"VK3XL" wrote:
Thanks for the reply. All very interesting, but I am thinking of
something a bit bigger ie 12 elements on 2 meters on a 6 meter long
boom and up to 27 elements on 70 cm on the same boom. I fear that
because 70 cm is almost exactly the 3 rd harmonic of 2 that the 2meter
elements will act as unwanted directors in the 70 cm beam, any thoughts
welcome.....

One method of solving that problem is to orient the 2m elements
at right angles to the 70cm elements. Of course, if you don't
rotate the antenna by 90 degrees between bands, you would
lose 3 dB on both bands (assuming you want horizontal
polarization on both bands).
--
73, Cecil http://www.qsl.net/w5dxp