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MFJ-1848 Hex Beam Tuning

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KJ7SJG, Feb 26, 2021.

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  1. KJ7SJG

    KJ7SJG Premium Subscriber QRZ Page

    Hi,

    I'm a new HAM and I need some advice on how to tune my MFJ-1848 Hex Beam. I have the antenna mounted 35 feet over my house and I really like it. I've made DX contacts to South America, Japan and Italy on 20 Meters. Workable right now but I'll like to optimize it.

    My question is about tuning 14,000 on the enclosed RigExpert SWR Chart. It shows a double bump around 3 and 3.75 for the low SWR. Any ideas on how to get a lower SWR on this band? The others have lower single bump SWR.

    Thanks,
    Jeff 2-19Hex.png
     
  2. NG1H

    NG1H XML Subscriber QRZ Page

    My experience is that adjusting a 6 band hexbeam can be a PITA. Adjusting one band seems to always affect the other bands. Seeing as this is an 8 band antenna this may be even worse.

    I found that the best result was from making sure all dimensions were as close to the manual as possible. Verify the wire lengths (even if they came precut) and the loop spacing along the arms. Make sure the tension on the arms is even. It is really easy to have the elements pulling the spreader arms out of shape as you assemble it. I ran two loops of line around the arms. One on the outer perimeter and one about halfway out. The line was fixed to each arm. This kept the spacing even and appears to give the antenna more strength.
     
  3. KJ7SJG

    KJ7SJG Premium Subscriber QRZ Page

    Thanks for the tips!
     
  4. KJ7SJG

    KJ7SJG Premium Subscriber QRZ Page

    A quick update. I found out that keeping the spacing of the 6 arms is crucial. The lowest dip of the SWR was high on all frequencies. Found that the gap in the arms of the forward direction of the antenna was wider than the others. The area between these arms don't have wires between them, so the pull on the wires pulled them apart.

    Fortunately, I had run a light cord at the tip of these arms. Tightened that up so they all looked about even. That dropped the SWR low point right on the bands. Much better than before.

    Still not sure why 20 meters SWR is 3.00 right now. All other bands are between 1.3 and 2.5. Any tips would be appreciated.
     
  5. NG1H

    NG1H XML Subscriber QRZ Page

    When you gave the 35' figure was that 35' above the ground or 35' from the nearest structure? From what I remember once the antenna was closer than around 20' to a structure the SWR started rising. But to go that high it had to be really close. Otherwise no idea. My problems were all on the higher frequencies.
     
  6. KJ7SJG

    KJ7SJG Premium Subscriber QRZ Page

    My antenna is 35 foot above the ground and it's 11 above the roof of my house.
     
  7. NG1H

    NG1H XML Subscriber QRZ Page

    Hmm. That may be some of the problem. Although I would have expected to see more of an issue on 40m.

    If you can get it higher I would try that. Otherwise I am out of ideas.

    Could you post a revised SWR v. Frequency chart?
     
  8. KJ7SJG

    KJ7SJG Premium Subscriber QRZ Page

    Here is the before and after SWR chart. The green chart was before I evened out the distance between the hex beams arms. The pink/red chart is after. 2-19Hex.png 3-18FullRangeHex.png
     
  9. NG1H

    NG1H XML Subscriber QRZ Page

    Well, I am at a loss. Hopefully someone else has seen this before and knows of a cause.

    This is just purely guesswork but if one of the 20m driven elements are poorly connected could that cause a similar effect?
     
  10. KD2CJJ

    KD2CJJ Premium Subscriber QRZ Page


    Would poorly cause high SWR yes, but would be far worse. Most likey the wire lengths need to be improved . There are some measurements out there but measure both the driver and the reflector and see if they are close to those measurements.

    Also make sure they are not drooping - they must be clearly separated from each other. Im not familiar with the MFJ but make sure the driver and reflector are parallel (near perfect) to each other. if your slightly off it will also throw off the SWR.
     

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