NVIS Screwdriver antenna

Discussion in 'Mobile Radio Systems' started by NT5CC, Sep 11, 2020.

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  1. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Ok, like we see in classical army photos on a jeep or tank where the whip is bent back and tied down for clearance purposes.

    Well, yes it would probably be better than a vertical whip. However, keep in mind that what you are really creating is an Inverted L antenna.

    Modern antenna analyzer programs have made it quite clear that Inverted L's radiate primarily from the short vertical portion and NOT from the longer horizontal element. Their pattern is still primarily low angle, vertically polarized. Not optimum for NVIS propagation.

    However, with a little tinkering, you could play with various configurations and see if it would improve YOUR particular setup. I don't want to discourage your ideas!

    Technicians approach (practical)
    -Bend your antenna back
    -Or, twist on some wire to the tip of your whip and run it horizontally.
    Compare signal strenghts on Tx or Rx.

    Engineers' approach (theoretical)
    -Use EZNEC or other program to model the original screwdriver ant.
    -Model various configurations modified, e.g. bent back, wire horizontal extension of various lengths
    -Compare! No need to "Debate" performance when you have hard numbers!
     
    AA5LS likes this.
  2. NT5CC

    NT5CC XML Subscriber QRZ Page

    I see now I should have been much more precise in my question...

    The idea is to bend the whip thus changing the takeoff angle, facilitating an NVIS signal for moving mobile (as opposed to portable) ops. Folding the whip of a screwdriver without something to relieve the strain would place undue stress on the coil/tuning motor resulting in rapid failure. A spring placed atop the coil might provide sufficient relief, if not tuned excessively. The obvious questions would be, "Can the tuning system handle the additional electrical length of the spring addition?" and "Will a light duty spring provide sufficient strain relief to prevent early tuning motor failure?"
     
  3. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Cross posted with you. See my edited reply above. b.
     
  4. NT5CC

    NT5CC XML Subscriber QRZ Page

    Military antennae systems where the whip is bent and tied off are not for "clearance" but for NVIS ops purposes. Folding over a long whip is a long proven NVIS practice. My concern is the ability to tune out the extra electrical length and undue strain on the tuning motor/moving parts.
     
    ND6M likes this.
  5. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Mobile whip antennas have been tied back for over 80 years: long before NVIS propagation was ever invented. They are STILL tied back for clearance purposes, when needed.

    ...wanders off......
     
    WG7X likes this.
  6. NT5CC

    NT5CC XML Subscriber QRZ Page

    NVIS was invented? Hmmm... And yes, I suppose military whips are occasionally tied back for clearance purposes but in almost all instances, the photos you see are in wide open spaces (I give you "The Sandbox" and they are positioned in the horizontal position to facilitate regional comms, ie NVIS..
    Being a military equipment buff, this is something you should know.

    ...wanders off...
     
  7. NT5CC

    NT5CC XML Subscriber QRZ Page

  8. WA4SIX

    WA4SIX Ham Member QRZ Page

    An STL in the bed of a pickup works well. 2 turn 7/8" heliax @ 7' 6" dia.
    It was ratchet strapped in place.

    Ed
     
  9. NT5CC

    NT5CC XML Subscriber QRZ Page

    I give up...
     
  10. NT5CC

    NT5CC XML Subscriber QRZ Page

    Yes, that probably would work well but doesn't address my question. Thanks anyway.
     
    WA4SIX likes this.

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