Dipoles fire broadside at their half-wave.

Those books are particularly helpful. Your large dipole antenna is a good swl approach which will also work well transmitting and will teach you something about tuning. To answer your specific question:

Dipoles are only really directional at their half wave. And there they fire broadside. As the frequency goes up, the number of currents or dipoles increases. Their alternating phase changes the shape of the radiation pattern into something that is somewhat omnidirectional.

I wanted to say this because I still hear it wrong listening to HF. If you're in doubt as to what to call the antenna, just call it a large doublet. It will only be 'a' dipole at its half wave.

Ideally, you build a half wave at the lowest operating frequency and use a quarter wave of twin lead. As you go up through the multiples, you'll maintain a high impedance input which your tuner will follow easily. Except for...but you're off to a great start! 73'