[ARDF] One difference between 2m and 80m courses...

[Kuon & Dale Hunt]

Ken Harker wrote:
>...
>In thinking about things after the meet, I don't think those antennas were
>good choices, not just from a mechnical standpoint.  They are optimized
>for forward gain, not for pattern.
>

    I look for an antenna design with all responses down
    at least 15 and preferably 20 dB or more from the peak.
    The VK3YNG sniffer uses 15dB steps, and the mobile
    rigs I've tested have about a 10dB usable meter range,
    so 20dB basically means a single obvious peak and
    the rest is off the bottom of the scale.

    Improving the F/B ratio to 30 or 50 dB makes no practical
    difference at this point.

    On the other hand, both of our Ron Graham antennas
    had a F/B ratio under 8dB, which makes it easy to
    get confused and go the wrong way!

    You also want to make sure the pattern holds across
    the range of frequencies that you are going to be using.
    An antenna optimized for 146.565 MHz may not have a
    good pattern at 144.5.  I usual scale my yagis
    to 145.25 or so to improve performance at the low end
    of the band for international hunts.

    I get my designs from W4RNL (www.cebik.com), who has
    descriptions and pattern plots of opimized 3-element
    yagis on diferent boom lengths.  I've built these
    into a spread sheet to make it easy to scale them
    for different frequencies and element widths.

    If you want to try computer modeling of tape measure
    yagis, you should be aware that most of the software
    available only handles round elements.  I've had good
    luck using the W2PV scaling equations assuming the
    equivalent diameter is 1/4 of the width of a
    flat element, but recent reading indicates that 1/2
    may be a more accurate factor.  (Or maybe I got the
    radius and diameter confused???)  Also the capacitance
    between the elements has an effect on the antenna -
    this will vary depending on how the tapes are oriented
    with respect to the boom, so you will still have to
    do some testing to make sure the design works for you.


    Good luck!

         - Dale WB6BYU


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