Modeling a trapped tri-band beam in 4nec2

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AE7F, Mar 4, 2016.

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  1. AE7F

    AE7F Ham Member QRZ Page

    Another model iteration. Good gain and improved f/b (still not 20dB!) on 10m and 15m but nothing interesting on 20m; shortened driver 20m end tips. Will implement tapering as soon as I get some time.
     

    Attached Files:

  2. KF6A

    KF6A Ham Member QRZ Page

    Yes. Your model is set for inches so a radius of "1" is a 2" tubing diameter.
     
  3. KF6A

    KF6A Ham Member QRZ Page

    Is this what you were looking for? About 1/4 down the page.
    http://www.w8ji.com/traps.htm
     
  4. AE7F

    AE7F Ham Member QRZ Page

    Dan, glad you are following this thread. Thanks for the helpful information.

    It had not occurred to me to search for TA-33 trap information... thanks for that.

    I spent the last hour playing with the model. I reduced the radius on each element to 0.25 and set the conductor type to aluminum. I then entered appropriate L and C values to cause the trap frequencies to be the same as the TA-33 frequencies from your reference site. The SWR curves on 10 and 15m shifted higher (out of band) and the 20m frequency was lower (out of band). Perhaps the "coax inside the driven element tube" provides appropriate matching in this condition?

    I've also experimented by trying to configure all 10m traps with the same values and all 15m traps with the same values. I also tried leaving the C values the same for all traps (assuming a fixed C value since this is not a variable tuning capacitor) and changing only L values.

    No matter what I have tried, I have not been able to coax the model into the ~20dB f/b ratio area.
     
  5. KF6A

    KF6A Ham Member QRZ Page

    Sorry I had mistakenly thought the model was a TA-33 and then realized it was CL-33. Should still give an idea though.
     
  6. AE7F

    AE7F Ham Member QRZ Page

    No problem. It did give me a lot of ideas and it is interesting to see what trap frequencies Mosley chose for the TA-33. When I plug them in, the CL-33 model SWR curves are too high. Still experimenting...
     
  7. E73M

    E73M Ham Member QRZ Page

    cl-33-trap-E73M-annot.jpg

    This is maybe the best way to point out how the dual trap works and why is important to have the correct length of a cavity for each element. The cavity is always part of a radiating element on 15 and 20m. By omitting second L15 coil, you can have single band trap (10m ) and such design was implemented by Fritzel (Hofi today) on its multiband designs. I have used same on MA5B mini beam for 12/17m dipole where the cavity has only one coil (12m trap) and another assembly does not have any turns on itself but a just short piece of wire to connect cavity end to rest of the element needed to resonate the dipole on 17m band.

    To properly model these traps, you need to measure C by lifting the wire connection on the edge of the cavity and use LC meter ( such as one in an article from QST 2015) .
    Same way can be used to measure the L . Always with coils in Cavity.
    QST article used cavity connected while measuring L but I believe a more accurate way is without cavity connected to wire coil.

    With that data and 12" cavity inserted in a model as the separation between two traps , best results close to real world antenna can be achieved.

    Problem is that you might expect to match the data from Mosley spec. sheet which was proven pretty much inaccurate by internal Cushcraft evaluation of CL33 as well as an independent report by K7LXC an N0AX years ago. As I recall, it had 1.5dBD gain on some band. HAMs always look to SWR chart as the most important parameter of an antenna but reality is much different.

    BTW when I was designing X7 and X9 tri banders I was able to easily get flat SWR response on all bands which will make many HAMs very happy. Truth is that antenna with such good SWRs was terrible performer both in gain as well as in F/B. Also, gain claimed on both these antennas as well as on Mini Beam MA5B was measured on the range compared to single band dipole. It was pretty much confirmed by Champion Radio antenna report (K7LXC & N0AX).

    73 Danny E73M
     
    AE7F likes this.
  8. AE7F

    AE7F Ham Member QRZ Page

    Thank you again Danny for your explanations and for the diagram. This really clarifies a lot and many will be able to benefit from your information.

    I (and others) would be very interested to know more about the Cushcraft evaluation of the CL-33. I actually came across some information regarding the K7LXC and N0AX evaluation on beams but I have not had the privilege of reading the book. I would like to see that information at some point. I'll look for the book. 1.5dBD is very dismal and I hope it isn't that poor.

    I will put my CL-33 model through some more iterations and see what I come up with. When my L/C meter arrives, I will test the traps as you describe and compare my results with those in the QST article!

    Thanks again.
     
  9. AE7F

    AE7F Ham Member QRZ Page

    Another iteration, this time with trap lengths of 5.5 inches each and a tube/cavity length between traps of 12 inches. 10m and 20m look okay but 15m is pretty much acting solely like a dipole....
     

    Attached Files:

  10. AE7F

    AE7F Ham Member QRZ Page

    Here are some pictures to evaluate:
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    15m is still off....
     

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