Building a 7-foot diameter QRO STL for 40M in my HOA backyard

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by NH7RO, Jan 25, 2017.

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  1. NH7RO

    NH7RO Ham Member QRZ Page

    Last year I started to build a two-turn (N-turn) small transmitting loop for 40M operation out of 32 feet of 1" Heliax that I scored at our annual hamfest. Suspended by a large PVC frame, the beast was about 5.25 feet in diameter yet still manageable as far as moving it around in and out of our backyard HOA common area.

    Yet shortly after acquiring a hefty 20-300pf Comet vacuum variable cap to tune it I found out that I would actually need a bit less capacitance to tune it to the Extra phone portion of the band than I originally thought.

    It seems that one cannot rely on conventional (single) loop calculators to design any multi-turn STL as the inductance and operating voltages multiply dramatically and capacitance requirements go in the other direction.

    "What to do?" I wondered.

    After some online discussion, back and forth I realized I could either cut the 32' down to 26' and cut the PVC frame down to 4.1 feet---or scrap the N-turn arrangement for the time being and just make the largest single loop that would fit in my covered patio instead.

    I chose the latter of the two options as it seemed the easier of the two methods and my Comet would be perfect; I could also benefit by some operation on 80M as another bonus as well.

    Besides having the big coils of Heliax mentioned above I also had about 27 feet of 7/8" soft copper tubing left over from my recent 15-20M STL project that was just completed a week or two ago. It is a lot easier to work with than the Heliax and since I always prefer to braze copper-to-copper with silver brazing rod it was an easy decision; no foam or insulation to remove or keep from melting (the Heliax copper shield is actually the same 7/8th" OD as the HVAC tubing so no tradeoff in efficiency there).

    I measured the height and width of our small condo patio space which is where I will store the loop when not in use 7 feet 2 inches wide by 8 feet 3 inches high---just enough to squeeze in a big 7-foot circle mounted on a wood stand about 8 feet tall.

    Running the KI6GD online loop calculator indicated that 22' of .875" copper yields a 7-foot loop with an estimated efficiency slightly over 66% on 40M; good enough for me especially since I have 600 watts to play with there. I can run QRO at that level and the peak voltage should be 1kv less than the 15kv peak rating of my Comet VVC ( I will probably run my amp no higher than 500 watts SSB just to play it safe). Comets are not exactly cheap!

    The first bit of work done was construct a non-metallic stand for the big loop and cap (the Comet alone weighs about 4 pounds or so);

    DSC_9998.JPG
    DSC_0001.JPG

    The 2x2 is a little flimsy but will suffice (I plan to augment it with some side supports soon).

    Next order of business was fabricate some better flange connections for the cap and loop ends; I used a piece of 1.625" pipe cut open and hammered flat for the top (supporting) connection and a 10" long sheet of .025" thick copper I bought at Hobby Lobby ages ago for just such a need (not as thick and sturdy as the other but this part wouldn't be "load bearing" for the most part.
    DSC_0011.JPG
    Brazed the top plate after carefully rigging up a real Rube Goldberg operating stand with an adjustable height stand, a brick and a 99-cent clamp with its plastic handle bits removed.
    DSC_0009.JPG
    DSC_0013.JPG


    The thinner bottom plate took a bit more effort to create but I proceeded slowly and carefully so as not to muck it up; it's a long roundtrip to Hobby Lobby to get another!
    DSC_0015.JPG
    DSC_0016.JPG
    DSC_0019.JPG
    DSC_0022.JPG
    Perfecto!

    Lots more to do and I also have more photos of the next steps accomplished but I will continue uploading and posting details when I have some more free time. Stay tuned!
     
    Last edited: Jan 25, 2017
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  2. NH7RO

    NH7RO Ham Member QRZ Page

    Next in the order of business was to roll out the copper and form a 7-foot loop with it before cutting the other end; this is what I devised with what I already had on hand:
    DSC_0024qrzcopy.jpg
    Plastic tent pegs from our old KH6 Wal-Mart (also a great source for fiberglass fishing pole blanks which are unobtainable here, oddly enough), metal rod in the center with thin Dacron line attached between it and one of the pegs (42" distance to make up a 84-inch radius) and my old "husband hammer" (what my xyl calls the big rubber mallet she'll use on me if I keep building so damned many antennas) to pound the stakes into our sandy "soil."

    After securing the already cap connection plate end to the first stakes I began to unroll the coil around the stakes as evenly as I could:
    DSC_0026qrzcopy.jpg
    Aerial view from our upstairs balcony. I was so excited at finally seeing my long-awaited 40M loop beginning to take shape I forgot to close our balcony door when I ran back downstairs. No worries, there were no unwanted critters or bugs crawling around when we discovered the door wide open late last night (although we found a large tarantula hiding behind our cat's water dish a couple mornings ago inside our downstairs entrance hall).

    View from the ground just before marking and cutting:
    DSC_0027qrzcopy.jpg
    Fairly circular with a few sections that need smoothing out; soft copper is not so soft once you bend it the first time...

    The leftover piece from the coil turned out very nice, though and I plan to eventually try it as a 6M loop about 18" in diameter:
    DSC_0028qrzcopy.jpg

    Today I am working on other things but before darkness falls I will go out and measure the big loop as accurately as I can and then try using my Imperial Model 102-F-14 Spring Tube Bender to see if I can bend the slightly misshaped sections back into round with it and my mallet.

    More photos to come once I make more progress in the coming days.
     
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  3. N3DT

    N3DT Ham Member QRZ Page

    Your neighbors must think you're loopy, if they can see what you're doing.
     
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  4. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Pondering....At 7ft diameter, 2 loops, this antenna may no longer be a "Small Transmitting Loop" and may begin to act more as an ordinary loop.
     
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  5. NH7RO

    NH7RO Ham Member QRZ Page

    No, it's a single 7-foot loop (I scrapped the N-turn 5-foot loop for the time being) and it is far from being a 1/4 or 1/3rd W/L on 40 Meters which would be much larger.

    If you're not too sure about all of this I suggest you read Leigh Turner's tome on the subject; he explains things very well:

    https://www.nonstopsystems.com/radio/pdf-ant/article-antenna-mag-loop-2.pdf

    73, Jeff
     
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  6. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Ok. I modeled a single loop. It's NOT an STL, since the circumference signficantly exceeds 1/10 WL. The model shows it no longer behaves as an STL, as it no longer has a dipole/figure 8 pattern. It's a vertical loop, with omnidirectional horizontal pattern and a high take off pattern.
     
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  7. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    loop 21ft Ant.png loop 21ft vertical.png loop 21ft horizontal.png
     
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  8. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Excellent article. It has several errors, but is definitely worth reading.
     
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  9. K4SAV

    K4SAV Ham Member QRZ Page

    You plotted it at 3.6 MHz, not 7 MHz.
    Note also that the azimuth plot of ANY antenna is a circle at 90 degrees elevation.
     
    Last edited: Jan 26, 2017
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  10. AA7QQ

    AA7QQ Ham Member QRZ Page

    Would you like to explain the errors in the article?

    Ed
     
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