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Thread: Tuning 40Mtr antenna

  1. #1

    Join Date
    Apr 2011
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    Dorset UK
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    69

    Default Tuning 40Mtr antenna

    Hi,
    My 40 MTR dipole antenna, tunes to a low SWR at 6.66MHz. How much should I shorten each end to tune to 7.1MHz please?
    Camerart

  2. #2

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    a little bit at a time.

  3. #3

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    Quote Originally Posted by camerart View Post
    Hi,
    My 40 MTR dipole antenna, tunes to a low SWR at 6.66MHz. How much should I shorten each end to tune to 7.1MHz please?
    Camerart
    Hi OM Camerart,

    Measure actual length on one side (assuming both sides are equal!) = L

    Length to be trimmed on each side = L x [1 - (6.66/7.1)]

    It would be wise to follow Obed's advice - 'a little bit at a time'!

    Repeat the process till you get the lowest SWR at 7.1 MHz.

    Good luck!

    73,

    Nandu.
    Last edited by vu2nan; Thu 18th Oct 2018 at 16:56.

  4. #4

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    I concur - never been able to make measurements reflect what you get in practice!

  5. #5

    Join Date
    Apr 2011
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    Dorset UK
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    Default

    Hi,
    Thanks for the replies, especially vu2nan, and the calculation.

    I have spent a while using the above suggestions, and failed so far. I am trying to avoid going up ladders, and one end is under the eaves of the house. In my earlier attempts I folded the ends back and wound them back round the antenna, in case I took too much off.

    I was talking to someone at my Ham club, and he explained the at the ends of the antenna, shortening/lengthening effects if an end is near to a building/tree, (for example) Also, winding the ends back, as I did also affects the length in a capacitance way. First unwrap and cut the folded back sections, while checking.

    When I asked the question, I thought a simpler % kind f answer would give me a guide.
    C.

  6. #6

    Join Date
    Apr 2011
    Location
    Dorset UK
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    Default

    Hi,
    I had to take about a foot of each end, now it's tuned.

    From #5 regarding antenna ends near a house or other. Previously, I had tried shortening the lower end, away from the house end, resulting in not much change. I folded back the extra. Once up the ladder I noticed that when I shortened the house end, the changes were most pronounced. Does this add up?
    C.

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