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HF NVIS 101

  • 11 Jul 2017 8:21 AM
    Message # 4968034
    Chris Doering (Administrator)

    NVIS - Near Vertical Incident Skywave

    I saw this Hummer online with the antenna farm on top. It has the tell tail whip that is bent forward and this is not for going through the drive-thru. We see this on many military vehicles. This position is not for passing under bridges, but it is meant to intentionally aim a portion of the antenna wave at a high angle upward so to get a skywave to bounce off the ionosphere. This is ideal for HF communications between stations up to a few hundred miles.


    On this topic we'd like to get the basics of NVIS, i.e. antennas, frequencies, ionospheric conditions affecting skywave propogation, etc...

    Please share the basics here....

    (note) the wild apricot forum is working on making photo posting by everyone. RIght now only administrators can add photos to forum topics. In the meantime, if you have a diagram or photo that helps us understand NVIS please email it to me and I can add it to the post.

    send to ki6qbm@landops.org

    Last modified: 11 Jul 2017 8:26 AM | Chris Doering (Administrator)
  • 12 Jul 2017 9:33 AM
    Reply # 4970991 on 4968034

    NVIS doesn't really work better at night than the daytime, you can just us a different frequency.

    This link is what i use all the time http://www.sws.bom.gov.au/Images/HF%20Systems/Global%20HF/Ionospheric%20Map/WorldIMap.gif

    For example right now at 9:20am we could use up to thru 5Mhz. By tonight it will probably be only usable thru 4Mhz. So you can see that at this time in our location 7Mhz (40 meters) will probably never work for NVIS.

    So that puts us at 75/80 meters, 3.8-4.0 for voice. Since we want to work NVIS, it should be horizontally polarized which eliminates vertical antennas. A good 75 meter antenna will be over 130 feet long. A lot of people don't have room for that. I just put up a wire 135 feet long 30 feet up in the air about a month ago. I "should" be able to talk all over SoCal if the other station also has an NVIS antenna.

    Then we get into mobile. The lower the freq, the long the antenna should be. Difficult to put a 130 foot antenna horizontal on my jeep. So just like the picture you showed above, we bend over a whip to make it horizontal and because it is about 120 feet to short, we load it with a coil. Efficiency is very low, maybe 1-3 %.

    I have one set up for my jeep, just need to test to another NVIS station.

  • 12 Jul 2017 9:49 AM
    Reply # 4971004 on 4968034
    Chris Doering (Administrator)

    ok, this is a great start.

    I passed the test but this will be great hands-on application training. Yes, I am interested in operating your base HF station at the Sierra Op. Please bring it. 

    So I guess you were running DX (vertical antenna) at Truckhaven to contact Emmanuel in Nor Cal. I'm really enjoying and fascinated the possibilities. After only being on 2 meters and 70 cm and being at the mercy of repeaters only, this opens up much more geagraphy for our ops to take on a whole new op style...concurrent!

  • 13 Jul 2017 6:56 AM
    Reply # 4974454 on 4968034

    I am very interested in NVIS for mobile applications.  Just found this military antenna on Ebay. AS-2259/GR Thinking about buying it for using on our Ops use.

    I am also looking into getting my Jeep setup with a whip bent over and reflecting off the top of it.  But the to is composite so need to come up with a reflecting channel below the antenna.  Maybe a half U metal tube mounted to the bottom of the whip antenna?

     

  • 13 Jul 2017 8:11 AM
    Reply # 4974603 on 4968034

    Chris, at truckhaven I used a vertical dipole. 2 hamsticks horizontal on my vertical mast, so the polarization was horizontal. But I don't think it was NVIS, just normal HF skip. I will bring my home base. We should be able to hook the middle of the dipole to your vhf mast or even to use trees up there.

    Warren, I don't think anything is needed under a bent over whip, my jeep has a soft top! There are some that don't even believe the bent over whip will work. I wouldn't spend much money on anything until we have done more testing. We will know much more in a couple of weeks at the Sierra Op.

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