This is the fastest and easiest way to get on 20m and above with an effective field antenna (vehicle required).  It's the very fun MFJ-1979 (17-foot) telescoping whip.  It covers all bands from 20m-6m. You just adjust the length to resonate on the band of choice.  Mark the whip at the various points (the length to extend it) only one time with a sharpie, using an antenna analyzer or SWR meter . After that, you can do a band-change in about 20 seconds.  The whip is around 18" when collapsed.

Unless you want to rig up a bunch of radials, the only way to mount this whip is with a tri-mag mount on top of a vehicle roof. The MFJ-336T works great. (A single mag-mount won't work. Not enough coupling-capacitance, and a 10mph wind will blow it over).  A tri-mag mount should be good up to about 20mph winds, even with full-extension to 17-feet.  And a tri-mag mount also works well with 40m and 80m Hamsticks, which need a high-capacitance ground-coupling (or better yet, a "hard" ground), to work properly.  Also, for best performance, you need a hard-ground to the truck body.  I drilled a hole in the plate that attaches the 3 magnets and ran a self-tapping #10 screw through it to the truck roof, with the tip punching through the paint to the truck sheet-metal.

 The quality of the ground with this type of antenna is important, so attaching the truck bed and hood to the cab body with heavy braid also helps.