Optimal 4:1 balun for unbalanced tuner

Discussion in 'General Technical Questions and Answers' started by ON3ZTT, Oct 3, 2008.

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  1. ON3ZTT

    ON3ZTT Ham Member QRZ Page

    I would like to build (wire) a 4:1 balun to use at the output of my Daiwa CNW-419 manual ATU, to tune an 80m long horizontal loop antenna for all-band use, with 450 Ohm ladder line as feedline.

    This 240 ft horizontal loop antenna will be hung about 13m high and more or less symmetrical, in a hexagonal setup.

    Sevick favors a Ruthroff-type 4:1 HD balun for this application, 21 bifillar turns on a 3 inch powdered-iron core. If the antenna system is well balanced, it should work optimally and have the best low-frequency performance.
    If the antenna is less well balanced, then a 2-cored Guanella type 4:1 balun should work better since it will force equal current and voltage on the ladder line.
    What is really the best choice of balun? I guess my horizontal loop that willpartly circle my house, be affected by surrounding trees etc will not be perfectly balanced.
    Could I use the same Amidon T-300 powdered-iron cores (with low permeability) as advocated by Sevick for the Ruthroff type balun, to make a twin-core Guanella balun?
    Which balun would offer the best wideband properties?

    PS: the 4:1 Guanella balun can also easily be transformed into a 4:1 Unun by strapping 1 outlet to ground, this is an advantage for this balun giving it more allround usage for other application eg. end-fed wire antennas.

  2. M3KXZ

    M3KXZ Ham Member QRZ Page

    I have good results with homebrew W1CG Guanella type balun - details at http://www.njqrp.org/balun/ . It's specifically designed for low power and was tested at 100W with only a very small temperature rise. A fellow ham over here tested one I constructed and was able to confirm low SWR and high efficiency even up to 2m band.
  3. KA4DPO

    KA4DPO Premium Subscriber QRZ Page

    A two inch mix #2 core with about 15 bifilar turns will do nicely and is less expensive. There are kits available in the 14 to 20 dollar range but you can find them cheaper at hamfests.
  4. ON3ZTT

    ON3ZTT Ham Member QRZ Page

    Still have my questions....

    Thanks M3KXZ, the link is interesting.

    I am not looking for a cheap solution but a good solution, hence my question.

    A Guanella current balun 4:1 should be superior, only to have high efficiency and withstand the heavy impedance and reactance swings when used at the end of twin lead/ladder line, at tuner outputs, and to tune a random doublet or loop antenna all-band, I suspect it is better to use low permebeality iron-powdered cores of large diameter like Amidon T-300 or T-400.

    But in Sevick's book all Guanella baluns are built with ferrite cores, and in the section on tuner baluns he advocates a single powdered-iron core Ruthroff voltage balun.

    So this is my doubt:
    - which balun will withstand best the abuse in the tuner balun use
    - which material will give best all-band use, both low bands untill 30 Mhz

    Please anyone with deep knowledge on baluns?

  5. KA4DPO

    KA4DPO Premium Subscriber QRZ Page

    Sorry, I should have said less expensive. Amidon Mix #2 is an excellent choice for a Balun trnasformer operating between 1.8 to 30 Mhz and provides better power transfer (less loss) than most other iron mixes. But the choice is yours. There are many articles that will tell you the G5RV is the best antenna in the whole world. Please feel free to believe that if you wish, many of us know better. I simply offered you what I believe is a better solution at less cost.

    Try an Amidon T300A-2 and around 12 to 14 turns. This will take all the punishment you can give it. I have run several just like this in home brew tuners and never had a problem even at legal limit power.
    Last edited: Oct 4, 2008
  6. W5DXP

    W5DXP Ham Member QRZ Page

    Do you know what is the minimum impedance the balun will see? If so, why would you choose a 4:1 balun?
  7. ON3ZTT

    ON3ZTT Ham Member QRZ Page

    TNX so far

    Hi KA4DPO, your 12-14 turns balun on Amidon T300-2: do you mean a single core Ruthroff balun (bifillar) or a twin core Guanella type 4:1?

    Hi W5DXP, no I don't I did not do any antenna modelling, and have no antenna analyser (yet) but I would expect quite high variance of impedance on a 80m long horizontal loop used from 3.5-30 Mhz.

    Why I am so keen on building a 4:1 Guanella dual core balun is that by simple strapping it can be changed into a Guanella 1:1 (choke, current) balun or into a very good 4:1 unun. Multifunctional. So if a 1:1 current balun would work on the loop, no need to transform down the impedances. If out of the tuners reach, simply rewire it to 4:1 current balun.

  8. ON3ZTT

    ON3ZTT Ham Member QRZ Page

    link to switchable 1:1 - 1:4 Guanella balun

    Look at this link:

    It is a QRP-design for a switchable Guanella type balun.

    Off course I would build it with large T300-2 toroids.

    In a 4:1 balun the wires should have typically Rload/2 impedance hence 100 Ohms, attained by careful choice of wire & isoltion wall thickness.

    If switched as a 1:1 Guanella current balun, each side of the transmission line is 2 x 100 Ohms impedance in parallel hence 50 Ohms which is again the optimal transmission line impedance for a 1:1 balun.

    When the "balanced out" lead on the bottom is strapped to ground, one gets a high quality wideband 4:1 Unun (ref. J.Sevick "Understanding, Building and Using Baluns and Ununs")

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