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80 meter dipole problems

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N7TDK, Jul 22, 2021 at 4:03 AM.

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  1. N7TDK

    N7TDK Ham Member QRZ Page

    I have a dipole cut for 80 meters, but is only about 15 feet high, and fed with a 1:1 balun. It used to load up OK on 80, with a low enough SWR, thanks to my tuner.
    For some unknown reason, it no longer loads up on 80 or 40 meters, but is OK on 20 and 10 meter bands.
    I made sure no tree leaves are on the 12 gauge stranded antenna wire, the balun connections are clean and tight. The balun checks OK with my ohm-meter......about .6 ohms resistance. Any ideas what the problem could be ??? 73, Bill, n7tdk
     
  2. M0AGP

    M0AGP Ham Member QRZ Page

    What was the SWR without the tuner when it used to work? Was it on the edge of tunability or was it already a reasonable SWR?

    What do you use for end insulators?

    How close are the antenna ends to anything?

    Do you have access to an antenna analyzer?
     
    KU3X likes this.
  3. N1IPU

    N1IPU Premium Subscriber QRZ Page

    Verify all your connections first. It should not go south without some change. Will second an analyzer.

    Check Coax for moisture infiltration, forgot to add that first time.
     
    WB5YUZ likes this.
  4. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    Most of us follow the same procedure for troubleshooting an antenna.

    (1) Close physical examination. Look for anything obvious.

    (2) Lower the antenna and check all connections for continuity with an ohm meter.

    These first two checks will resolve ninety percent of suddenly occuring antenna problems. But if they don't:

    (3) Put a dummy load at the far end of the feed line, and make SWR measurements. They should be completely flat. This test will find feedline problems that occur with RF but not DC.

    You can also temporarily remove the balun if it is not a transformative one. If the antenna loads up fine without the balun but not with it...

    Finally, there's this:

    If you were using a tuner, especially an automated one, to match a typical 80m dipole on 40m with a balun in line, you may have damaged the balun or the tuner. "Load up" is a very vague term, but if you mean you were using an auto-tuner to force operation on 40m, and something was damaged, it is possible the tuner isn't really finding a match on 20 or 10m, but that losses on those bands are high enough to mask the true SWR. We would not expect any but the best high-power manual tuner to make a typical dipole work on a frequency where it is a multiple of a full-wave long, and most experienced hams would not even try it for fear of damaging something.

    Good luck. If you can, please post back when the problem is solved, and let us know what you found.
     
    M0AGP, WA7ARK and K9WW like this.
  5. N7TDK

    N7TDK Ham Member QRZ Page

    I appreciate the responses. Will check all. Am using the white plastic insulators on end....about 2 inches long. Each end is within a foot from a tree. The coax is fairly new so didn't check, but will. Yes...I think a friend has an antenna analyzer, will ask him to assist. Don't know what the SWR was (without manual tuner) but think it was fairly close, as I cut it for the 80 meter CW part of band. It's the transformer type of balun.....will try without using it. The 12 gauge stranded antenna wire is continuous on each side of balun.....but will do a continuity test with the ohm-meter.
    Seems strange that it will work on 10 & 20 meters, (with a tuner), but not 40 and 80.
    Thanks for the suggestions....will follow up. Bill, n7tdk
     
  6. M0AGP

    M0AGP Ham Member QRZ Page

    Bill the reason I asked about the ends being near a tree is that the ends of a dipole are sort of the most sensitive part of it. Being within a foot of a tree can cause trouble as water flows into the leaves in spring and changes happen. But your problem doesn’t sound like a subtle issue. I agree with the notion of damage caused by using an 80m antenna on 40m.

    As you know, the ends of a halfwave dipole are high voltage points, and an 80m dipole will also have a high voltage point in the middle when used on 40m. This can cause arcing and this can damage the tuner, which is the concern here.

    If you want 40 as well as 80m you can attach a 40m dipole to the same feedpoint as the 80m one: this is known as a fan dipole, and you should also be able to use this antenna on 15m easily. There is a ton of useful information about fan dipoles on this forum if you are interested.
     
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    "With a tuner," almost anything might load up. The old "I can load the bedsprings" stories are pretty true, if it's a broad-range tuner.

    You may be loading up mostly the coax...who knows?

    Evidently, something changed. But if it's an 80m dipole, it should not natively (without the tuner) load up well on any band other than 80m. It "might" work on 30m, which is the third harmonic...sort of. But it should never load well on 40 or 20 or really any other ham band -- without the tuner.

    I'd take the tuner out of the circuit and see what's going on.
     
    WB5YUZ likes this.
  8. K8JD

    K8JD Ham Member QRZ Page

    Using an 80 M coax fed ant on 40 M will cause some very high voltages at the feedpoint , May have arced over the Balun and connections there. You would be a lot better off using a twinlead, high Z feedline for that aplication and a balanced line tuner in the shack, forgoing baluns that can get lossy and overheat.
    I have had success with a 20 M EDz type dipole on 60-17 M with a 450 Ohm feedline and a balanced line tuner.
    I bought a 100 ft roll of this balanced line for same or less than the length or RG8 type coax.
     
    KA9UCN likes this.
  9. KI8DJ

    KI8DJ Ham Member QRZ Page

    Remove the balun and try direct feed if vswr is good you found the problem.
     

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