It appears you are driving one end and the other is grounded via a capacitor (as in the diagram in post #3?)
A self-resonant antenna with one end grounded has theoretical infinite impedance at the other end.
It is just not a good way to design an antenna.
An antenna with a grounded end is matched by feeding a tap near that ground point, not the other end, which is the most critical point for resonance.
I do not think it can ever work well, especially in PCB form; I've never seen it used in any commercial product and they are usually optimised to the limit.
The usual PCB type are a simple end-fed type based on a quarter wave, whether straight or zig-zag, or a tuned loop with a tap at a suitable impedance point for matching, as ours above.
Or simple formed wire antennas, which have higher Q that simple PCB ones.
THIS is a commercial helical antenna for UHF modules - end fed with the other end open, a "compacted" quarter wave end fed antenna:
Edit - What exact RF IC are you using? NXP use the name Lizard in publicity material, but not in connection with IC data sheets...