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Eliminating some, not all traps from vertical.

Discussion in 'Amateur Radio Antennas' started by trapland, Apr 30, 2011.

  1. trapland

    trapland Member

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    Hi all,

    I have a question. I'm sure I'm opening a can of worms here and the answers are NOT as easy as I hope!

    I have an Hustler 5btv vertical and dont ever intend to use it on 10 meters, and maybe not on 15 meters. These antennas are also rather short for the depression that I have to have it installed in.

    I was wondering about removing the 10 meter trap and replacing it with enough tubing to make 1/2 or 5/8 wave up to the 15 meter trap. Also what about taking both 10 and 15 traps out and having 27 feet up to the 20m trap for a 5/8 wave vertical on 20 meters?

    My goal is to add length to the antenna while eliminating bands I dont want (I'll build a dedicated 10 and 15 meter).



    What am I missing here? Do the traps both act as loading coils (part of the length) and impedence to hold the RF below them? If they really only stop RF at a given freq range, I should be able to, in theory, replace everything below the 20 meter trap with tubing to create a 1/4, 5/8 (ya!) or even 1/2 wave. This will give me additional length(height!), a lower radiation angle at 5/8 and maybe less loss through the other traps.

    School me, please.

    73
     

  2. hookedon6

    hookedon6 W9WDX Amateur Radio Club Member

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    let me see if i have it correct,.... you DON'T want 10 or 15,.. just 20 on a vertical.


    my opinion,........ don't butcher the BTV, sell it and use the money towards a mono band 5/8 vertical like the zero-five;)
     
  3. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    If you have a fancy for beating your head against a brick wall then by all means start messing around with the traps and various lengths of tubing. :whistle:

    A trap is tuned to offer a high impedance on one band and in effect isolates the rest of the antenna above the trap from the section below it. When operating below the traps frequency the trap becomes a loading coil. If you tried to alter the electrical length below a trap and make it into a 1/2 wave or a 5/8 wave then the trap will still function however you will need a matching network at the base of the antenna and this matching device will be common to all bands that you wish to operate on.I wouldn't bother messing around with the traps if it were me and I love working with antennas.
     
  4. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    I enjoy building antennas, anything from hf up through uhf.

    If you try to modify that trapped vertical hope you have a big bottle of aspirin. They are designed to be used with a certain length of radiator to match the loading coil, and all coils need to be present for the antenna to be some what efficient.
     
  5. W5LZ

    W5LZ Crotchety Old Bastard

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    The only advice I'll offer is to measure/mark everything before you start. Then, you have a way of putting it back like it was before selling it and getting a mono-bander. ;)
    - 'Doc
     
  6. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    Doc sounds like you might have tried this before?

    I did not sell my one and only multi band vertical, Cushcraft R7.

    After I played with it for a few days and got all bands resonant, although it was a far cry from what their manual measurements were.

    I gave it away to someone who would use it in a restricted HOA, a vertical was all he could get away with in the back yard.
     
  7. trapland

    trapland Member

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    Ok, I'll clearly mark everything first. To be clear, I want to eliminate 10 and 15 mters and still use the vertical for 20, 40 and maybe 80.

    I dont understand how the traps in a BTV can also be loading coils. They are cased in aluminum. Oh, unless they are loading coils that arent intended to radiate. If they aren't loading coils, my scheme COULD work. If not, I just need about 100 free acres and 5 dedicated monobanders. :)
     
  8. RatsoW8

    RatsoW8 Supporting Member, W9WDX ARC Member - WD8T

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    I have the answer. Just don't switch your radio to 10 or 15 meters. You don't even have to hack up a well engineered antenna to NOT operate on those two bands.
     
  9. HiDef

    HiDef Active Member

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    Loading coils don't need to radiate.
     
  10. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    You need to learn about what a trap does and how it does it. They are parallel resonant circuits that offer a high impedance at their resonant freq. The coils is the inductor and the metal casing around it forms a capacitor. Above that frequency they appear as a capacitor and below their resonant frequency they appear as inductors and therefore act as loading coils for all frequencies below their resonant frequency.
     
  11. AudioShockwav

    AudioShockwav Extraterrestrial Admin
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    ( Read Above Post Again)
    CK Gets the Gold Medal.
    I do not want to discourage anyone from playing with antennas for fun, and if that is your goal carry on.
    If you want to get on the Air for now, just look around for 30 feet of tower, or even a solid push up pole and you will achieve more for your effort.
    Then already having your station on the air, you can start playing with home-brewing whatever you want to build.
    When you start "testing" your home brew, then you will have a feel for how the Hustler works and how well the new antenna compares to it.
    Have fun and let us know what you decide and how it goes.

    73
    Jeff
     
  12. trapland

    trapland Member

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    Just to dredge this up again....

    You've explained it perfectly. Thank you. To be clear, I never intended to do anything irreversible. I have an analyzer. I was just looking for starting points for experimentation. I am already on the air with a few rigs and antennas. I just wanted to see if I could customize this one to suit MY use. Obviously, I could spend a long time messing with it. Thats half the fun.

    A 10 foot hunk of coax, split to 2 alligator clips, a few yards of wire and a MFJ259 analyzer can be a whole afternoon of fun.

    I'm always a little surprised at what resonates and where. The aluminum fascia around my porch with the rain downspout as a counterpoise has almost a 20 kHz bandwidth centered at about 14.305 with 1.5:1. Go figure.
     
  13. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    Oh an antenna experimenter after my own heart.

    I tried so many different things, right now I use a section of Rohn25 and a 18 ft stinger out the top for an 75 meter vertical, then built a matching device and relay controlled for 40 meters, works great on both those bands.

    I have had the most fun and best results just experimenting with wires. Multi band dipo0les have been fun to experiment with also.

    So whatever you decide to experiment with just always have fun and enjoy.
     

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