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A 2 element Fixed, Reversible Yagi for 20 Meters

Discussion in 'Amateur Radio Antennas' started by RickC., Sep 15, 2011.

  1. RickC.

    RickC. Hopeless antenna junkie

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    You may remember my homebrew 20/10 homebrew yagi that met its untimely end in an ice storm:

    [​IMG]


    It was pretty clear that a push-up pole with a rotor and full size beam was not going to be a workable situation, despite the antenna weighing only 18 lbs. The cheap steel in one section of the mast apparently fatigued from the cold and it buckled like a toothpick.

    In the months following I had given up on having a directional antenna for 20 meters , due to the following:

    1. I cannot afford a tower and the associated heavy duty rotator, etc.
    2. There’s not room for a phased vertical array.
    3. I’m hemmed in by power lines on three sides and can’t safely put up a Bobtail, EDZ, etc., to favor the 45 degree azimuth I’m most interested in.

    So I threw together a 20m ground plane and it did well for what it was, but if you’re used to a directional antenna it’s really hard to be without one, particularly on 20 meters.

    Long story short, I decided a lightweight, monoband fixed yagi may just work. The wheels started turning, and I thought, “Why not make it a reversible beam?—Europe in one direction, New Zealand, part of Australia, and parts of Oceania in the other. Hmmmmm…”

    So I did- I repaired the mast by sawing off the bent section and putting the remaining piece (now 4 feet shorter) back into the mast. The antenna itself went together like this:

    The wooden boom from the dualband that died in the storm was still usable, this is it with the 10 meter reflector still attached:

    [​IMG]


    I decided to make the parasitic element a director with a switched-in stub making it a reflector, and leaving it in the reflector configuration normally—so a 10’ spacing was used to give a good direct match to 50 ohm coax (fed with a ferrite choke balun). The elements themselves are 18 gauge bare stranded wire, taped to fiberglass fishing poles as before—standard formulas were used for the initial length, expecting to have to tune them because of possible velocity factor introduced by being attached to the poles—turns out that was right.

    The first step was to resonate the driven element to the frequency I wanted (14.175 MHz)

    It was only 10’ off the ground (the length of the boom) but I had to start somewhere, and this was a lot easier than doing it on the roof.

    With that done, the next step was to build the director and tune it 5% higher (ballpark figure) to 14.880 Mhz- testing shown here—same setup as the driven element: 10 feet off the ground with a temporary feedpoint connected to a MFJ-259:

    [​IMG]


    With these so close to the ground I expected the tuning to change when it was hauled up on the roof (and I was right), but as I said, you have to start somewhere, and a couple of inches of wire had to be trimmed from both the driven element and the director to get them where they needed to be.

    Now, the relay box and tuning stub were built:

    [​IMG]


    Honestly, I just added what I figured I’d need to make the reflector resonate 5% below the driven element, didn’t do any testing on the ground.

    Details of the relay box:



    [​IMG]


    This isn’t complicated— since this is a parasitic element and the power across the relay contacts is much less than with a driven element, a plain old Radio Shack 12V DPDT relay was used. 10A contacts work just fine with the output of my SB-200. Since I’m more interested in the forward direction, the relay is wired with the NC contacts to the stub, while the NO contacts are shorted—So with no voltage applied to the relay that element is a reflector, and with 12V switched in the shorted contacts take that extra length out of the element making it a director. And there’s no reason it would have to be in the center if this were a wire beam hanging between trees or something—this is a wire beam supported by a non-conducting frame, anyway.

    I used the rotor cable from the Alliant rotor to tie into the pigtails from the relay coil, and switch the 12VDC from shack with the highly specialized switch shown here:

    [​IMG]



    Here’s the antenna in the air, boom height now only 33 feet:

    [​IMG]


    Now—since optimum element spacing is different for a 2 element yagi with a director as opposed to 2 elements using a reflector, the VSWR curves of the two are not the same (and most probably not the gain either), and I had to fiddle with the element lengths a bit once the antenna was up on the roof (where I could still reach it) to get a compromise bandwidth between the two. I’ll throw together a chart if anyone’s interested, but it’s still usable in both configurations across most of the band.

    So, how does it work? Well, since I have had two other 2 element 20m yagis mounted in exactly the same place and still have the dipole in the same place for reference—it does what you’d expect it to do. Signals directly in the azimuth of the main lobe are generally 1 to 1 ½ S-units stronger than when the beam is reversed, and that’s what you’d expect to see. However, in some cases the difference is much more dramatic—a lot depends on what angle at which the signals arrive and how propagation may be changing, and who knows what that might be at any given moment.

    But—even though 1 S-unit may not seem like much, it can make a huge difference on how well a signal can be copied in noise, and the QRM can sometimes totally disappear with the flick of a switch. This is not a big revelation to anyone who is used to directional antennas, but I say that to point out that even fairly inexpensive arrays can produce impressive results. In this case all I had to buy new was the relay/box and the switch/box, less than $20 total. Granted, the other materials were already paid for from other projects, but you get the idea.

    My biggest challenge is having to go back out and put the beam back on azimuth after a big storm, but there haven't been too many of those.

    Anyway, this is an idea worth pursuing if you have similar limitations to overcome. Is it a perfect solution to every possible situation? Nah, 'course not-- but nothing is.

    The summer heat's nearly over-- get out there and build something!


    73,

    Rick
     
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  2. gamegetter

    gamegetter Well-Known Member

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    that's for sure and nice job on the rebuild. sometime's you just have to think about stuff for a while and an idea comes to mind.
     
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  3. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    Good post and thanks for the pics, I have read about that being done with a 40 meter quad, switchable in two directions. Quad suspended from supports and trees. Relay switched in tuning stub to change resonant freq and direction of lobe.

    MOXON would be a good choice for light weight small footprint, but it would have to use a rotor for direction, nice thing about the moxon's is the wide bandwidth and uniform gain across the bandwidth.
     
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  4. RickC.

    RickC. Hopeless antenna junkie

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    Thanks folks-- and you know Wavrider, I did have a 20m Moxon up for a while, on a rotator. The f/b was better as advertised, but for whatever reasons the yagi always showed better gain over the dipole than the Moxon, and they were mounted the exact same places. Never could figure that out and I sure made contacts on the Moxon, but keeping that thing symmetrical in the winds got to be a pain and I took it down after a few months-- and I also wanted a directional antenna on 10 also, which led to the dualbanders I made.
     
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  5. n0zna

    n0zna W9WDX Amateur Radio Club Member

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    try a isotron,i have one for 75-80mtrs...and realy like it...it will take 300 cw/am and 1500 ssb...im ordering one for 20mtrs myself...73s de n0zna/John
     
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  6. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    Rick I can see where keeping a 20 meter moxon symmetrical would be a PITA if in a windy QTH.

    The gain for the yagi would be more as spacing of the elements helps with the gain of the yagi.

    I like the relay idea, good stable mount, no expense for a rotor and two directions with gain and rejection.

    Thanks for posting the antenna and I will keep that idea in the bag of tricks for future reference.

    Yes it is getting cooler weather and I have the antenna building big.
     
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  7. Moleculo

    Moleculo Administrator Staff Member

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    Very cool project and very nice pics. Thanks for giving me some new ideas!
     
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  8. RickC.

    RickC. Hopeless antenna junkie

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    You know, there are several ways remote relays could be used, and have been used over the years. I recall seeing an article from QST many years ago where someone had a 120 VAC relay at the feedpoint of something-- I don't think I'd do that!

    One thing I had considered (and may do it yet if it gets to where I have to re-aim this antenna a lot) was a parasitic ground plane array, using the same idea of a switchable director/reflector. After thinking about it for a few days and eyeballing the house and the yard I finally decided I just didn't want all those radial wires running all over the place.

    I also considered both a K1WA array and a 3 element switched vertical array similar to the old CB Super Scanner, and drew up a ground-mounted 1/4 wave version of it as well as a free-standing one using vertical dipoles. That's still a very interesting idea, but how to physically build that would be a challenge.

    Hey, all it takes is time. And money. Right?

    John, I didn't know the Isotron was still being made but I was sure eyeing those when I was a cliff dweller several years back. What I did instead was press a Texas Bugcatcher on the balcony using a piece of rotor cable for 20/15/10 radials-- worked a lot of 20m RTTY on that-- but I have a friend who had 3 of those things outside his apartment door and he swore by them in that situation. He had tried indoor antennas of all kinds, random wires, etc., and they did the job for him.
     
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  9. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    RickC,

    On the super scanner idea.

    Friend of mine is thinking about doing the same thing with three A-99.

    We are trying to figure out the best spacing and phasing harness to get it directional, or all co-phased.

    It was coming along pretty good until I relocated to another QTH 340 miles from him so now the project is sort of on hold.

    The initial results were promising on 11 meters. The problem we ran into was trying to get multi band coverage and maintain the same lobe patterns. Never got that one to work due to phasing issues with the coax. That frequency thing always seems to get in the way.
     
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  10. RickC.

    RickC. Hopeless antenna junkie

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    Waverider,

    That's very interesting-- how are you configuring the feedlines?

    On the Super Scanner (and I just had a look at the schematic of the control box to make sure), nothing's being phased, but in the directional modes, the two elements that are not being driven at the moment have shorted coax stubs which load them and make them reflectors. From the lengths mentioned in the manual, they come out really close to the 3/8 wave lines (assuming a .66 velocity factor) described in the K1WA array. Since the elements are center fed vertical dipoles, that works well. W4RNL wrote about a similar arrangement many times, though his switching was simpler because there was no LC matching network for an omni configuration.

    I don't know whether a shorted 3/8 wave stub would have the same effect on an end fed half wave that's fed through tuned circuit as with the A-99 though. Even running a pair of them in phase would be problematic unless the matching networks were set up exactly the same, and I don't know of a convenient way to verify that. I do know that some who have tried to use baluns in the K1WA type arrays have reported trouble with them and that they finally went back to direct coax feed-- the matching network in the A-99 would introduce random phase differences between the elements unless you just had a happy accident, it seems to me.

    Interesting idea, still-- and please point me to a thread if this has already been discussed-- apologies if I've missed it, I've been away for a bit. What have you seen in your testing? What bands are you using on the A-99s?


    Rick
     
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  11. RickC.

    RickC. Hopeless antenna junkie

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    Here's the way the VSWR shook out when the smoke cleared after 3 hours of snipping, measuring, soldering, measuring, snipping, cussing, soldering, measuring...

    [​IMG]


    The immediate reaction is to look at the curve of the beam when used with the director and think, "That's awful-- look at that, it's resonating too high."

    Well, yeah-- that's exactly what I'd think.

    BUT- every adjustment of the element lengths interacted with everything else, and what optimized the curve for the reflector totally jacked up the curve for the director, and vice-versa. It's possible a different element spacing could have helped that, but I had decided on a 10' spacing since the configuration I plan to use most often is the DE/Reflector, and figured if I could find a decent compromise for the reverse direction, that would be OK.

    To be honest, I did not expect the curves to be that different, but there you have the unvarnished truth.

    What I was after was a usable antenna across the 20m phone band, and all things considered, this is acceptable.
     
    #11
  12. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    RickC,

    All my notes are still in a box, somewhere.

    Memory recalls we visited ON4UN low band DX book looking at diffrent methods of feeding arrays, using different lengths of coax as a phasing line.
    The MFJ259B sure got a work out, that things eats batteries quickly.

    We manually hooked up the A99's using T connectors with the different lengths of coax to provide forward gain in whatever direction we were trying to achieve.

    None of this is scientific testing just to other fixed base stations, they did report a 15 to 18 db difference when we directed the signal to them and when we switched the phasing lines and directed the signal away from them.
    so we did accomplish some rejection as far as accomplishing any gain, never got that far as all the vert installed at his QTH were way up on top of a tower so nothing to really compare them to.

    The problem we were looking at is how to make it multi banded. The phasing lines for 11 meters were not the right length for 10 meters or 12 meters to get the directivity we were looking to get.

    http://www.eham.net/articles/20571

    This was the article that got it started. It was something to play with, just wished had time to finish it.
     
    #12
  13. RickC.

    RickC. Hopeless antenna junkie

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    Ah, excellent!

    You're chasing the same kind of thing that has kept me up at night, sketching things out... Christman phasing...

    I can see where the switching/phasing will get squirrely with that kind of endeavor. What would have me scratching my head is the physical spacing. I guess what you'd have to do is optimize one band and accept compromises on the others. That's a tall order for bands that aren't harmonically related, but it's a very interesting idea.

    That deserves its own discussion, sure enough.
     
    #13
  14. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    I accomplished the phasing with the 40 meter verticals using coax, 75 ohm and relays in a switch box, it worked well. Only a two element array not a three element.

    I used telephone wire from home depot for the control wires and junk yard 30 amp automotive relays for the switching.

    We thought to use three verts that way it would be all direction of the globe.
    One driven, one reflector Christman phasing, I think we tried 71 degree then 118 degree. 118 seemed to work best if I remember right or it might have been 71, can't remember. The third element would be grounded until switched around with the relay control.

    It would have been an extensive relay control box mounted at the base of the array but with some planning and little ingenuity I think we could have got it to work, as a mono bander only.

    The other idea was to use motor driven air variables and roller inductors at the feed point,sort of like a remote external tuner. But the Christman method of phasing would be all out of wack and all the matching unit would do is present a nice impedance to the rig.

    Yes this topic needs it's own discussion, and with that said the LAZY H went in the air today, results so far seem ok into 6 land with a 5-8 running 100 w on 15 meters. Not to bad for a set of wires. Will give it a work out tomorrow hopefully.
     
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  15. RickC.

    RickC. Hopeless antenna junkie

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    Hey, this is gettin' good!
     
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