Feedline for 80M loop

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AC2MM, Dec 3, 2017.

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  1. AC2MM

    AC2MM Premium Subscriber QRZ Page

    Hi folks,
    We just erected an 80M loop, hung on 6 points, in the clear, 280 ft. long, 35 - 45 ft up, and fed it with a 450 Ohm ladder line down to a 4:1 Current Balun.
    We used an estimated 45 - 47 ft coil (we didn't roll it out and measure it.) and simply hooked it up and went to the tuner. Beautiful. We tuned 80M, and it was almost flat across the band. 40M was perfect 1:1 with no tuning at all. 20M was also good, a little tuning required.

    The only problem was, the feedline was draped over our new deck posts, as we couldn't figure out a way to hang the excess anywhere else. So, knowing the wife wouldn't put up with that, I cut it back to 32 ft (approximately).
    Now according to what I read, 32 ft should be the correct multiple for 80M 1/8 wavelength.(31.9)
    But, it doesn't work as well now. We cut off 13ft, 8 " to give us a nice 20 ft run over to the house, and 12 ft down to the balun.
    I can still use all the bands, but 80M isn't flat across anymore, and we have to tune 40M a little.
    Since then, I've ordered another 50 ft of 450 Ohm, and intend, once it gets here, to take it down and measure exactly what was up there, and I have another tree I can use as a support rope to use that length instead on our new deck posts. Then I'll put the same amount back up.

    What did I do here? It definitely wasn't an odd multiple of 1/8 wavelength before, because that's either 31.9 ft (VF = .91) or 96 ft. I was somewhere around 45 - 50 ft.

    Now the Balun was from Balun Designs, brand new, that was designed for a yet to be erected 80M OCF antenna. It's a 4:1, Hybrid Voltage/Current Balun. It's what I had at the moment. Since then, I ordered a simple two toroid current balun designed for window feed lines from Balun Designs. Would that have had any effect on this do you think?

    The way I'm going to approach this is to measure the current feed line length, make it 32 ft., when I get the new balun and feed line. Then install the new balun and tune. See if it's different. (tuner settings required)
    If it doesn't improve, trim the new line to what it was before I cut it, hang it properly and see if things don't return to the original settings.

    You guys see anything I don't?

    Thanks,
    Robert AC2MM
     
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Sounds like it was nearly perfect and I would not have cut the ladder line at all. You can "use up" excess ladder line by using a messenger rope and coiling it on that rope, using large coils (maybe 12" diameter, spaced a few inches apart per coil) to provide "the perfect" line length occupying a lot less physical space. Paint it green and call it a vine.:p

    All the formulas in the world don't work unless you know exactly the R +/-jX you're dealing with on each band, as well as other variables.
     
    NH7RO likes this.
  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    Put the 1:4 current balun at the loop feedoint. Run any-length coax from the tuner to the loop. Throw away the ladder line.

    You will only need the tuner on 80/75meters. On 40, 20, 15, and 10meters, you wont need a tuner. If you want to work 60, 30 and 17m, the tuner will fix it.
     
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    Here is an EzNec sim of a six-sided 280ft loop, 40ft in the air. This is a plot of the SWR as measured right at the loop feedpoint, but with respect to a 200 Ohm source impedance. The SWR minima occur at:

    Freq SWR200 R jX
    3.6 2.01 111 j56
    7.15 1.086 213 j11
    10.65 1.68 333 -j26
    14.35 1.38 275 j7
    17.85 1.44 287 -j1
    21.4 1.14 213 j24
    24.85 1.56 296 -j52
    28.4 1.57 301 j47

    Here is the plot:

    swrp.png

    If you place the 1:4 current balun right at the feed point, then here is the plot of the 50 Ohm SWR with 25ft of coax from the tuner to the balun. Except for coax loss, then it doesn't matter how long the 50 Ohm coax is:

    SWRp2.png

    Continues in the next post....
     
    Last edited: Dec 4, 2017
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    Here is a detail of what the tuner has to match on 80meters, which should be an easy match, with low loss in the tuner itself, and no high voltages... Note that moving the Ferrite cores from inside the tuner (to make it balanced) up to the loop (to create the 1:4 current balun) means that there will be less net loss in the Ferrite, because it has to do less work up there:

    SWRp3.png

    The worst case SWR at 4.0MHz is 8.

    Now compare that to the same loop fed with 25ft of 450 Ohms Ladder Line. Which do you think the tuner would be happier with? Coax fed or LadderLine fed?
    swrp4.png
     
    Last edited: Dec 4, 2017
  6. K5VV

    K5VV Ham Member QRZ Page

    If you go this route, I would check the SWR50 with an analyzer after installing the 4:1 balun at the feedpoint. If the models are correct, they will be low and you can feed with coax; but if they're high, the coax will add a lot of loss. If you skip this step, you may have an antenna system that's easy to tune but very lossy.

    Remember, "easy to tune" does not necessarily mean efficient antenna system. Actually, inefficient antenna systems are often easier to tune!
     
  7. KQ0J

    KQ0J Ham Member QRZ Page

    I had great success with a similar feed setup you have - LOOP => 450 ohm Window line => W9INN 4:1 ( voltage ) balun => MFJ-915 isolator =>Coax inside house to tuner. I used it on 80-10 with a tuner so was not worried about the bandwidth on 80 but it worked fine at both ends.
     
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    Predicted worst case load at 4.0MHz is 36 -j100. With that load, 25ft of RG8 foam coax loss (shown by TLDetails.exe) is 0.46db. No one receiving the OP's signal will be able to detect that.

    The tuner losses predicted by the T-network simulator are quite small as well:

    tn.png
     
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    How long is the window line? That is the issue being discussed here. In order for the 450 Ohm feedline not to screw up the impedance that the tuner has to work into, the 450 Ohm feed line has to be a minimum of ~100ft or longer. A shorter 450 Ohm feedline transforms the ~200 Ohm intrinsic loop impedance into something most tuners have trouble with, either due to arc-over at high power, or core losses in the built-in balun.
     
  10. PA1ZP

    PA1ZP Ham Member QRZ Page

    Hi

    How long should be the windowline, so long it can reach your balanced tuner.
    The loop will not be 200 ohm on all bands forget it.

    450 ohm ladderline or windowline to a 200 ohm or another impedance will give impedance transformation on the windowline, so at the windowline feedpoint the SWR or impedance could be very far of 200 ohms.
    On different frequencies the transformation will be different as the windowline will be different in length in wavelength.

    The guy with the idea of the 1 : 4 balun at the loop and then go to reasonable quality coax is right just do that, but do not forget the 1 : 1 current ballun under the 1 : 4 balun if this balun is a voltage balun, otherwise you will have common mode problems.

    Or forget all this and use windowline or real ladderline all the way to a balanced tuner.

    I know several hams using an 80 mtr loop on all bands from 80-10 M and if its up about 40-50 feet it is a good multiband antenna.

    However all the guys I know here use homebrew ladderline and a balanced tuner, like a PA0LL double-L match or S-match , or homebrew tuners like a PA0FRI S-match.
    I build a 2 KW S-match for 20M - 160M for about $200.
    Its was a roller inductor of about 1-30 uH and a variable capacitor of about 15-500 pF (3KV) and a transformer build on a T-300A and 3 switchable doorknob capacitors of 300 pF ( 8KV) each to switch parallel to the variable capacitor if more capacitance is needed.

    Many hams also use a 135feet doublet fed with ladderline for a multiband antenna for 10-80 mtrs.

    I was able to build a 300 watt S-match 10-80M for 50 dollars with small roller inductor of 25 uH and a variable cap of 20-500 pF and a T-200-2.
    look at pa0fri website in ATU for how to build balanced tuners and see examples of many of these homebrew tuners.

    It seems to me that a lot of the Dutch hams havent forgotten how simple and effective ladderline can be.
    A lot of them even build their own ladderline with rose-clips these are snap-on clips for several wids of ladderline and several wire diameters, they cost about 0.10 euro cents a piece, I bought 1000 pcs. for 45 euro and that is enough for 750-1000 feet of ladderline.
    The rose clips are original produced as rose or tree-clips for agricultural use you can buy them at all hamevents or markets here, even online at GB-towers in Brielle.

    I use AWG13 braided insulated copper wire for my ladderline.
    It will beat all the 450 ohm stuff you can buy in performance and power handling and will have less loss.
    This ladderline is 10 yrs in use now still doing fine.
    And if its warn, i just will build me a new ladderline.

    I also used the snap on rose-clips for seperating cats whiskers dipoles.

    73 Jos
     
    KB9OFM likes this.

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