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Height of a 102 ft doublet for 20 meters

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KD3LT, May 23, 2020 at 9:45 PM.

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  1. KD3LT

    KD3LT Ham Member QRZ Page

    I've generally thought "higher is always better" with wire antennas, but EZNEC is showing me something interesting about my 102 ft. doublet: to wit, at the current height of 45', EZNEC shows a considerable lobe straight up in the air on 20 meters. And the higher I set the height, the more energy seems to be going straight up into the air. And when I lower it to about 30' the vertical lobe diminishes from what it is at 45'.

    I also notice that when I extend my doublet to 130', I get almost no vertical component at all on 20 meters. If that's indeed the case, I may just have to borrow my neighbor's tree....

    So is higher not always better? Or is it just that I don't know how to use EZNEC properly?
     
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    Post ur .ez file here. QRZ will think it is a potentially malicious file, so rename it something like “doublet.ez.txt”
     
  3. NN4RH

    NN4RH Premium Subscriber QRZ Page

    I’m guessing (without having done the simulation ) that you’ll find there’s been some trade-off. The lower angle lobes will probably have somewhat higher maximum gain and at a lower take off angle, but may also be a bit narrower. There may be limited situations where that is useful.
     
    K7TRF likes this.
  4. W9GB

    W9GB Ham Member QRZ Page

    When you use Metric measures, it is far easier to see the “wavelength relationships
    A height of 0.685 wavelength (20 meters) close to the 0.7 wavelength plots by AA3RL (below).

    Benchmark for a Horizontal Have-Wavelength Dipole, First Install is at:
    0.5 wavelength above the ground (For 20 meter dipole, 10 meters (~32.8 feet)).

    How High should my Dipole be?
    Dipole Antennas - the Effect of Height Above Ground
    by Mike Banz, AA3RL
    https://www.qsl.net/aa3rl/ant2.html
     
    Last edited: May 23, 2020 at 11:00 PM
  5. K7TRF

    K7TRF Premium Subscriber QRZ Page

    There are heights above half a wavelength where a horizontal antenna will have some high elevation angle lobes but as pointed out in the post above usually you still get a benefit as in a lower takeoff angle to those low angle lobes. Also don't get too caught up in flat earth, clear field patterns out of the NEC tools. Real world antenna installations usually have local clutter as in trees, houses and the like. There's almost always a real world benefit to getting up above that clutter even if it means some energy goes straight up.

    What's going straight up doesn't matter from a transmission or reception standpoint on 20m as long as you still have substantial gain and power at low elevation angles of interest. Basically the elevation angles of interest on 20m will always be fairly low as in 25 degrees above the horizon or less. That some of your transmit energy is going straight up or that when receiving you have antenna gain more or less straight up isn't a problem as long as you also have decent gain at elevation angles that are relevant to ionospheric propagation on 20m which is much too high for NVIS activity. IOW, ignore what's happening at steep elevation angles and focus on whether the antenna performs as you want at the lower angles.

    BTW, if you keep playing with antenna height you'll see that if you go even higher the high angle lobes will go away but you'll start getting multiple lobes at low elevation angles.

    Here's a classic view of what happens in terms of horizontal antenna elevation gain at various heights:
    [​IMG]
    From this website: http://www.astrosurf.com/luxorion/qsl-antennas-basics3.htm

    So yeah, at 5/8 wavelength high you get a big lobe going straight up but the low angle lobes are still good but get up to a wavelength high and now you lose that high angle lobe in favor of two lower elevation angle lobes. And as you further raise the antenna there are heights that give you high angle lobes or many more elevation plane lobes but as you make it higher the takeoff angle of those lowest lobes continue to drop to lower and better angles for DX work.
     
  6. KD3LT

    KD3LT Ham Member QRZ Page

    I started from the file Backyard Dipole and went--maybe downhill?--from there.
     

    Attached Files:

  7. W1VT

    W1VT Ham Member QRZ Page

    Most hams focus on the lowest lobe in the elevation plot, since that is the radiation that "does the work" these days. It would be nice to be able to get rid of the high angle lobe, but that is a compromise that many hams have to make.

    A 130 ft dipole feed with 100 feet of open wire is also a good compromise for many bands, particularly if you don't mind feeding it with a tuner designed for open wire, like the Johnson Matchbox.

    Tom W8JI has a web page on vertically stacking antennas to get rid of the high angle lobe.
    But most hams can't afford to do that.
     
  8. KD3LT

    KD3LT Ham Member QRZ Page

    From the look of this and from the diagrams posted at the link posted by W9GB, at some heights above 1/2 wavelength, there's almost as much power going up as going out.

    My trade-off now on the 130 ft. doublet for 20 meters is that I've noticed that the high angle radiation is almost negligible at my current height (45 ft), but there deep, deep nulls off the ends and exactly perpendicular to the doublet--again, if I'm modeling reasonably accurately. May have to have the neighbor move his tree....
     
  9. KD3LT

    KD3LT Ham Member QRZ Page

    Love my Matchbox, but I'm not smart enough to modify it to work on the WARC bands. So it's a 974B for me. Otherwise, I'm (usually) very pleased with my 102ft doublet--although with propagation the way it is these days, I sometimes have to go outside and look up to see whether it fell down again or not.
     
  10. W1VT

    W1VT Ham Member QRZ Page

    On 20 meters I use three dipoles strung between trees. And a vertical in the rose garden. I prefer to use dipoles no longer than an Extended Double Zepp so I can easily figure out where they are aimed.
    The dipoles are aimed at Europe, E/W, and N/S.
     

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