The Double Biquad is essentially the same as a regular Biquad, except that the reflector and the element is double the length, hence Double Biquad.
I'll only show the major differences from the original Biquad instructions, for the complete procedure check the Biquad page.
The new copper plate dimensions are 246.20 x 184.66 mm, calculations:
|Width = Full wavelength x 2 = 246.20 mm
Height = Full wavelength + (2 x 1/4 wavelength) = 123.10 + (2 x 30.78) = 184.66 mm
The lips should be bent at the 30.78 mm mark, make the bend sharp. It's important that you measure the inside if the reflector, not the outside or back. The last picture shows that the lips are within a tolerance of 0.2-0.3 mm, pretty close.
The wire length is doubled from a regular Biquad, the new length is 479,88 mm, calculations:
|Wirelength = (1/4 wavelength x 16 sides) - ((wire thickness / 2) * 14 corners) = (30.78 x 14) - ((18 / 2) * 16) = 479,88 mm|
When making an element for a Double Biquad you'll have to skip four bends and measure up 30.78 mm instead of 29.88. Measure from the center of last bend to where the wires will cross and mark this spot. Jump to the next bend and use the last spot as the 'center of the wire' to measure the next bend mark.
Doubling the Biquad element introduces a new crossing of wires, it's important that these wires do not touch. They should be 1-2 mm apart.
I found it easier to make them stay apart when making a round bend over the other wire, and vice versa. Just press the wire against the other with a piece of a pipe or similiar. The wires will stay apart and the entire element is relatively flat and within specifications (15-17 mm over the reflector).
I chose that the element center of the wire to run under the two crossings instead of over, I don't know if it has any effect but it's at least easier to hold it accurate under than over the crossings (support pipes is holding it in position).
Doesn't these make a nice couple =)
Double Biquad Antenna [martybugs.net]