long wire atu as a loop tuner

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by 2E0OKC, Jul 6, 2021.

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  1. W9IQ

    W9IQ Ham Member QRZ Page

  2. K1LH

    K1LH XML Subscriber QRZ Page

    Yes, at 10m its a vertical EDZ, each leg is a 5/8, at 30m it's just under 1/2 wave.

    40m it's a bit short but the AH-4 matches it no problem, same on 80m it matches, but I've not transmitted with it to see if it's any good on 40 or 80m, but it hears stations fairly well, same again on 160m, though the AH-4 only gets a 1.5:1 to 2:1 match, I doubt it's very efficient but usable if it comes down to it. Also works ok on the local 6m repeaters, but the lobes will be high angle. More info here.
     
  3. K1LH

    K1LH XML Subscriber QRZ Page

    I have an update on this one. I think you raised a good question here.

    It seems you can choke the coax and control cable on an AH-4, it'll work fairly well so long as the antenna is fairly balanced (thou thats a guess here, because I dont really know), or the RF fairy is playing nice.

    I more recently had an AH-4 on a 130 ft doublet via 100ft of 600 ohm open line. The doublet is not symmetrical, one half is flat top, the other more inverted V due to trees and supports I have.

    17m I got weird feedback into my IC-7300 that made the internal SWR meter freak out. Adding some ferrite to the shack end of the control cable, and coax helped.

    30m anything over 75W would upset the oven, it would beep, display error and get all angry.

    Using 8 turns and 4 mix 31 snap on ferrites on both the control and coax each, did not solve this. I suspect the 600 ohm line was radiating due to imbalance.

    When I received the 1:1 ATU balun from Balun Designs and put that between the 600 ohm line and AH-4 everything came right like magic, I got better matches, and the problems went away. Now even with a 1kW remote tuner in place (JC-4s), zero issues, I can dump 1kW up that on any band SSB or ~400W digi modes (except 30 and 60 which are power limited of course) and no RFI issues, no complaints from anyone, nothing.
     
    Last edited: Jan 13, 2022
    WA7ARK likes this.
  4. W1NJC

    W1NJC Ham Member QRZ Page

    An full wave loop should not present really low impedances on any frequency above the full wave freq. (well, I don't know what would happen going up to UHF).

    My recommendation would be to save the £300, feed it with ladder line, and use a home-brew 1:1 balun into an unbalanced tuner you might already own. You didn't mention power levels or bands of interest so you'll want to keep those things in mind. A loop fed with balanced line should give you quite acceptable multiband performance at and above the full wave frequency. Search G3TXQ on an image search for some balun charts for your bands of interest.

    Alternatively you can forego the balun and use an inherently balanced matching network like a balanced-L or a link-coupled, etc.
    Also, whatever frequency it's a full wave, you could run it against ground and use it on half of that frequency (it behaves like a vertical). You'd need a good ground/counterpoise for this to be effective.
     
  5. K1LH

    K1LH XML Subscriber QRZ Page

    EZNEC suggests for example, 160m full wave loop at 60ft height, fed in the center of one side = about 100 ohms, on the harmonics it goes up.

    Any given length of balanced line (300/450/600) will transform that up and down. Modeling will help know what to expect, even then when I've put an analyzer on the end of balanced lines to doublets, its been lower than expected!

    E.g. my 130ft doublet with about 70 ft of 600 ohm line (not sure of exact length, I just tweaked it with the analyzer until I got reasonable numbers on all the bands) gives me near 50 ohms R and some reactance on 15m:

    IMG_1686.jpeg


    The trap some seem to fall into is using 4:1 (or higher) baluns under the assumption the impedance are on the high side, when often its not. http://www.karinya.net/g3txq/tuner_balun/ has some good words on that. You can end up with very very low impedance at your tuner if you're not measuring anything, and especially if you go the non-ideal route and have some coax in the equation between the balun and the tuner.
     
    Last edited: Jan 13, 2022
  6. W1NJC

    W1NJC Ham Member QRZ Page

    @K1LH Thanks for that info. Your measurement is on a doublet so you would expect to see wilder swings. You have 57R but -179X for an overall Z of 187 ohms. On a loop the swings tend to be more constrained. Yes, the feedline will transform it of course. My big loop stays between about 200 and 5000 ohms at the feedpoint per 4nec2. At the shack end of my ~50ft of 600 ohm open wire it's not all that different. except I get almost a perfect 50 +j0 at about 3.9MHz.

    I agree 100% the 4:1 is not the greatest idea. Most people are using a T-match anyway which is good at matching high-z and not low-z. I suppose if you only want monoband performance and that particular band is presenting a very high impedance...but I prefer the multiband versatility. Plus a 1:1 is easy and cheap to DIY, and you can experiment with it for best performance.
     
    K1LH likes this.
  7. K1LH

    K1LH XML Subscriber QRZ Page

    Interesting, it surprised me too - 17m: R 35 X -161, 12m: R 136 X -114, 10m: R 162 X -93. I need to measure again and record the info 20 down to 160m. At one point I accidentally had a feeder length that gave perfect 1:1 on 60 and 30m, but 10 and 12m were quite low and 15 was border line, from there added about 8ft to improve those upper bands, but the magic match on 60 and 30m moved else where. Otherwise this doublet is the best wire antenna I've ever used, it keeps surprising me. Wish I had room/supports for a big loop!

    So I've seen one other case with 80m loops where about 20ft of balanced feeder = perfect 50 ohm match on 3.8 MHz. I recall someone posting here about how can this be?! I modeled it for them, and sure enough, they had accidentally found the sweet spot :cool:
     
  8. K9XR

    K9XR Ham Member QRZ Page

    For the last couple of months I have been using a 150 ft. loop in my seccond story attic. Most of the time it is pretty impressive, but it does not perform as well on 160 and 80 as an open ended loop that I had before. On 10-15-20 it is a toss-up between the loop and the driven element from a TH3 tied to my ridge beam in the attic. On 40 I have nothing to compare it to BUT, last night I worked 5T5, Ua9, and several South Americans with FT-8 in less than an hour. I am restricted to 80 watts because the SGC 230 tuner is only rated for 80 watts on FT-8. I say put it up and try it and as I always say you may be pleasantly surprised.
     
  9. K9XR

    K9XR Ham Member QRZ Page

    BTW I just fed it direct to the tuner, one end to the ground and the other end to the big white insulator.
     
  10. K1LH

    K1LH XML Subscriber QRZ Page

    Loops have a high(er) common mode impedance at the feed point, you have a higher chance of getting away with not using baluns/CM chokes at lower power levels at least - https://hamwaves.com/chokes/en/ > Antenna input impedances

    Near 30 years ago when I was horsing around on 11m before I got into ham radio, I built a quad loop with section of 75 ohm line to match it, no baluns nothing. That thing worked great, and no RFI issues either. Back then of course TV, phones etc were all analogue and even a 12W radio connected to an antenna that's making the coax radiate would cause problems with those and more :)
     

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