Collinear top section calculations from : mitglied.lycos.de/kleineronkelz/
Calculated dimensions are for DECT frequencies /1880 to 1900 MHz/.
Interesting for me here is top section.
“… The top is made from coax copper and lambda/4. Note!!! Because the top radiate without dielectric insulation no speed factor is needed! …”
This differ from usual wifi collinear designs.
Very good descriprions about calculating and building this antenna.