biquad/dish combination

Discussion in 'Wireless Internet' started by Edmund, Jul 28, 2006.

  1. Edmund

    Edmund Guest

    I don't see any problem with a feed pointing at only
    a part of a dish. The reflection of the dish still will go
    to the point where the disk is aiming to and no energy
    is wasted in a wrong direction.
    Correct me if a am wrong about that.

    One other thing I like to know is the influence of a reflector
    on a feed mounted on a dish. As said the reflector in some
    examples are still 110 mm square. While a the focus of a dish
    should be a about 1 mm^2. Does this mean a biquad with such
    a large reflector has to be mounted off focus?
    I really don't know anything about antenna's but I do know a little
    about reflection. So I expect since the biquad is much larger is size
    then a single point, it should be mounted off focus.
    Maybe a fair distance towards the dish or a little distance away from
    the dish. Hopefully this drawing makes clear what I mean.
    For the ease of the drawing the dish is pointed downwards.


    I S
    D H
    \ /
    \ /
    \ /
    \ /
    \ /
    \ /========\ / === biquad
    ____\/__________\/___ ___ reflector towards dish
    \ /
    \ /
    \ /
    \ /
    \ /
    ______\/__________ FOCUS
    /\
    \========/
    __\/____\/_____ reflector away from dish



    Remember I don't know nothing about antenna's :)
     
    Edmund, Jul 28, 2006
    #1
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  2. You're wrong.

    http://en.wikipedia.org/wiki/Dish_antenna

    In receive, the dish picks up all the signal it can get and sends it
    to the feed. If the feed has to wide a beamwidth (such as using a
    hemispherical feed that picks up from all directions), the gain will
    be the same as with a properly designed feed. However, if the feed is
    a spot beam, it will only pickup the percentage of area covered on the
    dish by the spot, and therefore have much less gain.

    In transmit, the situation is the same. A hemispherical (too wide)
    feed will spray much of the RF over the edge of the dish, where it is
    wasted. However, you're correct for the spot beam. All the RF
    generated by a spot beam will be reflected by the dish and sent to the
    remote target. In effect, this combination uses the feed to obtain
    its gain, and largely ignores the capture area (effective aperture) of
    the dish.

    There's quite a bit on the subject of feed design in:
    http://www.qsl.net/n1bwt/contents.htm
    It might be a bit too techy, but try reading just Chapter 4 (21 pages)
    which discusses feed design.
    Yes, it has to be mounted as an "offset feed". See above URL.

    The problem is that the feed is so large as compared to the effective
    area of the dish, that any feed blockage will cause a substantial
    decrease in gain. For very small antennas (i.e. 0.6m DBS pizza dish
    antennas), this is a serious problem. The dish would need to be about
    2m across before the blocking effect of the feed can be ignored.

    Please note that the DBS dish was designed to work at 12-13GHz, where
    the feed diameters are much smaller. The scale is linear so a 10cm x
    10cm biquad feed would be only 18cm x 18cm.

    The problem with a biquad feed is that it's to wide and therefore
    blocks quite a bit of RF going to the dish. A better choice is a
    dipole or 1 element yagi, which is narrow and does NOT block much RF.
    This is what's inside one of the MMDS (2.5GHz) dish antennas.
    http://home.alphalink.com.au/~vak/galaxy/8.jpg
    It's a circuit board 2 element yagi feed. The traces close to the
    circuitry are the dipole. The wide trace with the step is the
    reflector. No clue what the trace in between does.
    Finding the optimum location is difficult. All antennas have a phase
    center. That's the point where all the RF appears to be coming from
    if you measured it from a distance. That's also the point where you
    want the focus of the dish to appear.

    If you mount the feed off focus, you lose more than gain. There could
    be frequency selective cancellations (nulls), boresight errors, and
    excessively wide beamwidth. If you have a flashlight with an
    adjustable focus (Maglight), you can see the results as you move the
    position of the lamp through the reflector focus.
    The biquad reflector does NOT capture RF equally over its entire
    surface. Grossly over-simplified, the bulk of the signal is captured
    in a "ring" around the center coax cable feed at about a 3cm radius.
    (I'll see if I can grind out a model with 4NEC2). If the dish focus
    happen to hit on this "ring", you'll have maximum gain. If it misses,
    the gain will be less. If you move the biquad feed around, you might
    be able to see the effect.

    Anyway, only with the phase center of the feed at the dish focus, do
    all the reflected signals (at any frequency) arrive in phase. If you
    move the feed to some other place, you will see some rather strange
    effects.

    Also, please note that the maximum gain from a 0.6meter dish antenna
    is about:
    gain = 9.87 * Dia^2 / wavelength^2 * (feed efficiency)
    gain = 9.87 * 600mm^2 / 125mm^2 * 0.4
    gain = 91
    dBi = 10 log(91) = 19.5dBi
    The 40% efficiency (that's the 0.4) is probably optimistic.
    You're doing fine and asking the right questions.
     
    Jeff Liebermann, Jul 28, 2006
    #2
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  3. Edmund

    Edmund Guest

    Thanks for all info Jeff, your post makes real sense to me.
     
    Edmund, Jul 29, 2006
    #3
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