40 meter 1/4 wave vertical question.

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KB4MNG, Apr 20, 2019.

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  1. KB4MNG

    KB4MNG Ham Member QRZ Page

    I've been experimenting with a shortened vertical and ready to go to a full 1/4 vertical for 40 meters, 33 feet. I know what a capacitance hat does for a shortened vertical. Will adding a capacitance hat, say 3 ft diameter, to a 1/4 wave on 40, shorten the direct element from the 33 ft?

    If so, about how much....

    thanks
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Yes, a cap hat will make the antenna look longer electrically.

    How much longer depends on the specific design on the cap hat. You posted it will be 3 feet in diameter, is that radial spokes? If so, how many and what wire gauge? Or will it be a petal shaped design or perhaps radial spokes with a perimeter wire, maybe something like a hexagon pattern with 6 radial spokes and then a perimeter wire connecting the whole thing? Straight spokes will add less capactive loading than a hexagonal hat or a petal shaped design, basically the more surface area you emulate up there, the shorter the main antenna element will be to achieve resonance and closed shape designs emulate more surface area than straight spokes.

    FWIW, I ran a quick and dirty model of a 40m vertical made from 1.25" tubing over a field of 64 ground radials (16' long each so only about 1/8 wavelength long). Best match with and without cap hats:

    - 34 feet tall with no cap hat
    - 31.5 feet tall with six spoked cap hat made from 1/8" wire
    - 29 feet tall with spoked hexagonal cap hat (spokes plus perimeter wire) also 1/8" wire

    Actual tuning will vary with things like actual field of radials and exactly how you fabricate the cap hat as my model just has six spokes with nothing to account for the center hub and the larger that hub the more top capacitance you'll get. So don't take these results too literally but they demonstrate how adding a cap hat shortens the antenna but also how a cap hat that better emulates a large surface area adds more top loading.
     
    Last edited: Apr 20, 2019
    NH7RO, KD6RF and KU3X like this.
  3. KB4MNG

    KB4MNG Ham Member QRZ Page

    I've built the hat out of ground copper wire(large and stiff) and it resembles a wagon wheel. Thanks for this very informative post!
     
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    At what height will you have the radials? On/In ground? Elevated?
     
  5. KB4MNG

    KB4MNG Ham Member QRZ Page

    Unfortunately, ground mounted only option.
     
  6. K7TRF

    K7TRF Premium Subscriber QRZ Page

    FWIW, I ran the same model with an 8 spoked cap hat, each spoke 1.5 feet long (~3' radius) with a perimeter all made out of 1/8 inch wire. Best match occurs with the main antenna half a foot shorter than the hexagonal hat case. So a little more circular with the extra spokes, a bit more enclosed area and a bit more top loading capacitance. Here's a look at the cap hat and SWR sweep for this configuration:

    upload_2019-4-20_18-55-20.png
     
  7. W5LZ

    W5LZ Ham Member QRZ Page

    How much will it shorten the vertical? There's no set answer to that, too many variables. It will certainly do some shortening but the only sure way of finding out how much is by doing it.
     
    NH7RO likes this.
  8. W1VT

    W1VT Ham Member QRZ Page

    I have a tree supported 80M top loaded vertical. I have two wires for top loading. I just lower the antenna and adjust the length of the wires for my desired center frequency.
     
    NH7RO likes this.
  9. W9XMT

    W9XMT Ham Member QRZ Page

    To keep in mind...

    Using a top hat can greatly reduce the physical height of a vertical monopole needed for natural resonance of that antenna system (feedpoint reactance = 0 Ω), but also using a "loaded" radiator having a very short electrical height can greatly reduce its radiation resistance.

    Other things equal, the lower radiation resistance of such antenna systems produces less e-m radiation.
     
    Last edited: Apr 23, 2019
  10. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    Please explain why that is good or bad?

    New thing for me to learn.
     

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