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 ( I'll give the formulas below ).
The 3 half wave 15 meter doublet is almost exactly the same size as a conventional 40 meter dipole, so it works good from 40 on up, And exhibits some gain on the higher bands!

The formulas for the doublet are as follows: For the length of the flat top section use: 1428 / F Mhz = Length in feet, and for the matching section use: 468 / F Mhz X velocity factor = lenght in feet. Note: you can also use balanced line all the way to the transmatch and cut down on feedline loss.
Here are the dimensions of the 15 meter doublet. As always, try to get it as high as possible and try to get as much of the balanced matching stub as verticle as possible.
Flat top, 34' each side, 22' of 450 ohm ladder line terminated with 75 ohm coax going to the transmatch or just use ladder line all the way.