Home brewing a dual band VHF/UHF ground plane

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KJ7BNH, Oct 19, 2018.

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  1. KJ7BNH

    KJ7BNH Ham Member QRZ Page

    I recall reading long ago that a metal tube for a main element was more broadbanded than a metal rod. True, False, or It Depends?

    Is there any point in tuning the ground plane radials for the two different bands? Say, six or eight radials, three or four for one band and three or four for the other?

    Cheers!
    Neil the Noob
     
  2. NH7RO

    NH7RO Ham Member QRZ Page

    The outside of the elements are where the buffalo roam and the current flows---no difference or advantage to one over the other.

    A standard five-wire 19" GP can be used for both 2m and 70cm due to the harmonic nature of the bands involved. More radials the better, too (although four is sufficient).

    73.

    Jeff the OldOb
     
    K9ASE likes this.
  3. AA3EE

    AA3EE Ham Member QRZ Page

    There is a formula that relates bandwidth to the ratio of diameter to wavelength, so within reason, a 1/2" tube will have a wider bandwidth than a #14 wire.
    That's why we have "cage dipoles". A bunch of wires make a virtual tube which increases bandwidth.
    1/4" Diameter is probably plenty to cover the FM portions.
    4 radials per band is sufficient, drooped at a 45 degree angle will bring the feedpoint to 50 ohms.

    73
    Dave AA3EE
     
  4. NH7RO

    NH7RO Ham Member QRZ Page

    All else being the same tubing or rod makes zero difference.

    (He wasn't asking about 1/2" tubing over wire; he was asking about tube vs. rod).
     
  5. AA7QQ

    AA7QQ Ham Member QRZ Page

    Might as well just build a discone. No tuning....

    Ed
     
    NH7RO likes this.
  6. VU2NAN

    VU2NAN Ham Member QRZ Page

  7. NH7RO

    NH7RO Ham Member QRZ Page

    Better to connect both elements to center pin and splay apart slightly (like a tall V)---instant fan ground plane for two bands.
     
  8. N7EKU

    N7EKU Ham Member QRZ Page

    Hi Neil,

    If you don't change the vertical element, then radial cut for another band, won't make the antenna tuned on that band -- it will just make some kind of off-center dipole.

    If you are think of an antenna for 2m and 70cm, then a 2m ground plane will work fine for 70cm as that is the third harmonic of 2m.

    73


    Mark.
     
  9. VU2NAN

    VU2NAN Ham Member QRZ Page

    Yes, OM Jeff, like the 'The NA4IT "CHEAP" Dual Band Ground Plane Antenna'.

    https://www.qsl.net/na4it/dbgp.html

    73,

    Nandu.
     
    Last edited: Oct 27, 2018
    NH7RO likes this.
  10. NH7RO

    NH7RO Ham Member QRZ Page

    Yes, that's it---probably a slightly better match on both bands that way.

    73,

    Jeff
     

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