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?
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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.