135' Dipole Problems

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC9QQ, Oct 30, 2009.

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  1. KC9QQ

    KC9QQ Premium Subscriber QRZ Page

    I just installed a DX Engineering 135' multiband dipole and I am having problems matching it with a MFJ 998 auto tuner. My configuration is as follows:

    135' dipole installed between two trees at a height of approximately 60 feet.
    The dipole is fed with 30' 9" of 300 ohm ladder line
    The ladder line is connected to a DX Engineering 1:1 Current balun (as recommended by DX Engineering)
    I then have a 9' run of RG-213 to my tuner

    The one unusual thing about the installation is that I have a trapped dipole (80m and 40m) installed approximately 15-20 feet below and parallel to the 135' dipole.

    I connected my antenna analyzer to the 135' dipole and determined that it was resonant at 2.9Mhz, 6.0 MHz, 10.7 M, 16.0 Mhz, 24.6 Mhz and 28.3 Mhz.

    I am reluctant to reduce the length by 13' feet on each end (difference required to move resonance from 2.9 to 3.5 Mhz) since I know that many hams successfully use this type of antenna without problems. I have rechecked the element lengths and they are 67' on each side of the dipole.

    Is it possible that the second dipole 15-20 feet below the first is interacting with higher dipole?

    Any comments or suggestions would be appreciated.


    Last edited: Oct 30, 2009
  2. W4PG

    W4PG Super Moderator Lifetime Member 279 Volunteer Moderator Platinum Subscriber Life Member QRZ Page

    RG 213 is 50 ohm coax. You are connecting it to 300 ohm lead via a 1:1 balun. See the problem?

    Ideally, you would need a 6:1 balun to match 300 ohm twinlead to 50 ohm coax. Realistically, you can find a 4:1 easier and it will work nearly as well.

    Depending on your tuner, you may be able to bring the twin lead straight into the tuner without using the coax. See if your tuner has inputs for twin lead and use that instead of the coax. It's the same as the "balanced line" input.

  3. G3TXQ

    G3TXQ Ham Member QRZ Page

    That's a common misuderstanding!

    The impedance at the coax/twinlead junction can only be 300 ohms if the antenna impedance is 300 ohms, which it never will be. The impedance at the junction varies widely from band to band and a 1:1 balun is the appropriate choice.

    Modelling your arrangement, there are some extreme impedances which I'm not surprised the tuner cannot handle. They are occuring because of an unfortunate choice of feedline lengths. For example, on 20m the antenna is very high impedance because it's 2 wavelengths long; the twinlead is about a half-wavelength long and so replicates this very high impedance at the coax/twinlead junction; the coax is very nearly a quarterwave long and transforms the very high impedance to a very low one - 0.7-j18 ohms in my model. That's an impedance the tuner just cannot cope with.

    I see similarly extreme impedances on 30m, 17m, 12m and 10m.

    Try altering your twinlead or coax lengths and see if things improve.

    Steve G3TXQ
  4. G3TXQ

    G3TXQ Ham Member QRZ Page

    I forgot to mention that the resonant frequencies you measured are probably not the resonant frequencies of the antenna, unless you were making the measurement 60ft up in the air at the feedpoint :)

    You are measuring the resonant frequency of the antenna system - antenna+twinlead+coax, and the frequencies you report look consistent with my modelling.

    Steve G3TXQ
  5. KL7AJ

    KL7AJ Premium Subscriber QRZ Page

    Yipper, Skipper! Everyone needs to read "My Feedline Tunes My Antenna" by W1DX (SK)

    When in doubt you can always cut the transmission line to an exact half-wave. Then it doesn't matter what the Z0 is...it will still be the same as the antenna impedance at the feed input end. :)

  6. KC9QQ

    KC9QQ Premium Subscriber QRZ Page

    Thanks for all the input. Also thanks for taking the time to model the antenna. The antenna modeling gives me some comfort that the antenna is working as it should.

    As soon as the rain stops here in Indiana I will try to modify the feedline length and see if that solves the problem. I may have to zig zag the line a bit to get some additional length.

    I let everyone know how it works out.

    Thanks again,

  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Something doesn't add up here. You said 30'9" of ladder line, then 9' of RG-213/U to your tuner. That adds up to 40 feet. How'd you accomplish that with an antenna up 60' above ground? Is the tuner actually hanging 20' above ground, in the air?
  8. KC9QQ

    KC9QQ Premium Subscriber QRZ Page

    Actually it is. The ham shack is on the second story of our house. Also there is a slight slope to the antenna. The feedline drops down to the second story roof and from there I use the coax to bring it to the entry point to the shack.

  9. W4PG

    W4PG Super Moderator Lifetime Member 279 Volunteer Moderator Platinum Subscriber Life Member QRZ Page

    Which explains why I prefer resonant antennas fed with 50 ohm coax. Put 'em up, make sure they are resonant, then forget ever needing a tuner. Works for me!

    I hate having to use a tuner. To me, it's a sign of an improperly made antenna. If you can't built the antenna without the need for a tuner, then something is wrong or the designer is just lazy. And no I'm *NOT* talking about a Pi network for the final amplifier. Indeed, I would argue that the rapid rise in tuner use was related to the switch to transistors in the final amp stage, which don't tolerate higher SWRs as tubes do.

    Alas, there are always compromises to be made and certainly there is something to be said about the simplicity of a dipole fed with twin-lead . . . you can usually use a tuner to get a decent match . . . most of the time, without huge loses in the feedline (something you can't do with coax).

    I don't use twinlead of any kind for feedlines, thus obviously don't speak as any kind'a expert on the stuff. I tried a G5RV once and it was the worst antenna I'd ever used, all things (loading, etc) considered.

    .........Bob <------- quickly throwing out all his 4:1 baluns! :D
  10. KC9QQ

    KC9QQ Premium Subscriber QRZ Page

    Well I finally have gotten this antenna to work well for me. After receiving help from those of you that replied and reading the article' "My feed-line tunes my antenna" I have been able to adjust my feed-line to get good performance on 80-10m. I also purchased EZnec and have started to learn how to model simple antennas. I modeled my original installation which had a 80/40 trapped dipole installed about 20 feet below the new 135' doublet. The modeling showed me the significant impact the original trapped dipole was having on my new doublet. After I removed the original dipole I started to get much better DX performance from the new doublet.

    Thanks for all the help.

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