Recent discussion with a correspondent about the design issue of the so-called Co-Co collinears, vertical collinears made with alternating sections of common coax ranged onto the conflict between the phase velocity of the wave on the inside of the coax and the wave on the outside of the coax, and the difficulty in aligning both the outside standing wave pattern for optimal pattern, and the internal phasing to feed those sections with optimal phase.
The suggestion was made that this problem could be solved by dielectric loading the outside conductor.
I created an NEC-4.2 model of a 1m long dipole of 10mm diameter conductor and found its resonant frequency to be 140MHz.
I then altered the model to dielectric load the dipole with polyethylene (εr=2.3) and found the thickness needed to move resonance down to 75% of free space 1m wavelength (to align phase with a cable with internal vf=0.75). I needed 65mm of radial polyethylene to achieve that ‘synchronisation’.
It is not a very practical solution, the structure would now be 140mm outside diameter with a 10mm diameter conductor in the middle of it.
I have seen suggestion that the fibreglass radome on commercial antennas does just that, but it is unlikely that it slows the wave more than a percent or two.