Non-Friendly / Non-Permissive Environment: HF Operation

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KI5QMP, Sep 14, 2021.

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  1. KI5QMP

    KI5QMP Ham Member QRZ Page

    Hey guys,
    I just put together a half-wave 80mm antenna together with a 1:1 balun and my absolutely gorgeous IC-7300. I recently. Unfortunately, my antenna is at about 5' in height (60' is completely out of the question) and based on my SWR tests, I'm thinking that I may have shortened the antenna to have very low SWR between 3.850 MHz - 3.950 MHz. Does anyone have any recommendations for operations at this level?

    Also, could be interested in different kind of antenna that could operate both NVIS and other.

    Thanks in advance. I'm near the Waco, TX area if anyone wants to connect or help me troubleshoot.
     
  2. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    -Hoping that 80mm is actually 80meters. :)

    -Below about 1/2 wavelength any dipole will have a pattern that is horizontally Omnidirectional and vertically favor high angles.

    -So.... you can expect that your dipole will favor Regional communications, generally. But that does not rule out "DX" beyond say 300 miles at night when the band 'Goes long".

    -Work what ya' got. Continue to learn about propagation; work to improve your antenna(s) in whatever way you can.

    -Consider some sort of vertical as a next step. Depending on your limitations, perhaps a telescoping pole mounted on a tripod that can be easily deployed at night with a few radials, so as not to draw the attention of neighbors.

    -At my townhouse I used a fixed install dipole (open loop) for transmitting and a Pixel 1B active loop antenna on a tripod for receiving placed on my sidewalk at night, to hide it from neighbors. If asked, I told the inquisitive it was for receiving Hockey games or Basketball from a distance city!

    "Where there is a will, there is a way to get on the air".
     
    WB5YUZ, SWL37632, UT7UX and 2 others like this.
  3. AK5B

    AK5B Ham Member QRZ Page

    Suggestions:

    1) Install a vertical such as a Butternut HF2V or Hustler BTV series with plenty (30-60) of radials. If a HOA or other impediment exists, get out some flat paint and camouflage it so it blends into your surroundings.

    2) If #1 isn't possible, take up basket weaving or stamp collecting.

    Operating 80 meters with a horizontal dipole 5 feet agl is just too sad to even think about (even for NVIS that is still too low!).:(

    GL & 73,

    Jeff
     
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  4. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    The best antenna is one that is in the air and on air.
    But 5 foot is barely in the air.

    73 from,
    The K0UO " Rhombic Antenna Farm" 2 miles of wire in the Air & On the daily
     
    WA7F, WB5YUZ, K9UR and 5 others like this.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    NVIS antenna height on 80m is about thirty feet above ground.

    I'd try a different band and not 80 meters.
     
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  6. AA5MT

    AA5MT Ham Member QRZ Page

    I have a qsl from a guy using the Mfj portable antenna which has a 54 inch whip on it. I remember him well since I received his qsl card about a week before getting my Qst mag. The qsl card was printed in Qst up front section, congratulating him on getting WAS with the antenna. He had the whip and tuning box on his desk beside the radio, with the twenty feet or so of coax coiled up, under the desk.

    It can be done. The box does not do 80 meters though. I worked him on 40.

    Tom
     
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    First, here are the required lengths L (ft) for resonance (close, but not lowest Swr50) for bare copper at 5ft agl over average dirt at various 80/75m frequencies. This would be a few % different if the wire is insulated, or if the wire is laying on a fence or other dielectic:
    upload_2021-9-13_21-7-55.png

    Suppose we pick L = 120ft. Here is the Swr50 that the radio would see if the antenna has 50ft of RG-8X coax and a 1:1 balun at the feedpoint:
    upload_2021-9-13_21-14-38.png

    Now the bad news: @3.9Mhz (lowest Swr frequency), due primarily to its low height, this antenna radiates only 6% of the power fed to it, so 6W if you transmit 100W. This is -12dB, so it will be ~2 S-units below a dipole that is 65 ft agl...

    Here is the elevation pattern broadside to the wire:
    upload_2021-9-13_21-28-43.png
    Regional signals arrive from an angle of 30-45degrees elevation, where the pattern is about another S-unit down from the peak radiation at 90degrees (overhead).
     
  8. KI8DJ

    KI8DJ Ham Member QRZ Page

    Trim it match up on 40 meters. It will do a little better and you can use internal tuner to match 15m which should be fairly effective as it is so much higher in frequency there should be considerably less ground absorption.
     
  9. W4HWD

    W4HWD Premium Subscriber QRZ Page

    40mm is open to Asia in the early morning hours. At that height your antenna is 1500mm above ground so you should be reeling in the DX, though nowhere near as good as a loaded gutter - that's the gold standard these days...:confused:
     
    Last edited: Sep 14, 2021
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  10. NN4RH

    NN4RH Premium Subscriber QRZ Page

    I've made QRP CW contacts on 80 meters with a dipole as low as 8 feet off the ground. Field Day experiment several years ago. I just wanted to see how low I could go.

    I would not call it "NVIS" ... just "dang low".

    So your 80 meter dipole could "work" sort-of at 5 feet high but it would be very limited. You really ought to get it a lot higher if at all possible. Or as someone suggested consider a different band.
     

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