Multi-Band Wire Antenna Question

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N4LCV, Feb 22, 2013.

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  1. N4LCV

    N4LCV Ham Member QRZ Page

    I have put up a "Wireman Flattop" antenna. It's 135 feet long, & is fed with ladder line. There is a 4:1 balun where the ladder line transitions into a short piece of RG-8X to connect to an MFJ 929 Auto-tuner. It's suspended between two large trees and is up about 35 feet. When I connect my MFJ 259B Antenna Analyzer up to it, what I find is a very high SWR on 80, 40, 15 & 10. These are basically the bands I work. The only band where it's even fairly close is on 20 meters. There are some dips, but none of them, except on 20 is anywhere even close to one of the bands we can operate on. Is this typical of a Multi-Band wire antenna? If someone else out there has a "Wireman Flattop" I'd like to hear what the SWR in your shack is like. I can tune it with a tuner, but it takes a LOT of tuning on all of these bands to get a reasonable SWR. Another Ham told me recently about a friend who has an 80/40 dipole up and with a tuner, he supposedly is able to work all the other bands easily without a lot of tuning and no tuning at all on 80 and 40. When my antenna is tuned properly, it gets out pretty well, but I'm wondering if an auto-tuner is the best way to go considering the very high SWR that has to be tuned. I have an MFJ 941E Versa-Tuner II that I could use instead. What is the collective wisdom of the group about that?

    Thanks & 73

  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    The 4:1 balun may be the wrong choice, depending on the length of the ladder line.

    The antenna system you describe is complex and how it "matches" at the bottom end of the RG8X will depend entirely on the length of the ladder line and the balun. It's not an easy task to make a fixed-length dipole match well on even harmonic frequencies; a "trick" commonly used is to change the ladder line length for each band, and use the ladder line as a matching transformer. There are some fairly easy ways to do that, using a "patch panel" with ladder line sections installed between pairs of banana jacks (or similar), so just moving some jumpers around changes the line length.
  3. N4LCV

    N4LCV Ham Member QRZ Page

    Thanks. This whole system came as a kit, including the ladder line cut to length and the balun. It's all put together as the Wireman's instructions specified.
  4. WB3BEL

    WB3BEL Ham Member QRZ Page

    Maybe you could quantify what you mean by "Very high VSWR" by band.
  5. W5DXP

    W5DXP Ham Member QRZ Page

    Here's some info on just such an antenna:

    Interfaced to a 1:1 current balun, here are the optimum feedlengths per band for avoiding a tuner.

    The yellow region shows the lengths that I use. A fixed length of 100 ft. of ladder-line is a good compromise.
  6. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I couldn't find an antenna that fit your description on The Wireman's website. He does list a "Flat Top Antenna Kit," but that says it comes with 75' of ladderline and doesn't mention anything about a balun. He also does state it requires an antenna tuner.
  7. K8JD

    K8JD Ham Member QRZ Page

    My 250 ft "flat top " is center fed thru a 1:1 balun, 250 ft RG8x coax and has a 1.2:1 SWR at the bottom end of the 160M band, where I operate. NO TUNER.
    My 130 ft "flat top" is centerfed with a 1:1 balun, two hundred ft of RG8x coax and has 1.2:1 in the CW band on 80M where I use it. NO TUNER.!
    M y 90 ft (flat top) dipole antenna is center fed with 450 ohm twinlead right to my balanced input tuner with NO internal balun. It works from 60 to 20M with the tuner adjusted for less than 1.5:1 SWR on each band.

    Sticking a 4:1 Xfmr on the end of a random length of balanced line is guesswork as to the final Z at the radio.

    Measuring the SWR etc, with an analyzer, at the end of the short coax pigtail is futile since this kind of antenna is always used with some kind of tuner, it is NOT designed to be 50 ohms !!!!!.

    Get rid of the 4:1 and use a true balanced input tuner (the kind with NO INTERNAL 4:1 balun) at the end of the ladder line.
    Last edited: Feb 22, 2013
  8. KM3F

    KM3F Ham Member QRZ Page

    You got an assembly that is over complex in my opinion.
    I run 126 foot dipole with a 40m addon element otherwise known as a fan arrangement.
    Fed with 50 ohm coax and worked with an old Heathkit 2060 tuner.
    The feedline has a coil of feedline out at the remote switch to help limit any common mode.
    I can tune any band with this setup and have no feedback issues.
    May not be as efficient on every band but I do use it above 40m, having beams to switch to.
    Good luck.
  9. W1FBV

    W1FBV Ham Member QRZ Page

    Use your antenna tuner.

    73, Jim W1FBV
  10. K9ZMD

    K9ZMD Subscriber QRZ Page

    Bingo. Whenever I've used a doublet antenna with an auto-tuner (either an RT-11 or SG-239) I've always found a couple bands that could not be matched with a single length of balanced line. Changing the balanced line length moved the problem to different bands. However, when I used my Heathkit SA-2040 instead of an auto-tuner, there was no problem finding a match on any band.
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