RF receiver to hear all carriers at once? (to hear electrosmog)
If you have a signal:
$$x(at)$$
the fourier transform will be:
$$F[x(at)]=\dfrac{1}{\lvert{a}\rvert} X(\dfrac{f}{a}) $$
this is known as the scaling property of Fourier transforms.  Which also relates the inverse relationship between the time domain and the frequency domain.

So if you sample the broadcast signal at very high frequencies and then play it back (or down sample via decimation) the entire bandwidth will be compressed/scaled.

the challenge here will be to have enough Bandwidth on the front end, a high enough sampling rate and sufficient gain (that pesky \$ \dfrac{1}{a}\$ factor).  However, it need not be digital, just sampled.