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Mag loop antenna tuning questions.

Discussion in 'Home Brew' started by Alex Faught, Mar 15, 2017.

  1. Alex Faught

    Alex Faught
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    I have been doing some research on building a magnetic loop antenna. I built my first antenna (a half wave inverted v dipole), I am hooked and looking for another antenna project. My concerns are mainly coupling and tuning. As far as coupling is concerned I was thinking of using a gamma match system vs the common faraday loop, from what I have gathered, the gamma match system produces more consistent results. I intend on using an air variable capacitor for the tuning cap. My questions are these. 1. Is the gamma match tuned for desired impedance? My goal is 50 ohms. 2. If the gamma match is used for achieving desired impedance, the tuning cap is used to achieve desired swr or resonance? I have read pages and pages of material and I am still trying to wrap my mind around the tuning process using a gamma match system in conjunction with the variable cap. Any advice would be much appreciated. Thanks -Alex


     

  2. Road Squawker

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    the gamma will allow you to have enough bandwidth to cover about 3 bands (say 17, 20,30).

    after trimming the gamma, tune the air cap for max "noise" by ear.

    I hope you don't intend to run much power in to the air cap.

    have fun
     
  3. Alex Faught

    Alex Faught
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    From what I have gathered, I am going to have about 5.5kv at the cap, I intend on getting one that is rated a little higher so I have some leeway. My question is how to go about tuning. The gamma is match is for impedance and the cap for resonance?
     
  4. Road Squawker

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    The purpose of the Gamma is to match the antenna input impedance to the transmission line’s characteristic impedance, allowing max power transfer.

    the Cap brings the antenna into resonance where the impedance will be (mostly) resistive

    Z = R + jX
     
  5. Alex Faught

    Alex Faught
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    So once I tune the gamma first for the desired 50 ohms, I then move on to the tuning cap adjusting it for resonance?
     
  6. Captain Kilowatt

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    Other way around. The cap tunes the loop to resonance at the desired frequency while the gamma match transforms the impedance of the now tuned loop to 50 ohms. Resonance simply means there is no reactance at the feedpoint and the resulting impedance can be anything. It is the job of the gamma match to transform that impedance to 50 ohms. Good luck on finding a suitable air cap good for more than 5.5 Kv BTW. Most people use a vacuum variable for anything over 100 watts. How much power do you intend to run?
     
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  7. Alex Faught

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    Right on, I think I understand the just of it. Thank you both very much!!!
     
  8. 222DBFL

    222DBFL
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    Listen closely to these gents they know what they are speaking about!!
     
  9. Captain Kilowatt

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    Well as long as we are not discussing quantum dynamics I might be fine. :ROFLMAO:
     
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  10. Alex Faught

    Alex Faught
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    I was reading a little more on the faraday loop vs gamma match for coupling. According to what I have gathered, the radiation pattern of a gamma match is asymmetric because the gamma match is usually off center vs the faraday loop having a symmetrical radiation pattern due to the primary loop being symmetrically placed in the secondary loop. With that said could the gamma match be offset equally in both directions once tuned to provide a symmetrical loop thus producing a symmetrical radiation pattern?
     
  11. Alex Faught

    Alex Faught
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    Sorry for the bad drawing but this is what I am getting at. mag loop antenna.png
     
  12. Captain Kilowatt

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    Take no offense to this but you need to study up on some basic antenna fundamentals. You CANNOT move the gamma match. It has to be located where it is for tuning purposes. You cannot simply tine an antenna and then move it somewhere else. The gamma match taps the antenna element at the point where there is the proper impedance. Moving it throws that point out the window, down the street, around the corner and over to the bad side of town.
     
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  13. Road Squawker

    Road Squawker
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    what you are describing is a "hairpin" match, AKA a Beta match.... see above post #12
     
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  14. Captain Kilowatt

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    Especially the last sentence.
     

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