Question on doublet Matching / Performance

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by ND7IP, Apr 6, 2010.

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  1. ND7IP

    ND7IP Ham Member QRZ Page

    Hi all. From an FT-990 I am running 50 Ft. of coax into a 4:1, then 100 ft of twinlead to a flat-top 135 ft doublet configuration, center height at 50ft. Its doing well on 80-10 with the onboard auto tuner.

    I have an 'overkill' Nye Viking MBV-N-A that will match the twin lead in the shack, and I have a way now to get the twin lead straight from the flat-top center height into the NV.

    In your opinion will this be a more efficient antenna if I feed the twin lead directly to the tuner, and bypass the 50ft of coax and the 4:1 balun, or will the difference be insignificant?

    Thanks in advance for any feedback.

    73's, KK7YU
     
  2. AF6LJ

    AF6LJ Ham Member QRZ Page

    In a word; Yes.
    There will be lower overall loss.
    With the tuner at the far end of the coax, what the coax is seeing is often (nearly always in your setup) not the characteristic impedance the coax was intended for it will radiate and your losses will be higher.

    Getting the balanced line all the way into the shack is good. :)
     
  3. W5DXP

    W5DXP Ham Member QRZ Page

    Have you measured the SWR on your coax? That would probably answer your question. 100 feet of ladder-line is close to 1/2WL on 75m. If the feedpoint impedance of the 135 foot dipole is 50 ohms, that impedance will be repeated at the balun end of the feedline. The 4:1 balun will take 50 ohms down to 12.5 ohms for an SWR of 4:1 on the coax. Why would anyone want to do that when the 50 ohms would have been a perfect match using a 1:1 balun?

    The balanced tuner will not only eliminate the coax but will eliminate the 4:1 balun which is a detriment to 75m operation.
     
  4. K4SAV

    K4SAV Ham Member QRZ Page

    Probably the only thing allowing the internal tuner in the FT-990 to tune that antenna is the loss in the feedline and balun.

    Will you be able to hear the difference? I doubt it. Can you detect a 2 dB difference when listening to two different antennas at different times?

    Assuming you are using RG8X, the improvement should be a little over 2 dB on 20, 12, and 10 meters, about 1 dB improvement on 80 and 15, and less than 1 dB on the other bands.

    That's assuming the balun has insignificant loss, which is questionable.

    Jerry, K4SAV
     
  5. NA0AA

    NA0AA Ham Member QRZ Page

    It will be lower loss - but will you notice the difference? On some bands more than others most likely. I would say do it just because it's cleaner to skip unneeded items like the coax and balun.
     
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