NVIS Screwdriver antenna

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

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  1. KX4O

    KX4O Ham Member QRZ Page

    We have available some scholarship merging both approaches concerning this topic from the IEEE. Here is a comparison of both theoretical and measured values of vertical (left) and tilted (right) whips...
    [​IMG]
    B. A. Austin and K. P. Murray, "The application of characteristic-mode techniques to vehicle-mounted NVIS antennas," in IEEE Antennas and Propagation Magazine, vol. 40, no. 1, pp. 7-21, Feb. 1998, doi: 10.1109/74.667319.

    Further discussion is here.

    There's lots more to this paper beyond the above and a good investment to anyone considering mobile HF ops.
    This is a good question. It does seem the above research suggests it won't help unfortunately.
     
    Last edited: Sep 13, 2020
  2. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    Uhhh....Not necessarily. D layer absorption can be nearly total during the day. (Depends on your location, of course)
     
    Last edited: Sep 13, 2020
  3. KL7AJ

    KL7AJ XML Subscriber QRZ Page

  4. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    Of course, as our recent experiment on 60 meters showed us....all bets are off in Alaska. :)
     
  5. NT5CC

    NT5CC XML Subscriber QRZ Page

    Also, close hops resulting in regional QSOs does not NVIS make. NVIS is a method not a result.
     
  6. NT5CC

    NT5CC XML Subscriber QRZ Page

    My concern was more toward the effect of adding a spring on the ability to tune the antenna without needing an external tuner and if a spring would give sufficient relief to prevent excessive binding on the coil as it motors up and down. What was the word used... ah yes, "technician's" point of view. On that I should have been more clear. The radiation patterns of whips in various configurations is well documented. I have seen some documentation somewhat at odds with yours though. Specifically the military seems to be rather enamored with the idea.
     
  7. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    60 meters SHOULD be ideal for NVIS most of the time but for some strange reason it invariably sucks in Alaska...even when 80 and 40 are great. More inveastgation needed
     
  8. AG6QR

    AG6QR Premium Subscriber QRZ Page

    Perhaps, but then I suspect NVIS propagation was in use long before it was invented.

    NVIS is simply near-vertical incident skywave. Any time someone communicates via skywave that strikes the ionosphere at a near-vertical angle of incidence, they're using NVIS by definition, regardless of whether they were aware of it or intending to do that.

    I would say that NVIS was discovered more than invented, though the technique of deliberately choosing antennas and bands with the intent to take advantage of NVIS propagation can be considered an invention.
     
    N0TZU and KX4O like this.
  9. KX4O

    KX4O Ham Member QRZ Page

    Roger on the mechanical aspects. I've never owned a screwdriver antenna, but many friends do. Never heard reports of binding, but can certainly see where that can be a problem under wind load. I'll ask around because now you have me interested.
    There's a difference between documentation (often only instructing on good enough approaches) and research (the attempts to actually understand the physics of what is happening). Both have their place in their respective goals. Finding scholarship on mobile NVIS that includes modeling confirmed by measurement has proven difficult. The IEEE paper is a rare one. The paper's primary motive was examining the benefits of small loops making the whip measurements a mere sideline topic of comparison. My modeling of whips at the same angles on a vehicle do agree with the findings in the paper. Tilting the whip over the vehicle merely makes it less efficient with no appreciable effect of filling that zenith null. This is especially the case where the antenna system (vehicle plus whip) are so small compared to wavelength. Still the small loop approach does fill in the zenith null completely so the antenna system and arrangement do have a role to play.

    I wish they had performed a measurement with the whip tilted away from the vehicle. It would have been nice to confirm if this "more dipoleish" arrangement would improve zenith radiation as various documents suggest.
     
  10. KF5FEI

    KF5FEI Ham Member QRZ Page

    KX4O likes this.

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