Quite a few people who use WSPR use the tiny Microvert Antenna whose design seems to have originated in Germany. For example, this diminutive antenna is only about 30cms long on 28MHz. It uses the coax intentionally as a counterpoise with an RF choke the right distance down the coax. Performance is several S points down on a full sized antenna like a 1/2 wave dipole. With small antennas there is always a compromise! Versions have been made for most HF bands. The antenna looks ideal for portable or balcony use, being small and easy to erect. Perhaps I should test my (currently poor) build skills by making one for 10m or 20m WSPR use?
I wonder who reading this post has used a Microvert Antenna and how they found it?
See http://download.antennex.com/preview/archive4/Apr601/microvert.pdf .