Have you tried a non-conventional wire collinear, the Franklin-Spider?
It is actually a 1/4 wavelength groundplane (Spider) antenna, modified by adding an additional element at top of 1/4 wavelength whip. The additional element is a 1/4 wavelength hairpin (stub), on top of which is a 1/2 wavelength vertical element. Please see this post for simulation results & drawing:
Click here: My post #2601 dated Apr 7, 2015, in Planefinder Forum (In this post, click on thumbnails of Simulation screenshots to see full size images)
It had mixed results. It gave better results than 1/4 wave Spider to caius & xforce30164, both indoor installation, but gave poorer results than 1/4 wavelength Spider to jepolch (outdoor installation) and abcd567 (me) (indoor installation).
Later, I managed to improve it by adding an impedance matching network - I cut the feed coax at (0.85 x 1/2 wavelength x VF) from feed point, inserted a 3pF capacitor in series with core wire, and joined the cut shields so that it remained continuous.