I know that when you want to pick up low frequency radio signals, the longer the wavelength, the longer the antenna needs to be. Why is that?
Think of the vibratory modes on a string clamped at both ends. When you clamp a length of string at both ends, the allowed vibration patterns all have one thing in common; they go to zero at the ends.
So, the length of the string determines the natural frequencies of vibration; the longer the string, the lower the fundamental frequency.
Very roughly speaking, an antenna acts similarly. For example, the current must be zero at the ends of a center-fed dipole antenna in analogy with the clamped ends of the string.
So, for the antenna to resonate at a lower frequency, it must be longer, just as the string must be.