How to bring this horizontal loop to resonance on 160m?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by 2E0ILY, Mar 25, 2019.

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  1. 2E0ILY

    2E0ILY Ham Member QRZ Page

    450 Ohm window line
    At fed corner about 35 feet, other three corners about 30 feet
    About 460 feet in circumference
    Corner fed
     
  2. 2E0ILY

    2E0ILY Ham Member QRZ Page

    On LF the matching transformer is hardly needed at even without it the loop used with strapped vertical feeder and loop as a top hat exhibits close to 50 Ohms. The loading coil is tapped and hasa motor driver variometer at its base, in series. But for 160m meters none of that is used.
     
  3. 2E0ILY

    2E0ILY Ham Member QRZ Page

    As the loading coil for LF is very large it lives in what we in the UK call a council wheelie bin (Garbage bin). I don't think having it in the middle of the front lawn would get past the wife's sense of garden pride, so I am probably limited to corner feeding unless I have to go through hoops swapping between LF and 160 meters, but I appreciate it may not be ideal.
     
    KA9JLM likes this.
  4. SM0GLD

    SM0GLD Ham Member QRZ Page

    When modelling your loop it seams like it is longer than 460ft.
    The resonance frequency in my model is 2MHz and you have close to 1.7MHz.
    Please make a new scan and try to find the resonance without a balun or preferable with a 1:1 balun.
    Then mark the frequency you like to have the antenna matched to.
    Now we can read the Rs and Xs witch makes it much easier to come up with a matching L/C network.


    4to1-balun_data.jpg
     
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    Transposing the formula L=1005/f, your loop should resonate at 1005/480 = 2.094MHz. Using AutoEz, I created a square loop 120ft on a side, with one corner 35ft agl, the other three corners 30ft agl over average dirt. The feedpoint is at the high corner. Looking into the loop's feed point with no intervening feedline, it it resonant at 2.072 MHz, where the R=75 Ohms purely resistive. (The formula above is for loops that are mounted higher agl).

    Next, I generated plots of R and jX right at the feedpoint, and looking into the shack end of Wireman 552 Ladder line (nom Zo=380, Vf=0.918):
    480Lrx.gif

    You never said the frequency where you want this antenna system to resonate, so I am going to use 1.85MHz. Obviously, the way to make this loop resonant at that frequency would be to add wire to make the total wire length = 480*2.072/1.85 = 538 ft.

    Assuming that you do not have enough land to do this, the next best thing is to match it at the shack-end of the existing 35ft Ladder Line. Assuming that your ladder line is 35ft of Wireman 552, at 1.85 MHz, the impedance you are trying to match is 39.42 - j148. (Check that with your AIM). This is close to 50 Ohms, but is quite capacitive (because the loop is too short). This would be an easy match for a typical T-network antenna tuner with a built-in 1:1 balun.

    Assuming that you do not have a tuner, then the next best thing would be to put a simple low-pass L-network between the rig and the shack end of the Ladder-Line. That would be a shunt 850pF capacitor across the 50 Ohm coax input, and a series 14.4uH coil to the input side of a 1:1 balun. The output side of the balun connects to the twin-lead. The red trace is a plot of SWR50 that the rig will see looking into the L-network:
    480s.gif

    The other four traces are the SWRs that the rig would see if all you did was to put a 1:1, 1:2, 1:4, or 1:9 balun between the rig and the ladder-line. This obviously does not fix the problem.

    Notice that this is a high-Q resonant system and has a relatively narrow usable bandwidth, so I would make the L-network using adjustable elements (1000pF capacitor, 25uH roller inductor, voltage/current rated as needed). Otherwise, any commercial tuner that tunes 160m will work (not all do).
     
    Last edited: Mar 26, 2019
  6. WB5WPA

    WB5WPA Ham Member QRZ Page

    re: "How to bring this horizontal loop to resonance on 160m?"


    Ya know, I use a couple different series "loading coils" in series with what is a loop that is just a little short of a full wave loop on 40 meters ... just enough inductance brings the loop into resonance, then its a matter of using a 4:1 BALUN and an 'easy run' of coaxial cable into the shack ... the inductor is on one side of the 4:1 BALUN.

    Just tossing out the seed of an idea, if your loop's just shy of hitting 160 from above 2 MHz. On the flip side, I suppose some capacitance in series would 'raise' a loops F_sub_o if it were just a little too low in freq ...
     
  7. SM0GLD

    SM0GLD Ham Member QRZ Page

    Yes a 150pF series capacitor brings the resonance to 1900kHz for a perfect 50 ohm match.
    460ft long and resonant at 1700kHz! maybe we can't trust the measurements or the length.
     
    WB5WPA likes this.
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    The loop is too short. I have already showed that its feedpoint impedance is capacitive. How can adding more capacitance in series with it improve that?

    Here are two more test cases, Swr from the rig end, 12.75 uH in series with the input side of a 1:1 balun followed by 35ft of 380 Ohm twin-lead to the loop (blue trace), 19.3uH added in series with the loop wire right where the twin-lead connects to the loop (pink trace), compared to just the twin-lead (green trace).

    480i.gif

    Putting just the inductor in-place of the L-network I showed earlier is just cancelling the capacitance, leaving resitive term as it is, so the final SWR50 is a bit higher at 1.25.
     
    Last edited: Mar 26, 2019
  9. SM0GLD

    SM0GLD Ham Member QRZ Page

    Yes if it is 460ft it is to short but....
    Have you seen the added picture showing the 1700kHz resonance?
     
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    I see the newer plot with a 1:4 balun that shows the best Swr50 at 1.710 MHz. Something wrong with the "no balun" case.

    So I ran my model backwards to find what loop length produces a similar Swr50 plot with a 1:4 balun. A loop length of 564 ft would. Compare the blue trace below to the AIM swr plot with 1:4 balun.

    With the 1:4 balun in place, at 1.84MHz, the impedance into the balun is 45+j108 Ohms. An L-network of series 9.7uH and 1328pF shunted across the input side of the balun produces a match. A series 805pF capacitor followed by a 1:1 balun is almost as good. A 1:1 balun, 35ft of Twin-lead, with a 244pF capacitor in series with the loop wire near the feepoint also works, leaving an SWR50 of 1.6
    480n.gif
     
    Last edited: Mar 26, 2019

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