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Half-Dipole Antenna Question

Discussion in 'Electronic Basics' started by Wayne, Apr 26, 2005.

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  1. Wayne

    Wayne Guest

    I made a half-dipole antenna for my home stereo from wire scraps I had. It
    works fine, but unfortunately, my two favorite radio stations are
    geographically 90 degrees apart (East, and South). With the dipole being a
    directional antenna, that, of course, means sacrificing one station's signal
    for the other. Instead of dropping money on more equipment, can I just add
    a second half-dipole in series with the first, making the second in the same
    horizontal plane, but oriented 90 degrees from the first dipole? If the two
    dipoles are oriented 90 degrees apart, would the interference between the
    two dipoles be zero, allowing me to get maximum signal from both the East
    and Soputh?
     

  2. The problem with your method is that, in effect,
    the signal you pick up with one antenna will be
    partially lost (reradiated, actually) by the other.
    The net effect will be little different than if you
    had aimed your single dipole midway between
    the two directions.

    If you have only two stations that you really want
    to receive, you could build a pair of more directional
    antennae and sum their outputs in a (nearly) lossless
    splitter. The same effect as I describe above will
    occur, but you would have more signal to start with.

    BTW, aiming EW instead of E or W will only drop
    the signal about 3 dB for your antenna. You might
    want to make sure you can hear the difference before
    going to anything more elaborate.
     
  3. had. It

    I think you may mean a half-wave dipole.
    being a

    No problem point half way in between, and of course you point a dipole
    by the broad side of it, not the ends. Like Larry said, you really
    shouldn't notice a problem unless the signal is very marginal since the
    3dB beam width of the antenna is so wide. You could turn your dipole
    vertically and it will then be omni-directional wrt land stations. I
    believe that most FM broadcast stations transmit using horizontal
    polarization, but that doesn't mean the signal arrives at your location
    oriented that way. You will just have to experiment and then you will
    see how far the theory strays from reality. It's not that the theory is
    really so wrong, it's just that the nature of RF is so obtuse that you
    can't possibly factor in everything that is having an effect and
    sumarize it in a simple formula.

    Antenna theory is just that, theory. Reality often stands a good
    distance from it.

    I suggest you build yourself a two element cubical quad, or a two
    element beam. One of them will have so much more gain (hopefully
    without narrowing the beam width too much) than your dipole that you
    should have no trouble receiving both stations without re-pointing it.
    If you have the room for the 3-d nature of the cubical quad then I
    highly recommend you build one, you will be real happy with it. I once
    built an 8 element quad for VHF (2 mtrs) that really kicked serious
    butt, but it was a real bear to tune. Sadly, it was made from wood so
    nature eventually took it from me.
    You are not likely to have much luck with this kind of thing without
    learning a ton more than you probably want or care to know, but I could
    be wrong. The key is to experiment with antennas, you will be amazed at
    what works sometimes.
     
  4. Wayne

    Wayne Guest

    I am pointing the dipole by it's broad side, unfortunately, both stations
    are marginal. When I turn the antenna to get a decent signal on one, the
    other is too poor for me to tolerate.

    I will try this first.

    My antenna is mounted in a large attic, so I don't have to worry about rot
    or space. If the vertical orientation doesn't help I will try a cubicle
    quad. (I may try to build one just for the heck of it.) Can you recommend
    a web-site
    that would have specs or drawings to construct one? I don't really know
    much about radio electronics.

    Thanks to you and Larry for your assistance.
     
  5. Rich Grise

    Rich Grise Guest

    Look up a "halo" antenna. It's omnidirectional, but flattens out the
    pattern, so has some gain over a dipole, but in all (horizontal)
    directions. It's also horizontally polarized.

    You might also look for a "turnstile" antenna. That's typically two
    folded dipoles with a 1/4 wavelength transmission line connecting them,
    to give 90 degrees phase shift between the two antennas, making it
    omnidirectional.

    But since you have a random-wire, just put two little cuphooks in the
    ceiling, and hang the loose end of your antenna from whichever one
    works.

    Good Luck!
    Rich
     
  6. http://www.softcom.net/users/kd6dks/quad.html has a nice calculator
    program. Your measurements will not be that critical since you are not
    needing to transmit. Just build your antenna with the "driven" element
    and the "reflector" element. You should be able to play with your
    spacing to adjust the front lobe so that both stations can be received
    nicely.

    By itself, a full wave loop will have about 3dB of gain vs. the dipole.
    In fact you may get good results by just using a full wave loop without
    the reflector. You should try that first and then add the reflector
    noting the effect it has on sensitivity (gain). Adding the reflector
    will give you a front and a back side.

    You point a loop asif you are trying to have the signal pass straight
    thru the open hole. The optimal shape for a loop is a circle, but
    squares are usually more convenient and they work fine.
     
  7. JeB

    JeB Guest

    u might try a J-pole made from 300 ohm twin lead for your vertical
    project. I'd browse ham antenna sites for the calculations....
    they're typically sized for the ham 2m band (approx 144mhz)
     
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