The antenna resonates at a particular frequency
A whip (monopole) antenna (for example) produces an optimum output level for frequencies that are close to one-quarter wavelength. It will also produce a decent output level for frequencies surrounding that optimum frequency and the "spread" of those frequencies from "centre" can be considerable and be much wider than the nominal transmit channel spacing for the band that you are interested in.
It will also produce decent amplitude at frequencies corresponding to three-quarter of the antenna length and also at multiples of half a wavelength above and beyond. Take a look at the impedance of a typical monopole to understand this: -
You should be able to see that at 0.25\$\lambda\$ the reactive impedance falls to zero (tuned) and ditto at 0.75\$\lambda\$.
See also my answer here for the impedance as frequency rises and note that: -
- The length of the antenna (irrespective of aligning to \$\lambda\$) dictates how big the amplitude is i.e. longer antennas produce a bigger signal naturally
- The main characteristic change with length is the projected impedance
- That projected impedance may or may not significantly interact with your actual circuit interface to produce a wealth of unwanted "good reception" areas versus wavelength.
So, given the nature of most radio bands and the extent of interferers, you usually need tight control by using electronic/electrical filters.