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

    That is an interesting idea. I think this takes me back to your original thoughts about other rigging affecting the overall resonance? Lets say we could create a loop as you described, wouldn't all the other metal rigging like the forestay and shrouds negatively affect the loop?
    Andrew
     
    KA2RRK likes this.
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    Similar question:
    https://forums.qrz.com/index.php?th...boat-tuner-as-dipole-center-connector.757207/

    I had created an approximate model for that posting. Having no idea about your particular sailboat, here is a WAG at the critical dimensions for your boat:
    upload_2021-3-29_14-8-59.png

    Here is a schematic:
    upload_2021-3-29_14-15-15.png
    W1 is a short wire (3') that grounds the mast to a hull plate in sea water, for lightning protection.
    W2 is the mast (40' tall).
    W3 is the backstay, electrically connected to the mast top for lightning protection. Assuming bottom end is not grounded.
    W4 is a new, insulated wire added along the "deck" to create the "loop antenna"
    W5 is the existing forestay, also electrically connected to the mast top. I am assuming that there is no wire back along the deck to the mast?
    The (red circle) feedpoint is between the bottom of the mast (junction of W1 an W2) (as close to "ground" as possible) and the new insulated wire on the aft deck. That is where you will eventually put the balun/matching network.

    With the dimensions shown, the feedpoint impedance is 196 + j263. Easy to match, however, if you change the lengths of W2, W3 or W4, or the operating frequency (say to 20m), then that would change the feedpoint impedance.

    Now, let's see how this acts as an "antenna". First question is where does the current go? If fed with 1A at the red circle, here are the various current distributions along each "Wire":
    upload_2021-3-29_14-37-19.png
    Notice where the greatest current flows! (Into the sea water, almost double the feedpoint current). There is even a surprising amount of current in the forestay, and that would change if there was a wire along the foredeck from the bottom of the forestay to the mast.
    upload_2021-3-29_14-51-5.png
    This is mostly like a vertical antenna.
     
    KA2RRK likes this.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Seems it would be easier and more predictable to not use the backstay for anything and just install a Marine grade HF vertical with its accompanying tuner as far away from all the elevated metal stuff as possible.:p

    Then, I've never owned a sailboat.

    I did own a cabin cruiser (powered, inboard) where most of it was fibreglass or wood and a 21' long (or something) "Shakespeare" marine antenna with a tuner worked 40m through 10m pretty well, almost always within 30 miles of land. The farthest we ever ventured was Catalina Island, about 25 miles offshore and they have a huge marina with a lot of radio stuff for sale and it was common to bump into hams there...often from other countries.:)
     
  4. K8JD

    K8JD Ham Member QRZ Page

    Boats and Radios
    I have a 18 Ft ski boat I put on the lakes here, NO WAY is there a way to put any kind of antenna on it, except maybe a VHF or UHF GP bolted to a side railing.
    It has an AM/FM/WX radio but ignition noise from the motor drowns out all but a few strongest stations here. It's antenna is a wire running under the "dashboard".
     
  5. KC1KPW

    KC1KPW Ham Member QRZ Page

    What about a random length antenna wire running from the antenna tuner on the stern to the top of the mast? I'm not looking for perfection. Totally insulated from all rigging and the mast of course.
     
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    ...but that insulated wire is within inches of the grounded metallic backstay, correct?
     

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