Under the right conditions any frequency can bounce off the ionosphere. There is no magic frequency range (1-10mhz) that only bounces. Low Band public safety was 30, 42, and 70Mhz, and you can read the stories about CHP and others being received on the other side of the US. CB is 27mhz and can certainly have 'skip' (bounce off the ionosphere). Even up in the 155Mhz and 450Mhz and higher there can be 'skip'. I've received the 139mhz WISCOM public safety radio system in central Indiana. I've received 170mhz MOSWIN (Missouri), 450mhz SIRN (West Virginia), and 150mhz VA STARS (Virgina) as well; all with an antenna that is 25 feet above ground level. So, any freq can bounce off the ionosphere under the right conditions.