If what you are trying to do is a dipole type antenna for 40m and 80m,
you do not need any capasitors at all. You can use choke-coils as in this
design
but leave the 20m and 15m dipoles out.
http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=13422
73 Jouko OH5RM