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Elevated verticals - how low can you go?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by M0AGP, Jun 23, 2021.

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  1. M0AGP

    M0AGP Ham Member QRZ Page

    I'm thinking of taking advantage of a very sturdy concrete fencepost and a long wooden fence to put up a 80+160 "fan vertical" with 2 elevated radials each. The radials would be stood off from the wooden fence using something sensible like electric fence parts. The fence itself is about 7 feet high.

    I was thinking about having radials at a height of something like 5 feet above ground and then either a fiberglass or PVC support for each vertical element which I plan to linear-load.

    One person who I know has an elevated vertical for 160 has his 2 radials about 25 feet up - he thought my 5 foot elevation might be too low. Is he right?

    In this famous reference from N6LF:

    https://rudys.typepad.com/files/elevated-ground-systems-article-final-version.pdf

    A very helpful plot is this one, which shows dB gain (versus something) as a function of height for a 4 radial elevated vertical operating at 7.2 MHz:
    upload_2021-6-23_19-47-45.png
    So at 6 inches off the ground, you lose a dB at 40m. Scaling naively, that means at 2 feet on 160m you would lose a dB.

    Am I right in expecting that at 4-5 feet AGL my linear-loaded vertical would work much better than a ground mounted version (referring to ground losses)?

    I expect to find a very low radiation resistance for each of these fan verticals (ballpark 30 foot height), so depending on what I measure as the R value for each, I would use a (1:4 or 1:N ?) balun at the base of each.

    Safety: there are no small children or medium sized animals in this closed-off back yard.

    Thoughts please?
     
    AK5B likes this.
  2. W1VT

    W1VT Ham Member QRZ Page

    I have my radials at 5 ft. I am short so the low height isn't an issue. I use the same radials on 80 and 160. I have worked 205 countries on 80CW and 104 countries on 160CW in the last decade using it.

    I have wet clay soil in my back yard. When it rains the water ponds near the center. I've seen ducks swimming across my yard. I missed it, but my neighbor took apicture of a Blue Heron standing on my roof!

    Zak W1VT
     
    Last edited: Jun 23, 2021
    WB0PYP, N5YPJ, M0AGP and 1 other person like this.
  3. US7IGN

    US7IGN Ham Member QRZ Page

    People use two concepts - some raise 4-8 radials as high as possible and change their length to tune the antenna into resonance, others bury a lot of radials in the ground to improve its quality. Everyone does it differently because the soil and its conductivity are different.
     
    M0AGP likes this.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    I think Rudy was smoking something when he made that plot.

    That behavior is not supported by a simple simulation:
    upload_2021-6-23_12-28-57.png

    L is the length (ft) of the monopole required to bring the antenna to resonance. There are four 0.25wl (34.4ft) radials at a height G (ft) above average dirt. Here is the plot of Max Gain vs radial height G:
    upload_2021-6-23_12-32-27.png
    The shape is similar, but not the huge gains... I see a gain of ~1.2dB going from GL to 20ft agl.

    That gain is evident in the elevation patterns:
    upload_2021-6-23_12-38-54.png

    I remain to be convinced. Maybe he was measuring gains along the ground using a FSM. He wouldn't be the first that got fooled by that.
     
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  5. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    WA7ARK Graph is probably about right/ Good info.

    I did a lot of field testing on 40 m verticals and I never could see any significant changes after about 12 to 15 feet.
    But lower the gain suffered/ (tuning was harder and it did change), and course it was all depending on the dirt

    My 160 meter 4 sq has them at 16 feet which is low/ but it was a matter of practicalities.
     
    Last edited: Jun 23, 2021
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  6. AK5B

    AK5B Ham Member QRZ Page

    "Elevated verticals -- how low can you go?"

    As low as you want or need to at the sacrifice of gain and radiation angle, I suppose. I also suspect Rudy's gain figures are overly optimistic, too---but regardless of how little benefit one derives from a few feet of elevation---remember that even a small increase in height above ground puts your signal over any adjacent ground clutter and also reduces the angle of radiation slightly.

    I am of the opinion that every little bit can't hurt---and it is often a lot easier and simpler to install 2, 4 or 8 radials a few feet above ground than 32, 64 or 128 radials in or on the ground. Do all you can to reduce ground loss and you will likely benefit wisely.

    Besides Zack's dx achievements mentioned above, I know that Carl, @KM1H also swears by his 160 meter vertical whose radials are only a foot or so above his rocky New Hamshire ground. Anecdotal as they may be, these reports are inspiring enough to warrant trying out your plan firsthand and see how it goes.

    I am truly interested in hearing about your results when you do---and I have a hunch that there is a better chance of you being mildly pleased as opposed to mildly disappointed.

    73,

    Jeff
     
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  7. M0AGP

    M0AGP Ham Member QRZ Page

    Thanks all, especially Mike for that very interesting calculation! It sounds like theory and experiment both indicate this should work ok.

    In particular I expect it will work a heck of a lot better than my 6BTV trap vertical with 16 X 16 foot ground radials and with an 18 foot piece of wire clipped to the top of the 80m resonator - and which terminated 18 inches from a tree… (that last bit really didn’t help!)

    As this is an autumn/winter antenna I’ll be taking my time building it - will let the group know how it goes.

    Any thoughts on linear loaded vertical construction? I’m aware of the recent thread and a few links.

    How high can one go with guyed PVC - 30 feet? I suppose the wider the diameter, the higher…

    And I wonder how much efficiency I might gain by adding 2 wires to the top to act as a cap hat…?
     
    Last edited: Jun 23, 2021
  8. AK5B

    AK5B Ham Member QRZ Page

    PVC is a lousy support for anything above a few feet. I wouldn't even attempt guying PVC (i.e., wet noodle or fat wet noodle (large diameter PVC)) Go with fiberglass or wood or a line from a distant tree if you can.

    Cap hats never hurt, practically speaking. They raise the current node so it's also worth a try (maybe after the first iteration so you can have a comparison of sorts).

    73,

    Jeff (also looking forward to next winter)
     
    M0AGP likes this.
  9. KK4OBI

    KK4OBI Ham Member QRZ Page

    Some general elevation considerations about a vertical radiator with two horizontal elements:

    - Tuning is essentially unchanged with a feed point 1/4 wavelength or higher. Here the antenna has unity gain... rising when elevation is higher... becoming negative when lower.

    - At 1/4 wavelength the Vertical/(Vert. + Radial) ratio is around 0.7 for lowest SWR50.

    - At 3/16 wavelength the ratio is 0.68; gain -0.6 dBi. At 1/8 WL the ratio is 0.66: gain -1.2 dBi. At 1/16 WL the ratio is 0.625: gain -1.8 dBi. At 1/32 WL the ratio is around 0.58: gain -2.2 dBi.

    - For SWR, lowering the radials below horizontal raises impedance or visa versa.
     
  10. M0AGP

    M0AGP Ham Member QRZ Page

    Thanks for that - I expect an efficiency sacrifice for sure, but not too concerned about the SWR due to the low radiation resistance: I plan to use a balun built for high currents to match the impedance (approximately) to 50 ohms.

    And I just remembered someone gave me a copy of ON4UN’s book on low band DXing - time for some reading…
     

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