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Dipole with Ladder Line and Tuner

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N1ETL, Dec 3, 2018.

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  1. N1ETL

    N1ETL Ham Member QRZ Page

    I've installed an 80 meter dipole fed with around fifty feet of ladder line to a DX-Engineering current balun and SG230 tuner. It tunes fine on all the bands, including 160, which surprised me.

    Just for curiosity, I connected my Rigmaster antenna meter to to balun to see where it was resonant. Surprisingly, the resonant frequency is nowhere close to what I expected. Now I know that when using a tuner and ladder line, the resonant frequency doesn't really matter. But I would love to understand why the graphs are as they are.

    The dips (base on 50 ohms) from left to right are: 4.666MHz, 13.998 and up.

    upload_2018-12-3_14-6-56.png

    Phase measurements.

    upload_2018-12-3_14-7-24.png
     
  2. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    IIUC your set-up, your feedline is probably transforming the impedance of the system at the shack end to something other than the impedance at the feedpoint of the antenna. Changing the length of the feedline will move the low SWR points around.
     
    KD6RF likes this.
  3. WR2E

    WR2E Ham Member QRZ Page

    Check out this website for info on what you're seeing there

    http://www.w5dxp.com/notuner/notuner.htm

    Also, he has recently had a couple threads here on QRZ about this type of matching. Search for W5DXP and you should find them.

    @W5DXP
     
    Last edited: Dec 3, 2018
    N2EY likes this.
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    You've shown SWR and Phase angle, it would have been ideal to also show Complex Impedance (Z). SWR50 alone doesn't display resonance, it displays quality of match to a 50 ohm system. Phase Angle does indicate resonance but doesn't tell you the feed point resistance at resonance. You could take SWR50 and Phase Angle together that would reduce the feed point resistance at each Phase Angle zero crossing to one of two possibilities but you still wouldn't know which of those two possibilities is the actual feed point impedance at resonance nor would you know the actual feed point complex impedance off of resonance.

    Remember, resonance is achieved when the reactive term in a complex impedance goes to zero, not when an SWR50 of 1:1 is achieved. In some antennas those things happen at the same time but in general resonance and minimum SWR50 are not the same things. Phase angle indicates resonance when it crosses through zero but that doesn't tell you the value of the real impedance at resonance.

    So from your phase angle graph it looks like the antenna system, including the feed line, is resonant in the following ham bands:
    - ~3.5 MHz or the very bottom of the 80m band
    - ~10 MHz (in the 30m band)
    - Around 28.6 MHz (in the 10m band)

    But those resonant points may or may not be good matches to a 50 ohm system. For instance it looks like your resonance on 80m does not provide a good SWR50 match but your resonance on 30m isn't too bad but the resonance in the 10m band is again a bad SWR50 match.

    If your tuner can achieve a decent match on all the ham bands you want to work then you're in decent shape but one trick on doublets is to avoid cutting the antenna to be exactly a half wavelength long on any of the harmonically related ham bands. For instance with the antenna halfwave resonant in or very near the 80m ham band your antenna will present a very high feed point impedance on the even harmonic bands like 40m and 20m and perhaps 10m. If you cut the antenna a bit long or a bit short for 80m your match on that band won't be much worse and should be easily matched with your tuner but the match on the even harmonic bands typically becomes much easier. Convenient choice of feed line electrical length can change that a bit but it's generally easier to start out with an antenna that's intentionally non resonant on one of the harmonic ham bands.

    Given what you've stated and shown above it looks like you've got an 80m dipole that in and of itself presents a pretty decent SWR50. But you've also got around 50 feet of balanced feed line that when you take velocity factor into account is pretty close to 1/4 wavelength long so it's transforming that resonance and decent 50 ohm match near 3.5 MHz into a very high impedance that luckily your tuner can still match.
     
    WB5YUZ likes this.
  5. AA5CT

    AA5CT Ham Member QRZ Page

    Did you say how high up your dipole is?
     
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    You created (nearly by accident) what G5RV did on purpose....
     
    W2AAT likes this.
  7. W5DXP

    W5DXP Ham Member QRZ Page

    N2EY likes this.
  8. AG5CK

    AG5CK Ham Member QRZ Page

    Wasn't G5RV trying to build a 20m antenna?
     
  9. N1ETL

    N1ETL Ham Member QRZ Page

    The dipole is around 30 feet high with the last 15 feet of each leg pointing down due to limited space. Fortunately, it tunes fine.
     
  10. AI3V

    AI3V Ham Member QRZ Page

    Well, you got random results because you installed a collection of parts Willy-Nilly. :)

    Back up, and try a coax fed dipole, and choose a frequency where you can get the dipole a dozen wavelengths away from everything else and get back to us with your measurements.

    :)

    Rege
     

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