Antennae tend to give a multiplication factor to almost any input signal, no matter how low the initial power level might be. As a crude example, a 1 watt input

Antennae tend to give a multiplication factor to almost any input signal, no matter how low the initial power level might be. As a crude example, a 1 watt input signal might suddenly have an Effective Radiated Power in the hundreds of watts (yes, there are a vast number of variables in such a formula so take this as just a thought experiment). A bandpass filter has the effect of only allowing a desired sliver of the RF spectrum to be transmitted beyond it (i.e. Block All Except For...). There are cheap, passive, dumb analogue bandpass filters commonly available, and at the higher end of the cost scale there are active, smart digital circuits. Do your homework because not all bandpass filters can provide the sharp shoulder needed to give a clean slice of spectrum. A similar device is a bandstop filter, which blocks a specific slice of the RF spectrum as specified (i.e. Allow All Except For...).