135' Dipole Problems

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

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  1. K8JD

    K8JD Ham Member QRZ Page

    My approach for operation on multiple bands has been either to use traps on one antenna or multiple antennas tuned for close to 50 ohm Z at the feedpoint and coax feed. !
    The results are predictable, good performance and no shack RFI Problems or required external tuners !
    Similar views to W4PG...
     
    Last edited: Nov 15, 2009
  2. NL7W

    NL7W Ham Member QRZ Page

    I have run a home-brew 135 foot flattop, fed with 450 ohm ladder line for years and years. It works extremely well.

    Why it works well is because I tune it with a quality, legal-limit, balanced line antenna tuner from Palstar, the BAL1500A -- my 450 ohm ladder-line comes through the house with specifically designed insulators. My quality tuner tunes my "antenna system," consisting of the radiating dipole copperweld wire, low-loss ladder feedline, and the antenna tuner. My antenna system losses are very low, or lower than, just about every other "multi-band" antenna out there.

    Moreover, my ladder-line really doesn't radiate, as the current balance (voltage out-of-phase) is near perfect, given the quality tuner I use, that it utilizes an input 1:1 balun and totally floating network for balance line matching. The antenna system is a gem...

    At about 45 to 50 feet from tree to the other, it's "da bomb" on 17, 20, 40, and 75/80M bands, and handles my Alpha in fine style.

    73.
     
    Last edited: Dec 13, 2009
  3. NL7W

    NL7W Ham Member QRZ Page

    Hi Fred,

    Usually, the balun of choice for a 135 foot multi-band dipole, in your configuration, is a 4:1 current mode specifically made for your configuration.

    Let's see... MFJ makes one, and DX Engineering makes one. Others do, too. But, I'm getting lazy with the hyper-links...

    The 4:1 impedance step-up is the classic method when moving from 50 ohms unbalanced to either 300 or 450-ohm balanced line, then to a multi-band 135 foot dipole. On the HF bands, the VSWR on the balanced line can vary from 1 or 2:1 to 10:1 (referencing the 300 or 450 ohm line impedance -- which is no matter, cause the losses on this ladder-line are practically nil). The balun's 200 ohm output impedance presents a good compromise with the impedance range seen with this antenna's multi-band operation -- far better than the 1:1, 50-ohm balun.

    I would see if DX Engineering would accomplish some type of an exchange or discount, based on the faulty advice they gave you -- the wrong impedance ratio choice for a 135 foot flattop.

    You need a 4:1 or 6:1 TUNER-USE BALUN -- designed for the high voltages and impedance range your particular antenna system's balun will see.

    73 de Steve, NL7W



     
    Last edited: Dec 13, 2009
  4. G3TXQ

    G3TXQ Ham Member QRZ Page

    Take a look at the DX Engineering note "Choosing the correct balun" by Tom (W8JI). In particular read the sections on "Systems Requiring Antenna Tuners" and "Ladder Line or Open Wire fed Dipoles or Doublets". You'll see that their recommendation is for a 1:1 current balun:

    http://www.dxengineering.com/TechArticles.asp?ID={3E5220F7-2D0F-45B5-85F7-3B654F804C4F}

    The same note also explains why a 4:1 impedance transformation is unhelpful in this application:

    73,
    Steve G3TXQ
     
  5. AD0KI

    AD0KI Ham Member QRZ Page

    I'm just splitting hairs here but 30'9" + 9' does not add up to 60' above ground.... unless the middle sags considerably.
     
  6. W5DXP

    W5DXP Ham Member QRZ Page

    Here's some additional information that might help: www.w5dxp.com/notuner.htm
     
  7. AE1PT

    AE1PT Ham Member QRZ Page

    Steve (NL7W) makes an important point here. Remove the current choke at the terminus of the ladder line, and you have a very nice 135' doublet that can be tuned just about anywhere 80-10m. Add a good tuner (I use a Dentron MT-3000A) and you are good to go.

    If for some reason you cannot bring the twinlead in the house, it has already been recommended you use a 4:1 exterior balun to make the transition to coax. Inside the tuner, this is all that happens with the twinlead--it goes into the 4:1 balun and into the matching circuit.

    A G5RV is a wonderful thing. All this DX Engineering overpriced marvel is doing is creating another sort of the same thing--longer flattop, and different lengths for the ladder line and coax. Cut the coax off and do yourself a favor!
     
    Last edited: Dec 13, 2009
  8. NL7W

    NL7W Ham Member QRZ Page

    Hello,

    Normally I agree with Tom on most subjects, except this one. As mentioned, the impedances encountered across ladder-line while op'ing 10 to 75/80M with the 135-foot flattop discussed here, should (do) exist well above the low impedance 50-ohm area. A quality step-up, multi-core, Teflon wire wound, non-saturating, high-power balun should provide a better match/transformation and higher efficiencies overall.

    It would be interesting to determine an acceptable method to measure the difference in efficiencies between 1:1 and 4:1 balun use in the future. For example, I know my presently unused W9INN 4:1 ratio remote balun box from MFJ has measured to have poor performance at 21 MHz and above. So if I was to use this balun, I would likely limit it's operating range between 3.5 and 18MHz bands only. [Ed: I just noticed that Tom has made a similar bandwidth limitation statement in is write-up on tuner baluns -- his top end was 20M. :)]

    One should realistically determine the capabilities and limitations of each of the components used in a system, and use that system knowing the caveats.

    To each his own... YMMV.


    And N4PRT is right -- he agrees with me! Running the balanced line into the home to a quality tuner is, without a doubt, one of the most efficient methods to tune your multi-band wire antenna system for any specific HF frequency. :)


     
    Last edited: Dec 13, 2009
  9. G3TXQ

    G3TXQ Ham Member QRZ Page

    Steve,

    A couple of years ago I modelled a 135ft dipole with a variety of ladderline lengths, and looked at the whole range of impedances you would see at the tuner from 80m to 10m. I then used W9CF's tuner simulator to predict the losses for all those impedances, with and without a 4:1 transformation.

    Of course, sometimes the 4:1 helped, and other times it made things worse. Averaged across all bands and all the ladderline lengths I tried, a 1:1 was marginally the better choice; in addition it had the virtue of only needing the one core, and it also avoided the most extreme losses which occured when the 4:1 was transforming downwards an already quite low impedance.

    Of course, in a specific scenario where the ladderline length is known, the 4:1 may turn out to be a better choice; but I think I understand why DX Engineering would recommend the 1:1 in a general guide.

    I'll see if I can find the spreadsheet I recorded it all in :)

    Steve G3TXQ
     
  10. NL7W

    NL7W Ham Member QRZ Page

    Hi Steve,

    I would be interesting to see this, indeed. I have an open mind, and would be willing to change my beliefs with compelling evidence -- of course.

    BTW, I do not use this config at my home.

    ...

    Regarding my specific antenna -- which doesn't use a balun and coax between the antenna and it's matching network.

    At this time, my specific length antenna which is somewhere between 125 and 140 feet (I forgot what I settled on 8 years ago, after modeling and graphing several bands' impedances with W7EL's program -- it's probably a bit longer). The specific feed-line length of 16 ga., 420 ohm ladder-line was also a settlement on my part, of course, to serve the bands noted below. I do not tune up on 10, 15, or 160 meters due to wild impedances encountered, and tuner transformation difficulties and inefficiencies. I was expecting this...

    So, my operations are limited to specific bands where efficient matching takes place -- 75/80, 40, 20, and 17M. 10 and 15M are to be handled by a tri-bander when they come alive again. 160M is out with this short antenna.

    Remember, I bring ladder-line into the house to fully floating balanced line tuner. I know my system's limitations, and modeled them for higher impedances ahead of time. It's worked for me, and the efficiencies are fantastic. :)



     
    Last edited: Dec 14, 2009
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