15m Asymmetrical Antenna by G3EFY
This mono-band antenna that can be constructed with either with a 1 wavelength (44ft) or 1½ wavelength (66ft) top section. In either case the feedpoint is 11ft from one end.
https://rsars.files.wordpress.com/2013/01/15m-asymmetrical-antenna-g3efy-iss-1-31.pdf

21,28MHz shortened dipole antenna (pipe type) by JA1HWO
http://mk1502.web.fc2.com/ahf1/218dp_p.htm

21,28MHz shortened dipole antenna (wire type) by JA1HWO
http://mk1502.web.fc2.com/ahf1/218dp_w.htm

21MHz Hentena by JA1HWO
http://mk1502.web.fc2.com/ahf1/21hente.htm

21MHz small space 2EL YAGI by JA1HWO
http://mk1502.web.fc2.com/ahf1/15yagi_s.htm

21MHz squalo antenna by JA1HWO
http://mk1502.web.fc2.com/ahf1/15sqalo.htm

3 Half Wave 15 Meter Doublet (in phase) by KC8AON
For a good multiband ( 10 thru 40 meters ) antenna, try a 3 half waves in phase doublet for 15 meters.
It is derived by using the formula for the G5RV, but substituting 21 Mhz in place of the usual 14 Mhz.
http://fpqrp.org/15mdub.htm

A Light and Sturdy Quad for 10 and 15 meters by KA3PDM & W3/CE3VU
Fishin' for DX? A few fishin' poles and a hank of wire can help you snag the Big One!
http://www.arrl.org/files/file/Technology/tis/info/pdf/9107030.pdf

A Small Transmitting Loop Antenna for 14MHz and 21MHz by VK5BR
By definition, the small transmitting loop has a circumference less than one quarter of a wavelength. Because of its small size, it is possibly an attractive proposition for someone with inadequate space to erect a larger aerial.
https://www.qsl.net/vk5br/LoopAntennas/HFTXLoop.htm

The 10/15/20 Meter Trap Vertical Antenna by KGØZZ
This antenna project came out very nice. It's a rugged homebrew multi-band trap vertical antenna that works the 10, 15, and 20 meter amateur radio bands. The antenna can be mounted on the ground or on a mast.
https://www.youtube.com/playlist?list=PLeQk8lP4CRvJ4Qml7UOHiqzkSCXVjPh9U

The Cylinder Dipole by PA1B
The cylinder dipole has proven to be a very small indoor HF-antenna. The cylinder dipole with two Monster energy drink cans and a coil is easy to build and is working fine on 15 m.
https://a29.veron.nl/hams/pa1b/cylinder-dipole/

The HF Satellite Antenna by W4SAT
This is a no compromise dual band antenna. Instead of using traps, this design interlaces two separate antennas on one boom. This is a full size, four element beam on 10, and three elements on 15 meters!
http://www.qsl.net/w4sat/hfsat.htm

Ultra-short whip antenna for 2 bands by JA1HWO
http://mk1502.web.fc2.com/ahf1/218_tanhp.htm

Zippiro Vortex by IZ7DJR
https://www.qsl.net/iz7djr/vortex15.htm