160m inverted L

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WM3O, Nov 16, 2009.

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  1. WM3O

    WM3O Ham Member QRZ Page

    i'm planning on an inverted L for 160m. the antenna will start at the base of my 40' tower and rise vertically to the top of the tower. there it will extend out in a long slope for 90' or so.

    i was planning on terminating the coax shield to the tower base and then running radials outward from there.

    i am not planning on having a matching network at the base of the tower - do i really need one?
     
  2. R3BU

    R3BU Ham Member QRZ Page

    The calculation is made for perfect ground under antenna.The impedance is close to 5-6ohms only. For better efficiency having only 6 ohms to radiate, we need more efforts for organizing better ground for 160m under antenna.For non metal support tower.
     
    Last edited: Nov 16, 2009
  3. W0BTU

    W0BTU Ham Member QRZ Page

    Topband e-mail reflector

    You might want to also post your question to the Topband reflector.

    http://lists.contesting.com/mailman/listinfo/topband

    I'm constantly amazed at the wealth of knowledge some of those guys have, when someone posts a question about anything related to 160 meters.
     
  4. K4SAV

    K4SAV Ham Member QRZ Page

    There are two ways to find out how this will work, build it and test it, or model the whole thing including the tower and everything on it. Inverted L's close to towers will couple heavily to the tower. Depending on what is on the tower or attached to it, the results can be most anything. With antennas built like this, you may find an inability to obtain resonance anywhere near where expected, or may find extreme values of SWR. There is no way to guess the answer.

    As an example, I built an inverted L 3 ft from a tower recently. When I measured the currents, the tower had 97.4% of the current and only 2.6% of the current was flowing in the L wire. YMMV.

    Jerry, K4SAV
     
  5. K8JD

    K8JD Ham Member QRZ Page

    Try substituting a wooden or fiberglass mast for the 40 ft part. If you are near a quarterwavelength the base Z should be 35 ohms +/- flat top ground effect.
    The capacitance to ground may make the antenna look long (you will have to trim it some) and drive down the feedpoint Z.
    My effort to get on 160M led to a halfwave dipole. No radials to dig up the yard for and great match for a looooong run of RG8x coax. No tuner required and it WORKS.
     
    Last edited: Nov 17, 2009
  6. R3BU

    R3BU Ham Member QRZ Page

    Some more pictures.If the impedance at feeding point will happen to be let's say 10ohms, the efficiency will be: 5.307ohms :10.0ohms=53%
    Ra=R los + R rad Where 4.693 ohms is the ground loss.
    For impedance 20ohms, efficiecy will be 5.307 : 20.0=26.5% Ground loss=73.5% or 14.7ohms
     
    Last edited: Nov 17, 2009
  7. WM3O

    WM3O Ham Member QRZ Page

    no room for a 160m dipole. the L is as best as i can get. i'll build it and see what happens. at worst it doesn't work, at best, i make a few qso's, right? i'm not sure how badly it will couple to the tower.

    thanks for the images, the prefect soil is funny - we have nothing like perfect soil - sand and clay mix - not very conductive.
     
  8. R3BU

    R3BU Ham Member QRZ Page

    Perfect ground undermines random wires ~50-100 peaces .... Or two rezonant at some hight...
    And if the tower is metal-to use it like part of antenna but grounded version.There are some descriptions how to feed it.
    I know one guy-a tourist, who uses this type of antenna in outings but on 80m , when travelling. He even uses only two rezonant counterpoise lying !!! on the grass.... He says he is pleased with rezults.... !!?? !!
    http://translate.google.com/transla...&hl=ru&ie=UTF-8&oe=UTF-8&prev=/language_tools
     
    Last edited: Nov 17, 2009
  9. PE1PAL

    PE1PAL Ham Member QRZ Page

    inverted L 160meter and bad swr

    Hello, I have made an inverted L for 160 meter with ground network.
    I feed the antenne directly with 50ohm coax.
    On 1/4 dip the swr is around 2:1
    When I disconnected the ground, I receive a swr 1:1 !
    The vertical is 12 meters and horizontal 28meter (second high =4meters)


    Any suggestions or experience?
    73 Paul -PE1PAL
     
    Last edited: Nov 30, 2009
  10. G4LNA

    G4LNA Ham Member QRZ Page

    As most of the radiation will be in the vertical section, then the metalwork of the tower will have to be modelled into any calculation. I have found this can have a significant effect on the feed impedance radiation pattern. I have two metal masts and both of them have a marked effect when the masts are included in any of the modelling programs, you just cannot get the vertical section far enough away from the tower or mast so doesn't become part of the radiating element on 160.

    I notice one of the local BC antennas, they are using a top loaded vertical in the form of a Marconi "T" with tall wooden telegraph poles supporting it so as to not interfere with the radiation pattern,
     
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