Mobile antennas for NVIS at 80 meters

Discussion in 'Mobile Radio Systems' started by N0IOP, Oct 9, 2019.

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  1. KX4O

    KX4O Ham Member QRZ Page

    I often wonder where NVIS gets this mystical notion. This isn't rocket science.

    You said earlier...
    All it takes is just one example to disprove this absolute statement and many folks have many examples from direct experience and hard data.
    • We know a ground mounted vertical over a nice conductive screen will have the least NVIS performance... but not quite none.
    • Yeah sure an antenna designed specifically to launch straight up (horizontal dipole or STL) is going to be much better that a mobile vertical, but that null over the mobile vertical isn't as deep or narrow as idealized circumstances might suggest thanks to all the other currents running around the vehicle body.
    I'm kind of surprised at you Alan. It's your inspirations that have helped folks squeeze every little bit of efficiency from an HF mobile install... especially for the lower bands. People apply them and, behold, they are doing short hop skywave on applicable NVIS wavelengths. From what we see, the radio circuit behavior is such the antenna system inefficiencies aren't enough to prevent rather booming signals.

    Some folks arrange small transmitting loops on their vehicles for state QSO parties to maximize the skywave. They work quite well, but we find time and again, those with the larger, beefier, Scorpion-like screwdrivers (installed with your thoughts and ideas Alan) simply have no problem working the entire state on, of late, 80m for almost the entire event. The circuit viability fluctuates perfectly in sync with the ebb and flow all the fixed stations experience on their low horizontal dipoles or 1 wavelength horizontal loops. Put simply, if you are a fixed station working the VAQP and want to reach the HF mobiles with 40 and 80m, a skywave specific antenna is a must because that's where the mobiles, inefficient though they are, can be worked... in the case of W4GO in my article, with astounding margin.

    In summary, there are technically better ways to work NVIS on your mobile than a good ole' screwdriver, but if you pay attention to Alan's installation notes, a screwdriver will provide "good enough" service for this flavor of propagation providing a good return on the investment in money and hassle.
  2. K0BG

    K0BG Ham Member QRZ Page

    There is NO WAY you can bend a whip—in any direction—to achieve NVIS. Yes, the pattern is distorted. If you want to see how much, use EZNEC. All of this dates back to a flawed, and since debunked, Signal Corps document.
  3. WA4SIX

    WA4SIX Ham Member QRZ Page

    So, a STL in the bed of a pickup will not radiate vertically?
    Been there, done that. It works.

    KK4NSF likes this.
  4. KX4O

    KX4O Ham Member QRZ Page

    Yes quite well.
    KK4NSF and WA4SIX like this.
  5. KX4O

    KX4O Ham Member QRZ Page

    I should qualify my rather terse answer.

    If you can tolerate the high-Q, hence delicate tuning and quite narrow bandwidth (not necessarily easy), and orient the loop like this...
    [​IMG] then have the pattern of a horizontal dipole (with electrically vertical polarization interestingly) maximizing skyward radiation. Hamsexy baby. A few intrepid mobileers have put these on trailers and tow them behind during their VAQP treks.

    As Alan reminds us, this is only sensible for the low HF bands if NVIS is your goal.

    IMO the STL mobile options are more trouble than an optimized screwdriver given actual measured successes along with the relative ease of operating the screwdriver.
    KK4NSF likes this.
  6. K0BG

    K0BG Ham Member QRZ Page

    Yes, but loops like the one shown, have no appreciable high angle radiation.
  7. W5LZ

    W5LZ Ham Member QRZ Page

    ... if there's such a thing as a NVIS antenna, is there also a NHIS type antenna? (NHIS = near horizontal incident signal.)
  8. K0BG

    K0BG Ham Member QRZ Page

    No one said there wasn't a NVIS antenna. We're speaking of "mobile" antennas, and there is no viable way to generate NVIS with a mobile in motion installation.
  9. KX4O

    KX4O Ham Member QRZ Page

    Please cite a study that confirms your statement. NEC simulations will show plenty of high angle radiation.

    This study confirms via simulation and measurement the patterns from both a loop structure arranged as the previous photo and the vertical whip...

    The application of characteristic-mode techniques to vehicle-mounted NVIS antennas

    It also confirms the STL bests the vertical whip because of the whip's null, but also confirms both do indeed support NVIS communications.

    I look forward to your scholarship evidence to the contrary.
    KK4NSF and AK5B like this.
  10. AK5B

    AK5B Ham Member QRZ Page

    Roger that; that's part of the deal with STLs---they "spray" RF at all angles (including straight up and out low to the horizon) which is why they often excel at dx arriving at angles other than optimum for a given horizontal antenna at a particular height above ground).

    In a nutshell, they radiate and receive low, mid and high angle NVIS all at once. Think of half a donut sitting up vertically...
    KK4NSF and KX4O like this.

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