The Coco is very alluring as it is so simple and easy to make, and needs only a piece of coax, cutter pliers, stailey knife, and a measuring scale. This also applies to wirecollinears consisting of vertical limbs connected to adjuscent elements by coils.
The problem shows up only after it is built and put to use. Most builders get a performance far below what is claimed or expeted.
All Collinears are tricky to make, specially by a DIY novice. The collinear category includes Coaxial collinear (coco), the coiled whip, and Franklin dipole.
The reason being that collinears are multi element, which requires phasing elements to connect the vertical whip element. Franklin uses stubs, coiled whip uses coils, and coco uses core wire of coax to do the phasing.
The phasing elements are very sensetive to dimensions. Only a few mm error in ill-designed, or poorly built collinear can push collinear from excellent to poor category.
All antennas with one ¼ wavelength limb (68 mm for ads-b) pointing vertically upwards, connected to core wire of feed coax AND a support element connected to shield of coax, give good results compared to whip supplied with the dongle. These antennas are not very sensetive to dimensional errors. I recomend these to be used by beginners. They can try their luck with collinears at a later stage.
The supporting element connected to shield of feeder coax can be either of the following:
(1) Radials: 3, 4, 6, or 8 horizontal wires ¼ wavelength (68 mm).
(2) Disk: A sheet metal disk ¼ wavelength radius.
(3) Slanting Radials (Spider): same as (1) above, but legs bent down 45 degrees. Users reported 8 legs better than 6 or 4.
(4) Disk+Sleeve (Cantenna): A metallic can, 65 to 100mm dia, cut to height 69 mm, bottom up, with hole drilled in center for barrel connector to connect vertical up limb to the core wire of coax.
(5)Dipole: A second ¼ wavelength limb, pointing downwards. The feed coax arrives at the dipole horizontally at mid-point of dipole.