Mobile antennas for NVIS at 80 meters

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

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  1. N0IOP

    N0IOP Ham Member QRZ Page

    I am thinking of experimenting with mobile NVIS.

    I would like to install a long whip on my truck, with the tip secured down so that the whip forms a curve. It would be center loaded. Goal of getting reasonably close to unity gain, say, -3 dBi or better.

    I have seen Texas Bugcatchers and the Tarheel and other screwdriver antennas. Not what I want. I can use a longer whip because I'm tying the end down, and want something that is lighter weight and more flexible.

    Are there any relevant commercial products?

    Are there any good building blocks that can be purchased?

    I understand all the electrical theory, the challenge here is finding something that is strong, compact, lightweight, so it will go down the road at 75 miles per hour.
     
  2. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Forget it! There is no viable way to achieve NVIS from a mobile installation, regardless of what you read to the contrary.

    NVIS is a VERY special case, which in fact, seldom occurs on 80 meters. Fact is, most amateurs who think they've achieved it, are really relying on ground wave propagation. If you want to know how to do it correctly, read Robert Zavrel's (W7SX) book on antennas.
     
    WA8FOZ, K0UO and NH7RO like this.
  3. N0IOP

    N0IOP Ham Member QRZ Page

    I am curious, K0BG. Why do you believe there is no viable way to achieve NVIS from a mobile? Is it because you believe that it is not possible to build an antenna with a pattern that delivers enough signal skyward? Or because you believe that it is not possible to build an antenna with enough radiation efficiency? These would seem to be the two barriers. Or is there some other problem that I'm not aware of?

    I am aware of the techniques for fixed (including temporary "portable") installations, which presumably is what you refer to in W7SX's book.
     
  4. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    that it is not possible to build an antenna with a pattern that delivers enough signal skyward?

    Precisely! As I said, NVIS is a VERY (!!) special case of propagation. Few amateurs really understand the phenomena, and even fewer know how to accomplish it. It requires "DIPOLES" mounted very close to the electrical ground (image plane), and is highly dependent on the MUF (Maximum Usable Frequency). As the MUF goes up, the NVIS frequency goes down! In any case, seldom does NVIS capability exceed 5 MHz, and NEVER goes over about 8 MHz, under the best of conditions.

    Antennas for NVIS, consist of dipoles just a few feet over the electrical ground. The directors are always 20% to 30% shorter than the driven elements, and even the driven elements are rather short compared to the free-space length—perhaps less than 70% of their length. There is no way you could do that in a mobile, unless you were parked, and erected the necessary hardware.

    This is another case where it is easier to believe the myth, than the reality. That, and the inability to distinguish between ground wave, and NVIS. There is so much of this sort of pervasive mythology, it is all but impossible to dispel it.
     
    WA8FOZ, K0UO and NH7RO like this.
  5. KL7AJ

    KL7AJ Ham Member QRZ Page

    There is a fairly reliable means of determining if it's groundwave or skywave. If it's groundwave, the signal strength should be the same, day or night...if it goes away during the day, it's probably NVIS.
     
    K8AI and NH7RO like this.
  6. KL7AJ

    KL7AJ Ham Member QRZ Page

    The assumption here is that the D-layer absorption is dominant, which for NVIS frequencies is fairly certain.
     
    NH7RO likes this.
  7. KL7AJ

    KL7AJ Ham Member QRZ Page

  8. KL7AJ

    KL7AJ Ham Member QRZ Page

    NH7RO likes this.
  9. N0IOP

    N0IOP Ham Member QRZ Page

    Is that the mobile version or the portable version?
     
    NH7RO and KI7SYG like this.
  10. KL7AJ

    KL7AJ Ham Member QRZ Page

    I would call it a permanent mobile installation. :)

    NVIS usually has a rather small frequency window....you need to be below the critical frequency....usually between 4-5 MHz, depending on your location, and ABOVE the D layer absorption (Minimum usable frequency), which is between .5 and 2.5 MHz, most of the time. So it's not really great for the wee hours OR broad daylight....it's during the twilight zone that it works best.

    The reason HAARP can work well above the critical frequency is that there's enough power to wobble ions directly....that's a whole nother kind of science. At amateur power levels, its the free electrons alone that are the active ingredients.
     
    NH7RO, KX4O and N0IOP like this.

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