Biquad on Dish Question

Discussion in 'Wireless Internet' started by amdx, Jul 13, 2008.

  1. amdx

    amdx Guest

    I have what I call my reference antenna an MFJ-1800 spec says 15db gain (hmm
    best signal is -72dbm to -70dbm.

    My test signal is in a building about 100 yards away.

    I built the biquad with 30cm lips my best signal strength is -74dbm .
    when I mount the biquad on a 18"x36" offset feed dish (looks like this)
    the best signal I get is -64dbm, a 10 db gain over biquad alone.

    Is that the gain you would expect?

    I found the original feed position wasn't optimum, I had to raise the biquad
    about 2"
    and also back it away from the dish about 2" to peak the signal.

    What is are your thoughts about that?

    I question the the MFJs gain because the modified (110mm and lips) biquad
    is only 2 to 4 db less than the MFJ. If I did good, and got 8db gain from
    the biquad then that looks like 10 to 11db gain for the MFJ. I do understand
    antenna measurements can be difficult. That said, I have used the MFJ-1800
    for almost a year now and think it is
    a good buy at $30.00 plus shipping.
    amdx, Jul 13, 2008
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  2. Ok, let's play with the numbers. First, with a reflector type
    antenna, the gain of the biquad feed is not important. It's sole
    purpose is to "illuminate" the reflector. If the beamwidth of the
    feed is too wide, you lose gain in transmit. If the beamwidth of the
    feed is too narrow, you lose gain in both receive and xmit because
    you're not using the full area of the reflector. See:

    When dealing with elliptical reflectors, that are designed to handle
    multiple feeds (for multiple satellites), you don't get the benefits
    of the entire dish. The offset feed also means the effective cross
    sectional area is somewhat less than the physical area. I'm too lazy
    to work out the exact number, but my guess(tm) is that the DirecTV
    dish has an effective circular diameter of about 60 cm. In other
    words, the bulk of the reflector is not used.

    The maximum gain of a dish antenna can be calculated.
    Assuming you did everything perfectly, and the feed was properly
    matched to the dish f/D ratio, then:
    Maximum gain for a 60 cm meter diameter dish:
    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 for your
    home made feed. Again, the numbers assume that everything is
    lossless, perfectly matched, built correctly, and properly designed.

    Your 10dB difference is gain is very close to what I would expect
    (i.e. calculate). The typical biquad has 8-11dBi gain. See:
    So if I give the biquad a 9dBi gain and the dish perhaps 18dBi of
    theoretical gain, then an 9dB difference is about what you would see.
    Close enough to your 10dB difference.

    Some stuff on dishes and feeds from my bookmark pile:
    MFJ means "Might Fine Junk". They also no longer sell or support the
    MFJ-1800 antenna. The link:
    Ummmm..... yeah, that's an understatement. Actually, it's not
    difficult to make the measurement. The problem is finding a place to
    do the testing that isn't full of reflections and interference. I've
    found the deep dark forest to be fairly good as the trees absorb all
    the reflections. The only problem is that I have to elevate the test
    antennas well above the roadway to prevent ground bounces.
    I have a bad attitude about yagis. They have their place but are a
    PITA to build, maintain, install, and design. I much prefer a panel
    antenna which is much simpler, easier to mount, waterproof, wider
    bandwidth, and only slightly more dollars. How about 13dBi gain for
    Jeff Liebermann, Jul 14, 2008
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  3. amdx

    amdx Guest

    Thanks for all that info Jeff,

    I'll take some time to digest it, especially chapter 4. Also thanks for
    for the link to the panel antenna. I think I'll purchase that just to
    compare to the yagi. Oh, I find the yagi link does work.

    Two more questions.
    Regarding you formula;
    Where does the 9.87 come from?

    At my boat location, I find I need to have a signal strength of -76dbm
    to log on. If any weaker it won't work.
    At home I logged on with -90dbm with good speed.
    One big difference between locations is this, on the boat I have 10 to 14
    signals that I can see, at home it's one signal.

    Why do I need the stronger signal on the boat. (interference?)
    Thanks, Mike
    amdx, Jul 14, 2008
  4. So much for my bookmarks. Note that the MFJ-1800 is NOT listed on the
    antennas page:
    See formula at:
    Welcome to the joy of interference. You might be able to improve
    things at your end using a highly directional antenna. However, the
    interference will still be present at the access point end of the
    link, resulting in no real improvement. Also, high gain antennas have
    narrow beamwidths, which causes problems as the vessel rolls in the

    Also, see:
    Probably. The big question is interference at which end of the link?
    If it's at your end, there's plenty you can do with antennas. If it's
    at the access point end, there's not much you can do.
    Jeff Liebermann, Jul 14, 2008
  5. amdx

    amdx Guest

    Thanks Jeff,
    Sorry, I missed the Pi^2 on that page, I looked several places and couldn't
    find it.
    I'll study and try to leave you alone,,, for awhile!
    Thanks again,
    amdx, Jul 14, 2008
  6. amdx

    jimmie68 Guest

    I have some friends who tinkered around a lot trying to get a used sat
    TV dish to work. They had simliar results as the OP. I think they
    finally opted for a QUagi antenna consisting of quad driven element
    reflector and one director and several yagi type directors. They also
    experimented with some of the large TVRO antennas and the gain on
    these was awsome.

    jimmie68, Jul 15, 2008
  7. I've done it successfully. This is an early version:
    The only tricks are to make sure the feed matches the f/D ratio of the
    dish, and to *NOT* use dishes made from multiple LNB's.
    Reminder.... there is a legal limit to EIRP.
    Jeff Liebermann, Jul 15, 2008
  8. amdx

    amdx Guest

    I think I found the same antenna a little cheaper, but to late for me.
    amdx, Jul 24, 2008
  9. amdx

    amdx Guest

    Check out the data sheet on this page; (The 13dbi is the one we're
    It shows the 13dbi, 16dbi, and 19dbi Beampatterns. The 19dbi has a lot of
    Btw, I put my biquad on the 18" by 22" dish. The best I could get is 4 db
    over the MFJ Yagi. Might be able to tweak a little more out of it.

    amdx, Jul 25, 2008
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