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NVIS Ant for qrp

Discussion in 'QRP Corner' started by KD2RON, Mar 6, 2017.

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  1. W4OP

    W4OP Ham Member QRZ Page

    As K8JD noted, you are warming up the earthworms with NVIS that low. If running QRP why waste precious power into the ground. Get the antena up a quarer wavelength- still NVIS and less ground loss.

    Dale W4OP
     
    KA0HCP likes this.
  2. KB7NRN

    KB7NRN Ham Member QRZ Page

    But I like warm earth worms!
     
  3. KD2RON

    KD2RON Ham Member QRZ Page

    Was this a somple inverted V with a support in the center with coax to the FT 817? Did you use any "refector" wires on the ground ? Any reference where the military NVIS specs were? Did you use a balum or tuner? Thanks for your post.
     
  4. N8AFT

    N8AFT Subscriber QRZ Page

    K8JD; I agree totally. Seems some folks are way off with getting too near to the ground with the radiators.
    Signal Corps in Nam did the HF NVIS study with documentation still accessible from I believe W8JI's web site.
    Interesting to note; my close-in 40m operating is generally much better after switching to my ground mounted vertical, food for thought.
    Try it next time you wanna operate daylight or later HF locally or in town.
    VY 73 lane n8aft
     
  5. KF5FEI

    KF5FEI Ham Member QRZ Page

    I'm assuming this was directed at me. Here is the original design:

    http://www.qsl.net/ash_ares/Portable NVIS LG.jpg

    We built one to size, and then it dawned on us it was not resonant on the ham bands. We kept the same overall wire / guy length at 45', but trimmed one set of wires to be resonant on 20m, and one set on 40m. No reflector wires on the ground.

    We've pretty much worked the whole state of Texas on 40m from various parks around the Dallas / Fort Worth area with the 817. The 20m part works OK for the most part, although it is a bit low. Someone else has a variation of this design that covers, 80 / 40 / 20 / 15.
     
  6. KD2RON

    KD2RON Ham Member QRZ Page

    Thanks for the information. I had go luck with my 7" dipole with three ground reflectors to about 200mi.
     
  7. W4OP

    W4OP Ham Member QRZ Page

    KD2RON:
    Any reference where the military NVIS specs were? Did you use a balum or tuner? Thanks for your post.

    Let's try to get rid of the nonexistant word Balum. This came out of the CB lingo.
    The word is BAlancedUNbalanced=BALUN.
    Dale W4OP
     
  8. K8JD

    K8JD Ham Member QRZ Page

    ARES Portable NVIS Ant
    Kind of a strange antenna one dipole for 6.1 MHz and one for 10 MHz ?
     
    Last edited: Apr 13, 2017
  9. K8JD

    K8JD Ham Member QRZ Page

    Usually the best NVIS is with a HW dipole or FW loop up .15 to .25 Waves, to get a boost from ground reflections like a reflector element on a Yagi antenna.
    With a 40M dipole at 7 ft high, that's a ground reflector .o56 wavelengths, not an effective spacing for a reflector. It's more like having a big capacitor to ground across the dipole.

    Maybe Exeption to the Rule?
    Using a ground mounted vertical for close in work must be within DIRECT wave VERY close range (within less than about 20 miles), not any Sky Wave distances. A properly installed and tuned vertical should have a lower angle of launch , not suited to NVIS work.
     
    Last edited: Apr 13, 2017
  10. AD5KO

    AD5KO Ham Member QRZ Page

    I used a 240ft NVIS antenna with an FT-817, got very good reports. I now use a 240ft NVIS antenna with an FT-847 at 100 watts sometimes, not much different. The antenna was modeled using 4NEC2, it's just a OCF dipole with a 4:1 balun, it works on HF and 6m.

    [​IMG]
     
    Last edited: Apr 13, 2017

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