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    Vivaldi antenna behave different with and without SMA connector in HFSS

    Hi,all
    I designed a vivaldi antenna immerged in some kind of liquid(the Dielectric constant=37),with a simple rectangular lump port ,the antenna behave well,then I find if I put SMA connector in the simulation,the antenna would behave badly.The SMA connector is made for use in air only? Can someone tell me why?If I made this antenna,could it be usable?
    many thanks

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    Re: Vivaldi antenna behave different with and without SMA connector in HFSS

    whats the definition of "behaving well" and "behaving badly" ?



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    Re: Vivaldi antenna behave different with and without SMA connector in HFSS

    VAT.rar here is my HFSS file about the antenna with and without SMA connector,I'd appreciate it very much if someone could help check it.



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    Re: Vivaldi antenna behave different with and without SMA connector in HFSS

    Mistakenly you did connect both layers of the antenna (red and blue) to the center pin of the SMA connector.
    You have to connect one layer to the center pin, and the other to the ground, as in the example attached.



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    Re: Vivaldi antenna behave different with and without SMA connector in HFSS

    S11 : without SMA connector

    Click image for larger version. 

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    with SMA connector

    Click image for larger version. 

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    Click image for larger version. 

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    sorry I changed it but it seems useless.



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    Re: Vivaldi antenna behave different with and without SMA connector in HFSS

    Try to add ground plane near SMA connector, similar to first image in vfone's answer
    or this paper: http://jees.kr/upload/pdf/jees-2018-18-1-29.pdf
    Here is an image from paper:
    Click image for larger version. 

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    Potential problem: the way you connecting SMA to Vivaldi antenna looks more like double-sided parallel stripline to SMA (http://downloads.hindawi.com/journal...013/921859.pdf). Obviously parallel stripline and microstrip have some differences in impedance, especially over wide frequency range. This may cause impedance mismatch and strong reflection at the SMA port. I think your problem may be easily solved by adding ground plane. You can first design Vivaldi with lumped port as before with microstrip and large ground plane, and then connect SMA.

    If you do not want to use larger ground plane, here is some examples to reduce ground plane size:
    http://www.jpier.org/PIERL/pierl75/18.18032602.pdf
    https://www.hindawi.com/journals/ijap/2017/6259682/ - note triangular ground plane in inner layer:
    Click image for larger version. 

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    You may try similar triangular transition.
    Last edited by Georgy.Moshkin; 14th February 2019 at 05:19.



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    Re: Vivaldi antenna behave different with and without SMA connector in HFSS

    Yes, when I mentioned "connect one layer to the center pin, and the other to the ground, as in the example attached" I forgot to say that you have to make also the thick ground plane as in the picture.
    Otherwise your Antipodal Vivaldi antenna will be connected to the SMA ground through a thin PCB trace, which actually is an inductor.
    However, instead making an antipodal Vivaldi, you can make a well known microstrip-fed Vivaldi slot antenna, which may have a bit less bandwidth but gives better VSWR vs frequency.



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    Re: Vivaldi antenna behave different with and without SMA connector in HFSS

    Thanks for you guys' kind advice.I followed your advice to design a large ground plane as in the attachment. But same problem still exist.
    Problem may be the that the antenna is immerged in some kind of liquid(dielectric constant =37),when I design it in the HFSS,I changed the material of the air box into the liquid.Am i wrong doing this way? Or sma connector can not be used in the liquid?
    Cuz if I changed it into vacuum.The s11 of the antenna with and without SMA connector matches well. Click image for larger version. 

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ID:	151159Click image for larger version. 

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ID:	151160Click image for larger version. 

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ID:	151161



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  9. #9
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    Re: Vivaldi antenna behave different with and without SMA connector in HFSS

    The liquid dielectric could be the reason that you get this results.
    Standard SMA connector cannot be used in liquid. You have to search for underwater (or submersible) RF connectors.



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    Re: Vivaldi antenna behave different with and without SMA connector in HFSS

    Transition from ground plane to antenna looks too rapid on your image. Try to change it to trapezoidal one:
    https://www.edaboard.com/showthread....=1#post1640625

    Also on your previous images you've increased SMA pin dimension so it looks odd to me. This giant step in presence of high dielectric may cause some problems:
    https://www.edaboard.com/attachment....6&d=1550087568

    First I would redesign 3D model to be more realistic. Set SMA center pin cuboid dimension similar to real SMA connector. Connect ground plane to ground pins using few rectangular vias. Ensure SMA connector have right dielectric material defined inside. Test this SMA model using 50 ohm line, ensure it is in right place on Smith Chart. Also it may be a good idea to look at electrical field distribution to see where it reflects.



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