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Thread: Marine vhf antenna question (probably daft)

  1. #1

    Default Marine vhf antenna question (probably daft)

    Hi all, probably daft newbie question so please be gentle with me, I asked this on a boaty forum and got no response so thought this looks like a good place to try, please note I am not a ham radio user just marine band vhf user so have very limited knowledge of these things, the band in question is around 158mhz.
    I pulled apart my 8' vhf antenna today to see if I can repair it after using it to find a tree (I found one and it won!).
    I was surprised to see the coax inner stops 6" above the coax end and does no more, the outer of the coax becomes the main element with a small coil then carrying on halfway up to another coil before going all the way to the tip, I would have expected it to be the other way round.
    I this a normal thing?
    It has always performed fine both at transmitting and receiving (and finding trees.) so no complaints just confused.
    Thanks for any explanations.
    Dave.

  2. #2
    Super Moderator 5B4AJB's Avatar
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    Yes, quite normal for a collinear antenna.

    Marine antennas exposed to salt water are not worth the time repairing in my experience.

    Tree finding is a problem for bikes too, you can get springs that mount between the antenna and mounting screw, which will save you most of the time, but metal whips are the way to go...

  3. #3

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    You have to remember that RF behaves in a very non-common sense manner. Common sense says the centre conductor is the most important - but only in some designs - the simple vertical antenna type - but for marine this type would be 47cm or so long. Yours uses the other conductor, which is less common, but does the same job. If you think about most antennas - like tv antennas and other designs they use a dipole, which us often an out and back design - a squashed circle in effect. There should perhaps, be a connection between the centre and the screen - the typical VHF marine antenna, with cable feed at the bottom and a variable angle bracket uses a DC shorted design. If you put a meter across the coax it reads a short circuit, it's a grounded design.If it's open circuit it could be a DC grounded design - so a meter won't prove very much? The centre connection forms what is called a 'stub' - your antenna is effectively 4 quarter wave dipoles stacked end on end, with the coils providing matching. Antenna designs tend to be weird. Stacking elements gives gain, but messes up the electrical matching - so all the coils and Stubbs do is put it right, and let the antenna function efficiently, which makes the transmitter happy!

  4. #4

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    Thanks for the explanation guys, I barely understand it but pretty much expected that.
    Internally it's very clean with no salt corrosion it was just the damage at the base that was the issue as it snapped just above the ferrule it mounts on, as the splits were running along I've ordered new but repairing to use as a test antenna at home, it was just the layout that surprised me.
    Once again many thanks.

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    How can you hit a tree with a boat?

  6. #6

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    Quote Originally Posted by OH8GAD View Post
    How can you hit a tree with a boat?
    By mooring beside it and forgetting to drop the aerials in time.... (was on a river not open sea)
    The tree was an easy target, I managed to hit a washing machine once coming in on a flood tide, it had been dropped off a town bridge and was just submerged, that made for an interesting insurance claim.

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