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MFJ Moxon

Discussion in 'CB Antennas' started by Bearcat, Dec 25, 2015.

  1. Bearcat

    Bearcat Active Member

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    if stretched out to use on 11m what's the best way to strengthen it?


     

  2. w9cll

    w9cll W9WDX Amateur Radio Club Member

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  3. Bearcat

    Bearcat Active Member

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  4. w9cll

    w9cll W9WDX Amateur Radio Club Member

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    If you were really worried about it breaking I suppose you could slide some PVC over the joints to add some rigidity to them. Personally I would just tune it for the low end of 10 and go with it there. The SWR would still be in the 2.0 and under range which is fine.
     
    Bearcat likes this.
  5. tba02

    tba02 WOOF

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    It tunes to 27.385 with the need for additional support. I used the measurements below. Your situation may vary.
    [​IMG]
    A = 156 in
    B = 21 1/8 in
    C = 6 3/4 in
    D = 29 7/8 in
    E = 57 3/4 in
     
    Bearcat likes this.
  6. HomerBB

    HomerBB Sr. Member

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    I think this is a reasonable answer and may be all you need considering the Moxon is fairly widebanded.
    However, if you still wish to modify it to 11 meters dimensions purchase additional tubing.
    Below are two antennas dimensions, one 11 meters, the other 10 meters. They are only inches apart in overall lengths.


    10meters.jpg 11meters.jpg
     
    tecnicoloco likes this.
  7. Bearcat

    Bearcat Active Member

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    I think what W9 said was well put..get it on the lower side of 10m...and be happy
     
  8. vkrules

    vkrules Sr. Member

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    alternate method would be to leave it set for 10 meters and load the elements down to 11 meters with a wire tail mounted at each corner make the tail a little longer than required and trim down for VSWR.
    read this article https://antjab.wordpress.com/2012/06/07/even-simpler-moxon-tuning/ great idea takes the pain out of tuning any moxon.
     
    tecnicoloco likes this.
  9. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    tba02 I want to model a Moxon, what wire/tubing diameter did you use in your calculator?
     
  10. Bearcat

    Bearcat Active Member

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    The tail idea would work!
     
  11. Bearcat

    Bearcat Active Member

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    Model away Marconi!
     
  12. HomerBB

    HomerBB Sr. Member

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    I forgot about the tail idea. Should be very easy to do.
     
  13. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    Homer I worked with my Moxon models today, and again I find this antenna very troublesome if the dimensions are not very close to correct. The variations in wire diameters and using insulated wire or not...being among the most confusing to resolve...if this mix is not right.

    Because the tuning is typically made at the outer ends of this antenna, where the tip length and spacing is adjusted...I don't see this antenna as being easy to re-tune, unless you are working at or near ground level.

    The modeling of this one is equally not easy to make changes either, unless the tip length change is small and one just forgets about the resulting change in the (C) space between the wire ends. I did try some changes in wire ends without adjusting the spacing and found, within reason, the space may not be as critical as other dimensions on this design.

    Homer, I modified a model without a mast that I had...using the dimensions you posted. It does not show good rejection or match, but the gain and angle are pretty similar as compared to other 2 element Moxons I have. However I had to change your wire diameter to 1.00" inch to get the match looking good, and I'm sure you didn't use that large of wires even if you used aluminum tubing.

    Did you have to re-tune your Moxon made with this calculator after your original construction using the dimensions you posted?


    The dimensions tba02 posted look about the same except the rejection is much better. Tba02 did not indicate a wire diameter he used, so I set the wire diameter to .50" inch and that showed a good match. If his dimensions were as posted, then maybe he built his Moxon using .50" aluminum tubing. Just a guess base on my model.

    I asked tba02 earlier what wire diameter he used for the dimensions he posted, but he has not replied.

    I want to model a 2 element Moxon that Henry makes, but I'm waiting on an update on his dimensions first. I will go over all of these models later and if they look good...I'll post them.

    Homer, can you remind us a little about your success and issues with your Moxon?
     
    222DBFL likes this.
  14. HomerBB

    HomerBB Sr. Member

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    Howdy, Marconi.
    I used the Moxon Generator app that Tba02 looks to have used when I built my Moxons. I always used the size wire or tubing that I input into the generator. The results were very close to what I was expecting each time although the antenna had enough bandwidth to work with just fine. As my memory serves me, the antennas were slightly low of center frequency I had input into the application. The wire antennas were more fickle than the metal tubing antennas, that is, more off anticipated center frequency I was building for, and less easy to modify for the right lengths than the tubing antennas.

    The two different antenna sets of dimensions I posted above were from an online calculator, not the MoxGen app. I was less interested in them for their exact accuracy than in demonstrating that the 10 meters and 11 meters Moxons were not too different to quickly modify.

    I believe what you are seeing with the troublesome models is likely true to the way those antennas would behave if built.

    I used two different sized tubings that had an average diameter of 1/2" to make the two metal Moxons I made. That is why I chose 13mm, or 1/2", wire size.

    I tried the wires hanging from the corners of one of the wire Moxons I made but could not determine exactly how long they were to be. I found it easier to input into MoxGen a frequency above the actual center frequency I wanted equal to the distance of frequency below where I wished my antenna tuned as center to.
    After doing it that way I was able to simple build the next antenna for the new dimensions and be on target to center, in my case 27.385.

    I believe Henry's antennas are all metal tubing, too.
     
  15. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    Thanks for answering so quickly.

    Well tba02's, calculator shows to be entitled "Moxon Rectangle Basic Dimensions," not "Moxon Generator." The antenna images for both calculators may look somewhat alike, but you both got different dimensions, and you suggest here that you both used 27.385 mhz. If you go back and look at the calculator you posted it does not look like the one tba02 posted. Your model also shows a frequency used as 27.2 mhz for the CB model, not 27.385 mhz like tba02 noted above.

    There are several calculators out there, they all have different names it seems, and IMO they all seem to produce a little different results.


    If you used the same calculator as tba02 did and you both used the same 27.385 mhz frequency, and the same .50" wire diameter...then why aren't both your results the same?

    This antenna design is so unique that an operator almost has to have a calculator in order to get even close. Nothing about it simply uses the conventional 468/frequency, or whatever 1/4 or 1/2 wave factor one typically uses to figure dimensions, so a calculator comes in handy and are easy to follow for the Moxon. I guess then all you have to do is hope the real antenna you build produces the results you hoped for.

    Homer, I know you guys are not always thinking along the same lines of modeling that I consider important in trying to duplicate antennas accurately, but this is what I battled with before regarding the Moxon. Which calculator was used, and seldom did I ever get the wire diameter used...unless it was part of the calculator data input just like happened with the post tba02 made?

    I have a very good wire model that I'm tweaking, and it may show 30-40 dbi rejection with a little less gain than your model shows.

    I can't be sure, but to me Henry's Moxons don't look like they use tubing for the elements. He might use some metal in the structure however.His supporting structures all look like they are fiberglass bending up to support a wire.

    As related to durability what is you opinion of wire vs. tubing?

    I might try this Moxon calculator review idea again later. I would like to know which calculator works best. I wish I was up to doing this work, and we had some reliable traffic on the air...I would build a Moxon to test.
     

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