40m wire vertical poor performance and high swr

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KJ7DX, Dec 29, 2016.

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  1. KJ7DX

    KJ7DX Ham Member QRZ Page

    Just put up a 40m 1/4 wave wire vertical as an experiment. Triple checked measurement and it is right on. I have added 12 radials so far of random lengths (ranging from 6 - 30 ft due to space issues) and it is performing terribly. Analyzer shows swr of 6.5:1 at 7.120. In fact, the antenna isnt better than about 5:1 anywhere from 0-30 mhz per the analyzer swr scan. So trimmimg or adding a bit of wire isnt going to help.

    I know it needs some more radials but I figured it would at least be a semi decent swr with the 12 radials. I tuned it up with my manual tuner and got it to a usable point swr-wise but horrendous rx performance...S0 noise floor (not realistic here) and a station S9 on my dipole was only S2-S3 on the vertical.

    I am using a 1:1 ldg current balun at base of the antenna and brand new lmr240 coax so doubt there is a cable issue. Is this really all about the radials? Or maybe a bad balun, or ?

  2. AI3V

    AI3V Ham Member QRZ Page

    Loose the balun. It has no place on a coax fed Marconi.

    Ohm out the coax.

    A 33ft +/- Marconi and 12 ground wires and fed with 50 ohm coax will resonate somewhere near 40m with a better than 2:1 vswr.

    One other thing, how many wavelengths between your antenna and other objects?

    KD6RF likes this.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Based on the measurement results posted, you have an open circuit or a short circuit somewhere.

    I'd also ditch the balun for now and first check the resistance across the coax connector at the "shack" end of the cable. It should be an open circuit (infinity Ohms). If it is, then you don't have a short circuit.

    Next, attach a clip lead or some other good conductor directly across the feedpoint of the vertical, from the vertical wire to one of the radials, then go back inside and measure the resistance across the connector again. Now it should be a short circuit (0 Ohms) or very close to that, maybe 1 Ohm at most. If it's not, you have an open circuit somewhere.

    If the antenna was really "connected," SWR "everywhere" can't be that high and your noise level can't be S0.
    KD6RF likes this.
  4. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Once you get about 8 ground radials on a vertical one should have a fairly stabilized SWR curve.

    As we add radials the impedance continues to drop and approaches the natural impedance of about 35 Ohms (not 50!) which yields about 1.43:1. So, keep in mind that it may not be possible to obtain a 1:1 match, depending on antenna design and the surrounding environment.
    KD6RF likes this.
  5. KJ7DX

    KJ7DX Ham Member QRZ Page

    Just finished checking the items suggested. Has to be a balun issue. As soon as I took that out of line and hooked up radiator and radial hub directly to coax with clip leads swr is now 2.1:1 at 7.120. Analyzer shows a nice dip down to about 1.5:1 at around 6.9 mhz so I just need to do a bit of trimming now. That is much more reasonable. Noise level is now at about S4, which also makes a lot more sense. Thanks for the help.
    KQ0J, WB5YUZ, K7TRF and 2 others like this.
  6. AI3V

    AI3V Ham Member QRZ Page


  7. W8NSI

    W8NSI Ham Member QRZ Page

    For your ground mounted vertical, the ONLY balun you MIGHT need is a coil of coax or string some ferrite beads on the coax next to the feed point.
    It will be usable without. If you cant get a good swr in the band after removing that balun, then you have some other issue, like locating the wire near a metal obstruction or maybe a bad connector on the coax? Many on here can attest to that.

    A measured 1/4 wave using the formula will be in the ball park. the dozen or so radials should also work, more is better.

    I have a vertical mounted up 20 feet with drooping radials down to the ground and them running along the ground.
    In this configuration, I use only 4 radials as an elevated vertical with drooping radials. It works!
  8. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    Here's what a 1/4 wave vertical can do, if you raise the feedpoint at least a yard up (I was a bit over 2 yards up ), then slope the radials down to within at least a few feet of the ground, without touching. Mine are actually in a different configuration, to fit on the roof (see video from 8 minutes on). I recorded a nice 10w contact with my Elecraft KX2. This was a 15m contact, but on the 40m antenna. It did pretty well on 15, I should say. I forget the actual numbers, but 4 raised and tuned radials equals about the same as either 32 or 64 ground-laid radials. I think it's closer to 64.

  9. W6OGC

    W6OGC Ham Member QRZ Page

    The first thing to check when you encounter these conditions, or alternatively, the first thing to check when you start an antenna project (you pick), is to check the coax and connectors for proper operation.

    Other than not following the directions, it's the most important, and frequent, source if anomalies.
  10. K8JD

    K8JD Ham Member QRZ Page

    QW vert with a plane ground counterpoise system will be closer to 35 than 50 Ohms, 1.5-2:1 SWR will be about what to expect. An Elevated Vertical (maybe roof mounted) with fewer, drooping radials can be made to get FLAT SWR readings. Also a lower radiation take off angle if high enough,.
    Been there done that !
    W8NSI likes this.

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