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need help with a 2 meter whip

Discussion in 'Amateur Radio Antennas' started by Xy3lon4, Apr 6, 2007.

  1. Xy3lon4

    Xy3lon4 Member

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    I bought what I thought was a 2 meter 5/8 wave antenna made by workman. It is called the slim line 58. It turns out that it has an SWR of 1.9-2.1:1 and impedance of around 150 ohms at 144 MHz. I tuned the antenna analyzer to 6 meter and it showed an SWR of 1.2-1.5:1 and impedance of 50 ohms. Turns out that it is a 6 meter antenna. It is the same type of antenna as a Hustler SF2. It has 12 windings on the loading coil. What I want to do is to strip off the coil wire and wind it so that it will work with 2 meter. My question is, how many turns should I put on and what would be the spacing between the turns?


     

  2. Hamin' X

    Hamin' X Active Member

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    Hmm, With the impedance that you give, you should show an SWR of 3:1. Your measured SWR of about 2:1 should be tunable with the whip. Did you make several SWR measurements across the entire band?

    Rich
     
  3. Xy3lon4

    Xy3lon4 Member

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    Yep. We worked with it and it now is at around 1.5:1 from 144-148. Guess I can live with it. I also have a 1/4 wave paperclip that works great and I am in the process of making a cop-per cactus j-pole. Just have to get it tuned and the coax cut. I am also going to build or get a beam to try and talk to the ISS.
    And why was the word cop-per edited?
     
  4. freecell

    freecell New Member

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    "SWR of 1.9-2.1:1 and impedance of around 150 ohms........"

    "Hmm, With the impedance that you give, you should show an SWR of 3:1."

    not if reactance is present........your statement would only apply when dealing with a purely resistive, non reactive load.

    ....Z (impedance) is equal to the square root of the sum of R (resistance) squared and X (reactance) squared. impedance is a complex conjugate of both reactance and resistance.

    VSWR = 1:1 when R = 50 and X = 0.

    an undetermined value of X (reactance) and R (resistance) of other than 50 ohms is offsetting the VSWR.

    VSWR can only be directly proportional to Z when Z = R and X = 0.

    2 ohms of R (resistance) and 100 ohms of X (reactance) would provide a vswr indication of between 1.9 - 2.1 with Z = 100 ohms.

    Z = sqrt (R^2 + X^2)
    Z = sqrt (2^2 + 100^2)
    Z = sqrt (4 + 10000)
    Z = sqrt (10004)
    Z = 100.019 ohms Z (impedance)

    112 ohms of R (resistance) and 100 ohms of X (reactance) would provide a vswr indication of approximately 3:1 with Z = 150 ohms.

    Z = sqrt (R^2 + X^2)
    Z = sqrt (112^2 + 100^2)
    Z = sqrt (12544 + 10000)
    Z = sqrt (22544)
    Z = 150.146 ohms Z (impedance)

    the value selected for X was done randomly for purpose of example, your values may differ. any time X = something other than 0 then VSWR is not directly proportional to and is no exacting indicator of Z (impedance) match/mismatch.

    Z (impedance) is only altered when adjusting the stinger or whatever because X (reactance) and R (resistance) is what is actually being adjusted. as the value of X or R is varied, so is the impedance (Z).

    Z = sqrt (R^2 + X^2)
     
  5. Xy3lon4

    Xy3lon4 Member

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    I would like to get the thing down to 1.1:1. Is there anyone who can tell me how many turns I would need on the coil to make it a 2 meter instead of a 6 meter that seems to work on 2 meter?
     
  6. freecell

    freecell New Member

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    the only problem you have is that you think a measured vswr of 1.5:1 is a problem, you're wasting your time if you think that lowering it any further is going to make any difference in performance, it isn't. i see you were over on eham asking the same questions. if you're still bound and determined to do it anyway then just open up either end of the coil and try tapping it for a lower swr. you certainly don't need to remove it or strip it to accomplish that. no one here (or anywhere else) is going to definitively tell you how many turns to add or subtract one way or the other and you're way too hung up on vswr. of the several parameters that indicate any type of antenna efficiency it's the least important of the bunch. it's probably the only one you're aware of, not that you necessarily understand anything about it, you obviously don't. if you did you wouldn't be so worried about it.

    cutting it down for a 1/2 wavelength radiator isn't going to be any help as the input impedance of an end fed 1/2 wave is on the order of 3500 - 4000 ohms, your swr meter won't make you very happy with that either.
     
  7. Xy3lon4

    Xy3lon4 Member

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    Dude, you are just hateful. And you are arrogant and that is a bad combination.
     
  8. Beetle

    Beetle Sr. Member

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    Since all installations are different, it would be very difficult (if not impossible) to tell you exactly where to cut, how many turns to cut, or whatever.

    Get a couple of good antenna books and figure out the theory behind what you want to do. Then get or borrow an antenna analyzer (the MFJ-259B covers 2 meters). Take a reading, note the frequency that provides the lowest SWR.

    Make an adjustment, take another reading and note any changes.

    Repeat.
     
  9. Xy3lon4

    Xy3lon4 Member

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    I plan on getting the 259 or 269. Money's tight right now though.
    I just ran the numbers of the antenna through a design program and it is supposed to be 12.1 turns on the coil. It is at about 11.9 right now. I had a feeling that it needed more turns, that is why it wouldn't jive when I shorted the coil at different points.
     
  10. freecell

    freecell New Member

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    you go Beetle. if the modelling software doesn't provide the ability to model the antenna with the feedline included then there will be some additional error in the results. add an additional turn to the inductor and tap for whatever following the procedure that was outlined in the previous post. the 269 includes the ability to determine X even though the +/- sign is not given. X = 0 indicates resonance.
     
  11. W5LZ

    W5LZ Crotchety Old Bastard

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    Xy3lon4,
    Where you might be making a mistake is believing that any 'program' is going to tell you the exact thing you need to know. The difference between the number of turns the program said, and what's actually there (0.3% of a circle) is usually the difference between the 'perfect' circumstances and the 'real life' circumstances. That difference can be accounted for by the coil's housing, where the antenna is mounted, and all those other things like that.
    Don't misunderstand, though. Using formulas and a pencil, or a programs is certainly not unreasonable! If you can account for all the little variables it'll even give you an 'exact' answer. Otherwise, it'll just get you into the 'ball park'. Getting a 'good seat', popcorn, and beer is still up to you. (RATS!)
    - 'Doc

    Okay, 0.3% was wrong, more like 30%. I'm tired of correcting spelling mistakes, don't feel like re-doing it.
     
  12. Xy3lon4

    Xy3lon4 Member

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    I can pick up other hams at least 150 miles away. And they can hear me fine and no smoke has escaped my rig so I guess I am doing fine.
     
  13. W5LZ

    W5LZ Crotchety Old Bastard

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    Sounds like it to me. If my radio is 'satisfied' with it then I'm satisfied with it. I don't have to like it...
    - 'Doc
     
  14. Xy3lon4

    Xy3lon4 Member

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    The thing is that problems that will occur with an antenna with too high an SWR is that it is a gradual thing. But, I know it is well below 2.0 so I am cool. Just have to make sure I buy Hustler next time.
     

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