I write 'can', because unfortunately many commercial digital radio/TV stations don't do this to improve the viewing or listening quality but just to fit in more content. The benefit of this is that the digital signal requires (potentially) less bandwidth in the air so you can fit more signals in the same 'airspace' and that the digital signal can be less susceptible to noise. In a digital radio, the signal represents a digital version of the audio, just like a WAV or MP3-file on a computer is a digital representation which can be turned back into an analog signal you can send to a speaker. The rest is not really physics, but the difference between an analog and a digital radio comes in the circuits after this and basically an analog radio extracts a modulation from the intermediate frequency which is amplified and sent to the speakers or radio output. It is much easier to synthesize an exact mixing frequency to tune with than to change the resonance of the antenna circuit. It is in the mixer you tune the reception in a modern superheterodyne radio receiver. The antenna circuit should indeed have a resonance within the band of frequencies you are interested in but this wide-band signal is then mixed with an internally generated sinusodial signal in the radio in an analog component, this subtracts the frequencies and lets the rest of the radio operate on a much easily handled frequency band (called the intermediate frequency). All this without any battery! :)Д«ut actually the antenna in a normal modern radio is not the component that 'dials in' the selected broadcast frequency. The output signal is an alternating voltage, and by rectifying it with a diode (called a 'crystal' then.) you could extract a signal modulated as a varying amplitude of the carrier wave.

You didn't actually change the length of the antenna, but by changing the inductor (a coil) or capacitor connected to the antenna you tuned the resonance. In the early days of radio, the resonance of the antenna in combination with its associated inductive and capacitive properties was indeed the item which 'dialed in' the frequency you wanted to listen to.

How do tv tuner for pc work