A post on *<http://adsbexchange.com/forums/topic/beginners-2-cantenna-easy-diy-antenna-to-improve-rangeplane-count>* uses a $\frac{\lambda}{4}$ sleeve made of a pop can beneath a $\frac{\lambda}{4}$ wire protrusion made from the core of a coaxial cable (coax with shielding removed). The protrusion connects to an SMA F connector that's lodged in a hole through the base of the popcan which in turn connects to regular coax that emerges from the bottom of the pop can. 
[![enter image description here](https://i.stack.imgur.com/bFCGy.png)](https://i.stack.imgur.com/bFCGy.png)

**My question is** -- Why does the length of the sleeve (the pop can) matter considering that the coaxial cable enclosed by it is completely shielded? 

**EDIT** : Considering the comments below RE: inclined ground plane. It seems to me that the desire of the author is to impedance match to 75ohm coaxial cable. See the image below: 
[![enter image description here](https://i.stack.imgur.com/nYXp0.png)](https://i.stack.imgur.com/nYXp0.png)