Michael wrote:
According to the ARRL handbook the highest gain is achieve with a
5/8ths wave spacing between the upper and lower elements. The
handbook gives the following figures for estimated gain.

3/8 wave spacing = 4.4 dbd
1/2 wave spacing = 5.9 dbd
5/8 wave spacing = 6.7 dbd
3/4 wave spacing = 6.6 dbd

It seems to works on 14 MHz (stacked dipoles at 14 MHz), but on 10
meters the single 10 meter dipole blows it away.

Quoting the ARRL Antenna Book: "It should be designed
for the higher of the two frequencies using 3/4 lamda
spacing between parallel elements. It will then operate
on the lower frequency ... with 3/8 lamda spacing.

If you have 5/8 lamda spacing on 20m, you will have
5/4 lamda spacing on 10m with poor performance.
--
73, Cecil http://www.w5dxp.com