Re: [amsat-bb] 2.4 Ghz Helix--stacking distance

["John Stephensen, KD6OZH" <kd6ozh@xxxxxxx>]

I've been using the information published by John Kraus (second edition of
"Antennas") to calculate spacing.

    helix directivity = 12 * circumference^2 * turns * turns spacing

    effective aperture = (directivity * wavelength^2) / (4 * pi)

    helix spacing = 2 * sqrt(effective aperture / pi)

The antenna dimensions are in wavelengths. For three helices, I assumed that
the effective aperture is circular and mounted them one aperture diameter
apart. These equations reduce to:

    aperture diameter = diameter * sqrt(12 * turns * turns spacing)

The diameter is in the same units as the spacing and the turns spacing is in
wavelengths.

For a square array you could assume that the effective aperture of each
helix is square and get a smaller spacing:

    spacing = diameter * sqrt(3 * pi * turns * turns spacing)

The calculated spacing works reasonably well. I haven't been able to detect
any sidelobes when listening to the AO-40 beacon.

73,

John
KD6OZH


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