DIPOLE SWR PROBLEM

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KB8YBG, Jan 21, 2018.

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  1. KB8YBG

    KB8YBG XML Subscriber QRZ Page

    Hello my name Jim call KB8YBG. This fall I put up a new tower with a beam and on that tower I have 3 wire dipoles for 17, 40, and 75 meter. Swr was beautiful on all 3. In the last couple weeks my 40 and 75 have both climbed from a 1.1 to a 3. My 17 is still 1.1. I thought at first it was ice but it did not effect all 3. Now we had a melt and swr is still high on both. For example my 40 was cut for 7.200, now the swr is 1.1 down at 6.850. My 75 was cut for 3.850 and now swr is 1.2 down on 3.752. I am getting frustrated with these and not sure what to do. They are pretty high on tower and up in trees. Any advice would be great. Thanks again. Jim KB8YBG
     
  2. G0JUR

    G0JUR Ham Member QRZ Page

    looks like your wire has stretched some.
     
  3. KM4DYX

    KM4DYX XML Subscriber QRZ Page

    Could be stretch. What kind of wire? Insulated or no? I'd lean toward moisture in the connectors or coax. Sounds like you're gonna have to drop those dipoles and check them out. Good luck.

    73,
    Al
     
  4. N7WR

    N7WR Subscriber QRZ Page

    Wire stretching as a result of ice loading is possible. Even after the ice melts the wire will still be stretched. Your SWR results do indicate that both of the antennas that changed are too long and that's why the SWR is better at a lower frequency.
    What type of wire did you use? I live in an area that gets heavy ice loading and my wire antennas (all home brew) were constructed using wire that has very minimal stretch (if any). I use 14 ga copper-weld steel which is available at a number of ham supply outlets.
    If the ends of those dipoles are easy to get to a little trimming should bet you back to the best SWR where you want it to be.
    As for the comment that it could be moisture in the connectors or coax I guess that is possible but IMO not the most likely cause. In my experience water in coax and/or connectors has a much more profound effect than just moving the SWR point lower in the band.
     
  5. KB8YBG

    KB8YBG XML Subscriber QRZ Page

    I used solid 14 guage coated wire. These dipoles were purchased from a company in Ohio and I have had great luck with them. I can understand 1 out of three acting up but 2? They are the longest ones and they are tied into trees. I guess stretching is a possibility. If it was water in coax I think all 3 would have same problem. I am going to take 75 down since its the easiest to get at and measure it out.
     
  6. K5RT

    K5RT Ham Member QRZ Page

    That is a lot of stretch! Check the coax and balun before trimming.
    Disconnect the coax from the balun and check continuity of the center conductor and braid.
    If the coax is good, remove the balun and direct connect the wires to the coax.
     
  7. KB8YBG

    KB8YBG XML Subscriber QRZ Page

    I agree....I am out looking at them and there is no sag in wires...I think if stretched u would see dip in wire
     
  8. N7WR

    N7WR Subscriber QRZ Page


    I guess I'm not clear on something. Are all three dipoles fed with one coax through a balun or does each have its own separate feedline? If they are fed with one coax and the SWR on one of the 3 antennas has not changed then I think you can rule out moisture in the coax or balun or a problem with the balun. Solid 14 gauge coated wire WILL stretch...been there and done that. And it does not take a lot of stretch to make the best SWR point drop down in the band. That is why I use copper-clad steel wire as there is little to no stretch to it. If you wind up taking the whole thing down scrap the balun and just make an RF choke out of 8 turns of coax, 8 inches in diameter. A dipole really doesn't need to be "matched" to 50 ohm coax. It simply needs an RF choke to keep RF from traveling back down the feedline.
     
  9. KB8YBG

    KB8YBG XML Subscriber QRZ Page

    No all separate coax...these are just monoband. ...I used paracord to tie in trees...I think that would stretch before wire would
     
  10. WA9UAA

    WA9UAA Ham Member QRZ Page

    Good Day,
    The change from 7.200 to 6.850 shows a change in length of 3 ft. and about 4 inches on the 40M dipole. The 80M dipole shows a change of just over 3 feet as well. That seems like a lot of stretch. I use 14 ga. stranded but feed with ladder line so no worries until the pattern changes. :eek: If the ends can be lowered and the wire taken up 1 foot and 1/2 on each end that will solve the problem. The other alternative would be to use a tuner as the matched loss would be so low the station on the other end will never notice the difference. hth
    73,
    Rob WA9UAA
     

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