Which uses the most spectrum: transmitters or receivers?

When the author first got involved in spectrum management more than 30 years ago he was surprised to hear from colleagues at FCC that "transmitters don't use up spectrum, receivers do". This at first seems to contradict what one learns in basic electrical engineering courses where it is taught how to calculate the bandwidth of a signal as a function of its modulation. However, we also learn that radio signals that transmit real information (in the Shannon sense) also must have infinite bandwidth. Finite bandwidth channels must of necessity distort the signal somewhat. Realizable receivers can not have a finite bandwidth with a "brick wall" frontend that excludes all power outside the intended bandwidth. And finally practical receivers cannot have infinite dynamic range and exhibit nonlinear characteristics if signal limits are exceeded. When all these issues are combined it turns out that that achievable minimum separation in space and frequency between multiple transmitters and their receivers depends critically on receiver characteristics in most cases.