I currently use a ~35' (~10m) fiberglass telescoping mast, and I love it. It's great when I hoist my 10m window line j-pole (thank you KB9VBR!) and now that I'm messing around on 20m, it's just too short. My SWR is a bit higher than it ought to be on account of ground reflection since I can't get the base of the radiating element more than about ~1m (~3') off the ground.
The antenna I use (currently, at least) is fairly lightweight, so I'm not worried about mast flexion since the wight will largely be just outside mast center.
I am not to deterred by cost, and I am trying to avoid metallic antennas for fear it will mess with my radiation patterns and SWR in a new way. I am greatly interested in telescoping options.
Are you familiar with fiberglass or carbon fiber masts which get the top to a definite 15m (~50')?
Nearly all of my operations are /P, so it really has to be fairly convenient. I recognize that these requirements may be a little mutually exclusive.
How are you hanging your antenna? Dipoles for 20m and lower are typically set up in an Inverted V configuration, which should work fine with the 10m mast you already have.
The window line j-pole is a monoband vertical, a half wave radiating element and a quarter wave impedance matching section. On 20m the overall length of this thing is 14.4m.
On my current mast, the radiating element comes down to about 1m off the ground and then I have the matching section strung perpendicularly across the fence to keep it off the ground.
Have you tried a 1/4 wave vertical with 4 radials sloping towards the ground? (all wire)? or even a vertical dipole fed halfway up? J-poles show significant coupling to their feedline and mounting arrangements and, as you discovered, are taller than they need to be for their gain.
I haven't tried any radials, and tbh I'm not certain to where I would attach them given the j-pole design has a matching network in-built. I've got a 1:1 balun at the feedpoint and an inline choke at the radio. My 10m antenna of identical design is great, and this one is great too (I see ~1:1 at 14.250), it's just much higher in the band than I had planned and I strongly suspect it's simply ground reflection since the radiating element is way lower than the halfwave it ought to be.