Looking for models of verticals with small radial angles

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W0RIO, May 24, 2019.

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  1. W0RIO

    W0RIO Ham Member QRZ Page

    Now that the snow is finally melting around here, I'm looking into making a 10 meter vertical antenna that
    can be easily hung from a tree and held vertical by the weight of the feedline. I'm thinking of making
    a spreader at the bottom of the radials out of plastic pipe to keep the radials, feedline and choke in place.

    KK4OBI has an excellent article here: https://www.qsl.net/kk4obi/Elevated Radials.html
    where he models a vertical antenna with radials at 20 degrees from vertical,
    it has an impedance of 50 ohms with a vertical/radial ratio of 0.522 and a
    resonant length that is 93.3% of a dipole.

    That's very close to what I had in mind, but not quite a low enough angle on the radials
    to make it easy to hang from the top. I'm thinking that an angle of 3 to 5 degrees would be
    a good mechanical design.

    Too bad (for me) that he didn't model the radiation pattern as the angle of the radials was
    stepped from (say) 90 degrees down to 0 degrees, and at the same time,
    the ratio of vertical/radial length for a 50 ohm feedline impedance at those angles.

    Going to the limit of zero degrees on the radial angle, the "vertical bazooka" design
    uses 1/4 wave of feedline coax wrapped back over the feedline with a choke at the
    bottom of the foldback part.

    Of note, the 2 meter AEA Isopole design is similar to what I want to build, but has too many levels
    of radials for a practical 10 meter antenna in my location.

    So, to the root of the question, is there any advantage of having the radials fan out at the
    bottom vs the straight bazooka vertical design, and what happens to the vertical/radial ratio for 50 ohms as
    the angle goes to zero?

    I'm thinking of making a test antenna that I can connect my MFJ459 analyzer to,
    trimming the vertical element for a 50 ohm match, then scaling the final results for the 10 meter band.
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Here's a quick look at an antenna like that with different radial angles:

    As always, these are clear field models that assume no objects like houses or even the tree you'll hang this from nearby so don't take the patterns too literally but look how they do or don't vary. These aren't optimized beyond adjusting the radials at each spreading to maintain low SWR across the band.

    With the base of the 4 radials spread 6 inches from the antenna center axis:
    upload_2019-5-24_12-39-45.png ===+===


    With the base of the radials spread 1 foot from the antenna wire center axis:

    upload_2019-5-24_12-31-5.png

    With the base of the radials spread 2' from the antenna center axis:

    upload_2019-5-24_12-25-32.png

    With the base of the radials spread 3' from the center axis of the antenna:
    upload_2019-5-24_12-34-41.png

    With the base of the radials spread 4' from the antenna center axis:

    upload_2019-5-24_12-37-23.png

    Bottom line, across a range of fairly steep radial angles it doesn't make a big difference from an antenna matching standpoint or from a clear field pattern standpoint. I haven't added a transmission line nor looked at possible common mode coupling to that line which obviously becomes an issue if you take this to the extreme and instead of building a coaxial dipole style antenna with an outer 'radial shell' you go all the way to a vertical dipole with coax running downwards from the center.

    This won't cost much to build nor take much time. I'd just go ahead and build it in a way that works for you with at least a few inches of radial spread at the base and short insulated spreaders. Then test it and see if it fits your needs.
     
    W0RIO likes this.
  3. W0RIO

    W0RIO Ham Member QRZ Page

    Wow, thanks for the instant analysis.

    IW5EDI has an interesting page on the Vertical Bazooka here:
    http://www.iw5edi.com/ham-radio/?the-vertical-bazooka-antenna,18
    That's the limiting 0 degree radial angle case of the dipole with radials.

    He cautions about having anything near the ends of the antenna, including the bottom of the "radial" coax braid,
    which would be a high voltage/high impedance point. I'm wondering if the Q of the antenna drops a lot when the
    radials get very close to the feedline. He reports a fairly wide bandwidth and not-great SWR results, which sounds to
    me like a low-Q antenna.
     
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Yeah, for a wire antenna like you described I'd probably stay away from the pure bazooka sleeve design and have a bit of slope in the radials. Seems like there's lots of options for radial spreaders like a hula hoop if you want a wider spread or something like a 1' disc of plastic if you want something real narrow.
     
  5. W0RIO

    W0RIO Ham Member QRZ Page

    I was thinking of using a 1' loop of PEX tubing for the bottom loop, or possibly a plastic top from a plastic paint bucket,
    although that would be more likely to catch the wind and move around. A simple X made of PVC pipe would also work
    as a spreader for a 4 radial antenna.

    I was wondering if I'm interpreting the wire lengths from the EZNEC+ output correctly.
    It looks like the feedpoint is at 20' above the ground, the vertical element is 8.2' and
    the radials are 11.8' ignoring the lengthening effect of the slight angle on the radials.
    The radial/vertical length ratio would be about 0.7.

    Using the normal 468/F half wave dipole measurement formula, at 28.2 Mhz, a dipole
    would be 16.6' long, compared to the 20' total height for this model.
    Am I on the right track here?

    Lastly, the impedance shows Z=45.05-J25.82, is that reactive enough to require a matching
    network at the feedpoint? If I recall the theory, would that be a series inductor?
     
  6. K7TRF

    K7TRF Premium Subscriber QRZ Page

    You might want to check your math. The feed point of the modeled antenna is at 20 feet. The main element is 8.2 feet long and the radials shown have a vertical height in the range of 6.6' to 7.1'. So the overall antenna ranges from roughly 14.8' to 15.3' tall. I'm not sure where your 11.8' length for the radials came from but the vertical height of the radials is (20-h) feet where h is the Z axis height of the bottom of each radial. So the actual wire length of the radials works out to: sqrt((20-12.9)^2 + 0.5^2) for the first case of 6" bottom spacing on the radials.

    As mentioned previously I did little to no optimization on this model. I picked the main radiator to be approximately 1/4 wavelength in the band, set the radial lower end spacing and adjusted the height of all four radials the same amount to get reasonable SWR in band. You can find a good match with various combinations of main element length and radial lengths but I didn't spend any time trying to find an optimal solution, just one that worked. That's really not terribly different than what you'd do in the field when tuning this antenna but you'd likely pick radial lengths, perhaps what are shown in the model and then tune the length of the main radiating element to achieve a match.

    For a wire antenna I'd choose radials that are similar in total length to what's shown in the model, cut the wire for the main element a bit longer than you expect and fold excess back through the top insulator that you intend to hang from the trees then either lengthen or shorten that main element as necessary for tuning purposes. Hanging this in close proximity to a tree as you proposed will likely alter the tuning a bit anyway so build the antenna with the intention of field tuning it.
     
  7. W0RIO

    W0RIO Ham Member QRZ Page

    I always need to check my math ;-) I get 7.1' for the radial lengths on the narrowest model this time,
    I'm not sure how I came up with the other length.

    Thanks for all of the info,
    its time to start cutting some wire.
     
  8. W0RIO

    W0RIO Ham Member QRZ Page

    I managed to build a test version of the antenna, it is hanging from a telescoping PVC "wonder pole"
    between some trees, I used RG58 as the element wires and RG8 as the feedline.
    The radial spreader is made from 3/4" pvc pipe.
    The choke coil is 4 turns of RG8, which I based on a chart of recommended coax chokes
    done by G3TXQ.

    The radials could be straighter but they stay away from the feedline at the
    bottom. The mechanics could definitely be improved, but this is only rev 1.

    After trimming the vertical part, the antenna resonated at 28.3 Mhz
    and with an approximately 40' length of RG8 feedline, my MFJ459 analyzer shows
    a 75 ohm impedance and good swr from about 28 to 28.6 mhz.

    The vertical/radials ratio is 1.12, the KK4OBI models mentioned above seem to indicate
    that a longer vertical and shorter radials would make a better match for a 50 ohm feedline.


    00006.jpg

    Here's the center piece of the antenna, there's an SO239 connector inside.
    Rev 2 will probably use a bigger cap and screws on the sides to avoid metal fatigue issues.
    00005.jpg
     
    K7TRF likes this.
  9. W0RIO

    W0RIO Ham Member QRZ Page

    I did some more experimenting with my narrow radial angle vertical antenna for 10 meters.
    The version shown above had a radial angle of around 3.9 degrees from vertical, it tuned up ok,
    but as can be seen in the above photos, the tips of the radials were too close to the feedline for
    a good mechanical design.

    This time I decided to try the 20 degree radial angle and 0.6 radial/vertical length ratio that the KK4OBI
    models mentioned.

    My first attempt started out way low in the resonant frequency at around 25 Mhz, trimming it
    caused the frequency to rise but the minimum SWR also rose unexpectedly.
    I tried changing the feedline and nothing changed.
    The antenna was positioned so that it was only a few feet from tree limbs and the limbs were
    on several sides.
    Then the thunderstorms moved in and the test was on hold.

    My next attempt involved moving the concrete base that supports the wonderpole to a clear
    portion of my yard. Now the antenna tuned up with low SWR and luckily, it was still cut too long.
    This time, as I trimmed the vertical part a couple of times the antenna resonant frequency rose and
    the SWR remained low. The MFJ analyzer showed an impedance of around 55 ohms at resonance.

    As luck would have it, 10 meters was open Sunday afternoon. I hooked up my Heathkit
    HR-1680 RX and heard beacons, WSPR signals and SSB signals coming in strong.

    I learned an important lesson about keeping the antenna in the clear. Unfortunately, my
    original goal of hanging the antenna from a tree limb seems to be mostly impractical.
    It may be possible to take the antenna into the woods and find a particular tree with
    a high branch that is in the clear, but none of my trees are shaped like that so for now,
    I'm stuck with using the temporary wonderpole setup.

    Experimenting with antennas is kind of fun.

    00003.jpg
     
    WZ7U and K7TRF like this.
  10. W0RIO

    W0RIO Ham Member QRZ Page

    Here are the final measurements for the second version of this antenna after it was
    trimmed for the lowest SWR at around 28.25 Mhz.

    Vertical: 97"
    Radials: 87-1/2"
    Lowest point of radials: 174" (14.5 feet) from ground
    Radial spread at bottom: 4' (2' from center to each radial)
    Radial angle from vertical: 19.8 degrees
    radials/vertical length ratio: 0.667

    It's interesting that KK4OBI said that his models converged at the 20 degree angle
    and the K7TRF models showed the lowest SWR for the 2' center to radial spread, which is
    also close to 20 degrees.
     

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