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Need help with MMANA two band yagi

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by SA6NIA, Jun 12, 2021.

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  1. SA6NIA

    SA6NIA Ham Member QRZ Page

    Hi!

    I'm trying to simulate an interlaced 21/28 MHz yagi. 2 elements on 21 MHz and 3 elements on 28 MHz.
    See attached model.
    I'm not sure if it's correct to enter sources on both 15 and 10m drivers at the same time. It doesn't give good result.
    When I simulate 10 and 15 m yagi separately, it works OK.
    When I put them together, only 15 works and 10m is changed (bad SWR).
    How do you feed 2 interlaced antennas? In reality I would like to run the coax from TX to one of the feedpoints and then continue to the next feedpoint.
    The antenna is supposed to have direct 50 ohm feed.

    I would appreciate any help!
    Thanks es 73
    Oscar SA6NIA
     

    Attached Files:

  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    Not MMANA savy, but in NEC, if your antenna has two feedpoints, you should only have one source active at a time (unless you are building some sort of "phased" array).

    The non-active feedpoint has to be set to reflect the reality of what is happening there. For example, for a Yagi with two feedpoints, is the non-active driven-element open at the feedpoint? is it shorted at the feedpoint? is there a coax feedline connected to the feedpoint? if so, how long? what is happening at the rig-end of the feedline? is it open? is it shorted? is it terminated into a load? etc, etc.

    The interaction with the non-active driven element is effected by what is connected across its feedpoint.
     
    Last edited: Jun 12, 2021
  3. UA3TW

    UA3TW Ham Member QRZ Page

    As you described its feeding it can not be made in MMANA because MMANA does not support coaxes. It can be made in AutoEz and it is not so simple feeding. One way - coax from TX goes to wire 3 and from wire 3 to wire 4 at actual distance if the coax is 0.8 velocity factor. Wire 3 is shunted with L=1.42mkH wire 4 is shunted with L=1.035 mkH. SWR 21.0=1.57 21.2=1.01 21.4=1.32 28.0=1.37 28.3=1.07 28.6=1.23
    AutoEZ model is attached.
    Forgot mentioning that the length of driven elements does not the same as in original model - wire 3 will be +- 2.598m wire 4 will be +-3.321m
     

    Attached Files:

    Last edited: Jun 13, 2021
  4. UA3TW

    UA3TW Ham Member QRZ Page

    Perhaps it would be more practical to stretch the line of two wires between wire3 and wire 4 let say 550 Ohm. All can be calculated in MMANA in this case - MMANA calculates two wire lines reasonably well.
     
  5. SA6NIA

    SA6NIA Ham Member QRZ Page

    Thank you so much for your reply and I will try to make it work with your thoughts on the problem.
    //Sa6nia
     
  6. UA3TW

    UA3TW Ham Member QRZ Page

    The attached model is in MMANA. The driven elements are connected with the 2 wire line.
    The wires are 6 centimeters apart. Perhaps if the wires are closer or made of lardger OD wires
    the bandwidth will rise. Of course the coax 50 Ohm line gives better bandwidth but I do not know what to do for the simmetry
    in this case. Some people immitate 50 Ohm line by putting 3 wires in parallel in close proximity in MMANA, kinda two parallel ribbons, each ribbon is made of 3 parallel wires - one is in the middle,
    the other slightly (2 cm or so) higher and one more lower.
    The coil of 2.82 mkH is not convenient constructively, though it gives the halvanic connection between the halves of the antenna besides the match for better SWR. One can try the model without the coil if the poorer SWR is not minded.
     

    Attached Files:

  7. UA3TW

    UA3TW Ham Member QRZ Page

    X fed variant with a hairpin
     

    Attached Files:

  8. SA6NIA

    SA6NIA Ham Member QRZ Page

    Thank you for this design!
    I think I'm going to build this antenna.
    You've must been putting down quite a lot of time on this and I appreciate that really much as a new ham!
    Thank you for all the help and best 73
    De Oscar
     
  9. UA3TW

    UA3TW Ham Member QRZ Page

    Hi, Oscar
    Its been a pleasure to play with this model. I am retired and have plenty of time.
    I wish you luck with this project.
    If i find a better model i will attach it here.
     

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