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6M Wire Dipole - Electrically Short, Physically Long?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC7RAD, Jun 14, 2019.

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  1. KC7RAD

    KC7RAD Ham Member QRZ Page

    Hi all,

    My first attempt at building a dipole for >10M has left me in a conundrum.

    I calculated each side of a 6M dipole for 51MHz is about 4.59 feet (234/f). So, I cut each wire to five feet, soldered it onto the 50Ohm feedline, tied the ends of each leg to ropes in the trees and raised her up.

    The results are 3:1 SWR on the low end of the band and 2.5:1 on the high end. So... The antenna is physically longer than the calculated length but... appears to be too short electrically?

    Do I solder more wire onto the end of the legs to make it longer? Have never had this issue on all the HF dipoles I have built. Thoughts?

    TNX!
    -Ken
     
  2. KI8DJ

    KI8DJ Ham Member QRZ Page

    Add wire that formula is only a rough guidline. Also vhf behaves a little bit differently.
     
  3. KK4OBI

    KK4OBI Ham Member QRZ Page

    For a half wave at 50.1 MHz I get 4.81 feet using a velocity factor for copper wire of 0.98. That should be slightly long for tuning because no adjustment for end effect.

    Changes in elevation over ground affects impedance only a few Ohms but resonant frequency changes quite noticeably with height. Frequency is higher at multiples of a half wave and lower in between half waves so you might be able to jigger your tuning this way. Tuning by adjusting sag is ok too.

    Your hook-up is fine because common mode currents for a resonant center-fed dipole are trivial. No chokes were used for decades (even though they are popular now).
     
  4. W9KEY

    W9KEY XML Subscriber QRZ Page

    I'm trying not to make a dumb suggestion here, but did you sweep the entire 6 band with an antenna analyzer? Where is the minimum SWR point? Could there be more than one dip? The band is 4 MHz wide and (not having built a 6 meter antenna) might not tune the entire range with one simple dipole. You didn't say how far the dipole is from the trees. If too close, might affect the tuning.
     
    NH7RO likes this.
  5. K7JEM

    K7JEM Ham Member QRZ Page

    I would shorten each leg by around 3" or so, by folding back the ends without cutting them. Then check again. An antenna analyzer would make it easier.
     
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    Not at all trivial. At certain coax feedline lengths they are huge!

    Suppose his 6m center-fed-dipole without balun at the feedpoint is 18ft agl. The coax runs straight down, and is grounded to the earth straight below the feed-point. Further suppose that the ground rod resistance is 35 Ohms. Here is a model:
    6mm.png

    A coax length (antenna height) of 18 ft is shown with the antenna resonant at 51.00MHz. At this height, the common-mode current on the coax is benign.

    But watch what happens as the coax length (antenna height) is varied from 18ft to 32ft (see the X-axis in both plots):
    6mDipole RandX.gif 6mDipole SWR.gif

    Notice the effect on R, jX and SWR. The SWR varies from 1.25 to 1.8. The effect is periodic. Bad stuff happens when the coax length is a multiple of a half wavelength (20, 30ft, ...)

    This is happening because there is no common mode choke at the feed-point which effectively connects wire 3 (antenna leg) in parallel with wire 4 (the common mode current on the coax shield), and at certain lengths of wire 4 it becomes a resonant grounded monopole (i.e. vertical antenna), which profoundly unbalances the current in the antenna legs.

    Here is an animation that shows the current distribution in the three wires (1,2 and 3 that comprise the antenna wires, the first 37 segments) and wire 4 (the coax shield) as the length of the coax shield steps from 18ft to 32ft, hitting resonance near 20ft and again at 30ft. Watch what happens to the current balance in the dipole (on the left), and watch the standing wave that forms on the coax shield (on the right side):

    ezgif.com-optimize.gif
     
    AH7I, KC8VWM and NH7RO like this.
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Definitely sweep the entire band if you're using an SWR bridge and a transmitter, and take data every 250 kHz or so to see what's really going on.

    If you have an antenna analyzer, sweep way outside the band, maybe 49-55 MHz or something and take data points or if you have a fancy one, let it plot that sweep.

    If you're looking to work weak-signal modes (SSB-CW-Digital) you wouldn't want to optimize for 51 MHz, you'd want to optimize for about 50.2 MHz. There ain't much at 51 MHz, and for FM work above 52 MHz you'd want a vertical, not a horizontal dipole.
     
    NH7RO likes this.
  8. VK3YE

    VK3YE Ham Member QRZ Page

    Video shows a successful wire dipole I made for 6 (and 10) m.

    Dimensions shown about 1/4 way in.

     
  9. KC8VWM

    KC8VWM Ham Member QRZ Page

    Try choking the living crap out of that feed line... lol
     
    NH7RO likes this.
  10. W5LZ

    W5LZ Ham Member QRZ Page

    The common antenna length formulas are a fairly close approximation of the 'end length', they are not exact by any means. They also tend to have a 'fudge factor' built into them, just in case you know. So, adding a little bit of that length to make up for making connections (mechanical and electrical) is a pretty good idea (another one'a them "you know?" thingys). Having a new antenna a little long is better than having it a little short! You can always shorten one, it ain't easy to add to them. <-- Cheap wisdom learned expensively!
    You will always make mistakes. Sometimes small ones, sometimes not. Stick to the easy ones when you can.
     

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