Matching/Tuning 2.4 GHz Yagi Antenna

Discussion in 'Wireless & RF Design' started by madatr, Apr 3, 2019.

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  1. madatr

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    Feb 24, 2017
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    Hello All,



    I am building a 2.4 GHz Yagi antenna for a project, its bandwidth must be cover 2.4 to 2.5 GHz.

    I have already constructed the antenna and assembled it, what i need help is to properly match it to standard 50 Ohms to ensure maximum efficiency.



    What i have gathered so far is that i need to use a balun to match the resistive part and a reactive section to match the reactive part of the impedance.



    I am going to purchase a ready made balun for the resistive part leaving me with the reactive part to be matched, please send your opinions, prayers and possible easy solutions to match/cancel the reactive part.



    Here are the results from a VNA :



    2.40 GHZ: 89+50i Ohms:

    [​IMG]
    2.45 GHZ: 140-26i Ohms:

    [​IMG]
    2.50 GHZ: 58-70i Ohms:
    [​IMG]


    Thank you :)
     
  2. BR-549

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    Sep 22, 2013
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    Power level? Type and length of feed line?
     
  3. madatr

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    Feb 24, 2017
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    Thank you for your reply.
    Im using RG-316 which should handle the frequencies at hand, length 4 cm so it is less than one wavelength, the connector is SMA type.
    I do not understand which power level do you mean?
     
  4. BR-549

    AAC Fanatic!

    Sep 22, 2013
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    The power level that you are putting into the antenna.

    Your feed line is only 4 cm long?
     
  5. madatr

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    Feb 24, 2017
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    Yes it is, i'm using the antenna as a part of a handheld project, the feedline is temporary to measure the parameters and tune the antenna.
    I am not sure about the exact power i'll be using but approximately it is 3.6 watts ~ 35 dbm
     
  6. BR-549

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    Sep 22, 2013
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    Ok, well, to tune the antenna, I would use a grid dip meter.

    450MC is the highest I ever worked with, other then plug and play. If dip meters at MW aren't available, then a signal generator. You can frequency sweep the yagi.

    If tuning the antenna is your only objective......then a sweep generator.

    Matching a tuned antenna to a feed line......is a different story.
     
  7. madatr

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    Feb 24, 2017
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    I will look more into that, thank you :)
     
  8. BR-549

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    Sep 22, 2013
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    I think it would be hard to tune an antenna, with that short of a feed line.

    The yagi needs to be high up and by itself(isolated) to resonate properly.

    Does the close proximity of the amplifier, or your hand and body capacitance interfere with your operation?

    There might be other methods, with more of your application information.

    People who want a yagi, usually want gain. Height is gain. Feed line length is loss.

    It's always something.
     
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  9. madatr

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    Feb 24, 2017
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    Ok, i feel that i need to elaborate a little more to allow you to see the into the reasoning of my design.

    I am basically designing a jamming system in which i inject noise into the 2.4 GHz band to disrupt all communication, the device must be portable and small in size.

    You were right in your guess that i choose yagi because i need gain and that's self explanatory; more gain == better jamming, also i picked it for good directionality meaning less "collateral damage" in form of interference to other devices.

    I will look into the potential interference of the noise generating circuit nearby to the operation, maybe devise some form of faraday caging.

    My plan is to purchase a balun to match the resistive part: like this one.

    I need advice on matching the reactive part.

    [​IMG]
     
  10. Yaakov

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    Jan 27, 2019
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    I can’t think of a single case where designing a device to produce malicious interference would be a legal thing to do. I have no idea where you are, but in every jurisdiction I am aware of such a device would be prohibited.
     
  11. madatr

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    Feb 24, 2017
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    No sir this is for final year university project, it is always kept at University in a technical lab designed for this and is supervised. I am going into the theory as well as in practice, i will not publish detailed instructions whatsoever in my final released report.

    I'm in the UK, masters of regulation, and this all has been looked over and approved.

    Please take it from a purely scientific perspective, I choose to give more detail just to inspire discussion.
     
  12. Yaakov

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    Thanks for responding. Are you designing the transmitter as well?
     
  13. madatr

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    Feb 24, 2017
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    Yes, Noise source -> amplifier -> bandpass filter -> matching section -> yagi. All either fully or mostly "in-house" designed and made.
    A showcase of all we have learned in our course..
     
  14. Yaakov

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    Jan 27, 2019
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    Does the yagi design you used include a matching section, like a gamma match?
     
  15. BR-549

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    Sep 22, 2013
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    Wow.....I wouldn't want to help build a jammer.

    But if I were building a tracker, I would mount the yagi on a 6-7 ft pole.

    I would use a folded dipole(300 ohm) for the driven element. I would use 300 ohm tv lead for a feed line, connected to dipole, up thru the pole. I would mount a 300 to 50 ohm balun at amplifier output.

    Of course, for UN-conventional uses, I have always wondered.....about mounting an amplifier in the feed point of an antenna.
     
  16. madatr

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    Feb 24, 2017
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    That is the last cog in the machine that i have not been able to figure out.
    I have been recommended by a senior lecturer to get the balun i mentioned earlier to match it (resistive part), but i also know that the smith chart needs to be matched to center to have best performance so i'm left with the reactive part as i understand.
    I read up previously on gamma match and i think that physical based matches will be harder to achieve at such short wavelengths.
     
  17. madatr

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    Feb 24, 2017
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    The antenna uses a folded dipole already, 4 mm thick aluminium rods were used to improve bandwidth.
    So assuming you've built and mounted your antenna for the tracker, how would you properly match it to increase efficiency?
     
  18. BR-549

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    Sep 22, 2013
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    A dipole is 50 ohms. A folded dipole is 300 ohms. Coax is 50 ohms. TV twin lead is 300 ohms.

    Connect twin lead directly to folded dipole.

    A 50 to 300 ohm balun will be the load for your amplifier. The 50 ohm side is amp......the 300 side is connected to twin lead. This port is the output of amp on the board.

    No adjustments should be necessary if the yagi is tuned. And if you want to be really picky....adjust length of feed line for the lowest SWR. Probably won't help that much.

    But it sounds like you have a lab of equipment, you can check it out.
     
  19. Yaakov

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    Your antenna design might be flawed. Most yagis at this frequency use a gamma match on the driven element. Can you show your physical design?
     
  20. madatr

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    Feb 24, 2017
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    Here you go: https://imgur.com/a/I5b53p4, i have used aluminum rod which is next to impossible to solder to, so i've used a metal clamp configuration to connect the cable to the radiator/driven element.
     
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