Which balun for horizontal loop?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AE5KG, Feb 25, 2021.

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  1. AE5KG

    AE5KG Ham Member QRZ Page

    I have procured 4 utility poles roughly 40' long. So 34' or so above ground.

    The idea is to bolt a 20' section of fence top rail to each, and get +- 50' above the ground, and run +-556 of wire horizontally. Skyburner on 160, but 80-10 should work well. I think...

    Anyway, the plan is to run an ldg rt100 at the corner of the feed point. Ladder line down to a balun to the tuner.

    I'm gonna dig out my old manuals, but which would be better? 1:1, 4:1, or something like http://www.trueladderline.com/model-4116t-4-1-hybrid-balun-1-5-54mhz-3kw-ships-free/

  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    Since the LDG remote tuner is coax in/coax out, I would install the tuner at the loop corner. Use a 1:1 current-mode ferrite balun between the tuner output and the loop corner, and use 50 Ohm coax from shack to the tuner.

    AFAICR, the feed impedance at that loop corner goes from about 80 Ohms at 160m to ~350 Ohms on 15m. The impedance on 20m is the only one that comes close to 200 Ohms... 40m is about 120 Ohms. This is without having a section of ladder line or OWL screwing up the natural feed impedences that the loop offers intrinsically.

    My 558ft 160m loop is fed via a -998RT 1kW remote tuner that is located 55ft up my tower, just inches from one corner of the loop. I have tried both 1:4 and 1:1 baluns between the tuner and the loop, and couldn't tell any difference. The tuner effortlessly tunes the loop anywhere on all harmonic bands; 160, 80, 60, 40, 20, 15, and 10m. It tunes up easily on 17 and 12m too...

    I found it best to prune the loop to resonance at its fourth harmonic (7.15MHz). That optimally aligns the other harmonics into their respective bands...

    It is really nice to have almost instant access to any part of any HF band just by sending 10W of RF up the coax to the remote tuner for about 3 seconds...

    btw, after following your link, I see I underpaid even though I had to pay for shipping. It cost me about $12 to build 800W ferrite baluns...

    Ps: found the previous post that listed the FP impedances:
    Last edited: Feb 25, 2021
    KG7QJB and K0UO like this.
  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    I recommend not making the loop a perfect square. The azimuth pattern gets some really deep nulls on the higher harmonic bands if the loop is a perfect square, less so if the loop is made rectangular (or even triangular). If using four supports at 60ft agl, I would aim for 104x175ft=558ft total. That means the poles would have to be set in a rectangle about 120x190ft.


    If you feed a corner and adjust the wire length to resonance at 1.803MHz (or ~7.2MHz on the fourth harmonic), here are the feedpoint impedances at the resonant frequencies with the loop at 60ft agl over average earth (except 17m, which is not resonant):
    If the tuner is placed near the feedpoint, you can see that either a 1:1 or 1:4 balun will work with those feed resistances, but a balun is needed to covert the tuner's unbalanced output to the loop's balanced feedpoint.

    If you were to put the tuner at the base of the pole, it would take about 65 ft of ~380 Ohm window line to reach the loop corner. This is what the tuner's balun would see after 65ft of Wireman #552 window line is added:
    The tuner/1:1 balun should be happy with that, although it may run out of capacitance/inductance range on 160m...

    If you use 65ft of 600 Ohm OWL instead, notice what happens to what the tuner/balun sees:
    Now, the tuner/balun will hate that on 80m and the lack of capacitance/inductance range problem on 160m is even worse...

    For a long time, I ran my 160m triangular loop without a remote tuner. I just used the tuner built-into my radio or a desktop manual tuner instead.

    If you just run coax the whole way and use a 1:4 balun at the feedpoint, here are the SWRs on the 50 Ohm coax run for the 104x175ft rectangular loop:
    On 160m, the swr is ~3 because the starting feedpoint resistance on that band is the lowest, but that is also the frequency where the swr-induced loss on coax is insignificant.

    Since I have saved the model, what else do you want to know?
    Last edited: Feb 26, 2021
    WB5YUZ likes this.
  4. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    If DX work on the harmonics is the primary goal of your antenna, I don't think you can beat the feed scheme Mike suggested above.

    Too many articles about loops on the web draw the intuitive but not necessarily correct conclusions that (1) the loop should be fed with balanced line to keep unmatched losses down compared to coax, and (2) since coax is 50 ohms and balanced line is nominally 450, using a 9:1 balun at the junction will automatically proved a good match.

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