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Thread: A λ Ground Plane for 70 cm

  1. #1

    Default A λ Ground Plane for 70 cm

    A λ Ground Plane Antenna for 70 cm I built years ago!

    https://nandustips.blogspot.com/2011...for-70-cm.html

    73,

    Nandu.

  2. #2
    gnuuser's Avatar
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    other than experimentation building a 1/4 wave for 70 cm will work but the length of a half wave would perform a heck of a lot better.
    granted some home owners associations may be a royal pain in the arse. a 70 cm halfwave is not that big!.
    too much blood in my caffeine system.

  3. #3

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    I like that antenna. But now I want to get technical for a moment...

    Quote Originally Posted by gnuuser View Post
    other than experimentation building a 1/4 wave for 70 cm will work but the length of a half wave would perform a heck of a lot better.
    granted some home owners associations may be a royal pain in the arse. a 70 cm halfwave is not that big!.
    Once those radials are bent downward such that they are not perpendicular to the vertical element, IMHO, it is no longer a ground plane antenna. "Plane" suggests planar, that is not. What those antennas more closely resemble are dipoles with one element split open and fanned out. Bend those buggers just a little further down and that's exactly what it would be, a center-fed dipole with no balun.

    Gnuuser, how would another 0.9 inch in the vertical dimension be "a heck of a lot better"? In fact, with respect to the height of the top vertical portion and making the other dipole element the same, that is less than a half inch shorter than a dipole would stand.
    Last edited by brandon lind; Thu 25th Jun 2020 at 06:55.

  4. #4
    gnuuser's Avatar
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    Quote Originally Posted by brandon lind View Post
    I like that antenna. But now I want to get technical for a moment...



    Once those radials are bent downward such that they are not perpendicular to the vertical element, IMHO, it is no longer a ground plane antenna. "Plane" suggests planar, that is not. What those antennas more closely resemble are dipoles with one element split open and fanned out. Bend those buggers just a little further down and that's exactly what it would be, a center-fed dipole with no balun.

    Gnuuser, how would another 0.9 inch in the vertical dimension be "a heck of a lot better"? In fact, with respect to the height of the top vertical portion and making the other dipole element the same, that is less than a half inch shorter than a dipole would stand.
    oops
    I meant to say a 70 cm full wave is not that big.
    many of the hf antennas sizes make it impractical and prohibitively expensive to make them full wave size or even half wave size unless you own a lot of open land and a hefty bank account. but when you get to 2 meter and higher frequencies it does not make a lot of sense to go with anything less than at least 1/2 wave for efficiency.
    but then again i haven't taken into account how much power is being fed to the antenna either.
    too much blood in my caffeine system.

  5. #5

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    Quote Originally Posted by gnuuser View Post
    oops
    I meant to say a 70 cm full wave is not that big.
    many of the hf antennas sizes make it impractical and prohibitively expensive to make them full wave size or even half wave size unless you own a lot of open land and a hefty bank account. but when you get to 2 meter and higher frequencies it does not make a lot of sense to go with anything less than at least 1/2 wave for efficiency.
    but then again i haven't taken into account how much power is being fed to the antenna either.
    Full wave antennas are tricky things to feed (at least from the center or either end) due to the extremely high impedance at those points. That ground plane he made was almost a perfect half-wave in height ~ almost ideal (short of a 2 element beam or better), that was my only point.

    Another issue is when you exceed 3/4 wavelength on a straight radiator, the pattern gets a bit odd and one better understand what is happening to know what angles the radiation leaves on. This is why people don't make 3.5 wavelength antennas as single runs of wire (short of long wires where things get more defined again). It turns into chaos with many sharp nulls in unexpected places. I do believe a nearly full-wavelength dipole (if you managed to feed that high-impedance bugger) would have a cloverleaf pattern and nothing perpendicular to the wire itself. I haven't modeled enough antennas to say for sure though, simply (and quite possibly erroneously) recalling the days I spent in the arrl antenna book.
    Last edited by brandon lind; Fri 26th Jun 2020 at 06:09.

  6. #6

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    The only balanced antenna is a half wave antenna also referred to as a Hertzian antenna or a Di Pole.

    Full wavelength antenna is not balanced. Most verticals uses a form of stacking - ex Diamond X510

    https://www.diamondantenna.net/x510series.html


    X510HDM Base Antenna

    Specifications:
    Band: 2m/70cm
    Element Phasing: 3-5/8l/8-5/8l
    Last edited by R2D2; Tue 30th Jun 2020 at 22:13.

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