Some Elements of WiFi Rubber Ducky Antenna Design

In the article Anatomy of a 2.4GHz Rubber Ducky Antenna by Martin Pot, I observed the following discrepancies with theory. The "decoupler" indeed acts as a quarter wave ground plane however the plastic spacer between the braid and the end of the quarter wave 'ground plane cylinder' is used as an insulator for the quarter wave ground plane and could not however be made of a conductor, for instance. This antenna is not a simple dipole but rather a quarter wave stub over a quarter wave ground plane. The plastic spacer's functional purpose is insulation from ground potential on the coaxial braid and not "to keep the coax centrally located inside the metal casing."

Furthermore the signal velocity of propagation between the two quarter wave stubs of the antenna dipole is NOT the velocity of light in air (or effectively in a vacuum) as the dipole elements are separated by not air, but by an insulating rubber compound with a dielectric constant differing from that of air. The waveguide structure formed by the dipole elements embedded in rubber, has a velocity factor VF<1, is in fact the source of the requirement for shortening dimensions of both the quarter wave driven stub and quarter wave ground plane to, lambda(Dipole) = lambda(air) * VF for VF<1, and that's why the stubs are shorter. The VF of the coax actually has nothing to do with the length of the subs.