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B-Square J-Pole

Discussion in 'Amateur Radio Antennas' started by Need2Know, Feb 18, 2012.

  1. Need2Know

    Need2Know KK4GMU - The Villages, FL

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    Does anyone have experience with this 2 band J-Pole?

    Quality ham radio antennas built to last a lifetime

    Thoughts?


     
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  2. RatsoW8

    RatsoW8 Supporting Member

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    Looks good. Since I've been looking for a j-pole like this I think I'm going to buy one. The price is good.
     
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  3. Need2Know

    Need2Know KK4GMU - The Villages, FL

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    Are these gain figures what are typically expected from J-Poles:

    VHF: 3.4 dBi UHF: 4.3 dBi
     
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  4. W5LZ

    W5LZ Crotchety Old Bastard

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    Those gain figures are probably about average for any 'J'-pole. Just keep in mind that they are isotropic figures.
    - 'Doc
     
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  5. Need2Know

    Need2Know KK4GMU - The Villages, FL

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    Here is my stab at the meaning of dbi:

    The "i" stands for isotropic.
    Isotropic means "identical in all directions."

    dBi equals the decibels over isotropic. This is the power in the strongest direction divided by the power that would be transmitted by an isotropic antenna emitting the same total power.

    That defintion makes little sense for an antenna that is supposed to be omnidirectional.

    Is there a better way to describe dbi?

    "Isotropic figures" vs. what other method? Is there a better/more meaningful way of specifying antenna gain?
     
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  6. W5LZ

    W5LZ Crotchety Old Bastard

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    The best way I know of is to say that an isotropic antenna is the 'ideal' antenna in an 'ideal' location where nothing around it can affect it. It's the 'best' possible for whatever style of antenna it is. It's a 'virtual' antenna, doesn't have to put up with the 'real world' conditions and limitations. The first thing to do with any isotropic gain figure is to knock off about 2 dB for a closer 'real world' gain figure (2.12 dB I think). It's not exact, but that'll get you close to what you might expect if you're lucky and everything works just right. (If you are that lucky please buy me a lottery ticket!!) Isotropic antennas give you an idea of what a particular antenna might do, it shows a trend. It's what you can expect if you do everything exactly right, sort of, but you ain't never ever going to get there for real.
    - 'Doc
     
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  7. KC9Q

    KC9Q Supporting Member

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    The term dBi stands for an isotropic point source. To visualize this look at a ball and find the exact center point inside the ball. That center point is analogues to the isotropic source. The surface of the ball is the isotropic reference point(s). This is something that cannot be physically accomplished since the mast holding the antenna is protruding through the ball. This would cause interference to the antenna pattern and cause distortion.

    There are two other dB references in the two-way field: dBd (dB reference to a ½ wave dipole) and a lesser known dBq (dB referenced to a ¼ wave vertical, sometimes called just dB). The truest reference is dBd since it can be physically measured with an actual ½ wave dipole antenna.

    The differences in reference values are well known:

    dBi = -2.15 dBd
    dBd = +2.15 dBi

    dBq = -0.7 dBd
    dBd = +0.7 dBq

    Many antenna manufactures like to use the dBi reference to gain an extra 2 dB: Makes the antenna look better when compared to a true dBd measured antenna.

    Mike
    ex Motorola Systems Engineer
     
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  8. RatsoW8

    RatsoW8 Supporting Member

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    So the true gain figures for this one are.

    VHF - 1.25db
    UHF - 2.15db

    Unless I operate from the moon of course. This is fine as I live within the primary coverage area of all the repeaters I use.
     
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  9. Need2Know

    Need2Know KK4GMU - The Villages, FL

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    OK. My confusion was between ominidirectional and isotropic. Omindirectional is focusing on horizontal dispersion/reception; isotropic is equal dispersion in all directions, horizontal, vertical and everything inbetween.

    An omnidirectional antenna attempts to flatten the isotropic theoretical (as in the "ball" example.) The energy that would have gone up and down with an isotropic antenna is directed to goe out "fairly" uniformly in all horizontal directions. So the gain of an antenna is the degree to which it can redirect the energy from being wasted going "up" or "down" and focusing it outward horizontally (if that is the desired direction of the signal.)

    A directional or beam antenna focuses the signal even more compactly in a specific direction thereby achieving a higher db in the focused directions than what an omni could achieve.

    I need to recall some of Mr. Wizard's experiments.:oops:
     
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