originate from the mast?
    Are any/all of the stays also electrically connected to the grounding plate system at the hull end. (e.g. via railings)?

    Think of the forestay, backstay and mast system of creating a grounded electrical loop. You are inserting a "monopole" conductor driven against some unspecified part of that "loop", where the monopole wire is physically located inside the "loop", and likely runs parallel to one of the legs of the "loop". The monopole cannot be more than a tiny fraction of a wavelength from the sides of the loop.

    Ball is in your court. Tell us how you think this creates an HF "antenna" worthy of the name?

    The only scenarios I can think of that turn this into a real HF antenna system is cutting a stay (installing a insulator to create a two-terminal feedpoint) or shunt feeding a stay or the mast....

    If the boat hull is non-conductive, is there a wired connection between the hull-end of the backstay and the mast or is the hull-end of the backstay intrinsically electrically isolated? It may be possible to drive RF between the hull-end of the backstay and the grounded mast.
     
    Last edited: Mar 29, 2021
    AB1YN likes this.
  9. KA2RRK

    KA2RRK Platinum Subscriber Platinum Subscriber QRZ Page

    Backstay insulators are cheap.

    A feedline choke is recommended.

    RRK
     
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  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    If the boat hull is non-conductive, is there a wired connection between the hull-end of the backstay and the mast (along the deck) or is the hull-end of the backstay intrinsically electrically isolated (due to being anchored to fiberglass)?

    It will be possible to drive RF between the hull-end of the backstay and the grounded mast by adding a coax run along the deck from mast to then figuring out how to feed the backstay (balun +matching network). Then the "antenna" would consist of a current loop; up the backstay, down the mast, horizontally across the deck back to the backstay.
     
    Last edited: Mar 29, 2021
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