Uwe,
'Bigger/longer' isn't necessarily 'better'! A longer
dipole -may- resonante on the desired frequency, but the
65 feet (approximate) is really all you need. Twice that
length would probably work for you, but feeding it in the
center would also be a very bad impedance match for 50 ohm
coax. Not to mention will produce a radiation pattern
different from what you might expect. A full wave, instead
of a 1/2 wave, dipole can be made to work, but all things
considered, why bother?
'Doc