This comes up often in some use cases, such as the bridges of ships, where the radios are mounted overhead, and the antennas are only a few feet above them. The higher the power of the radio, the more separation you want between the radio and its antenna and any other radio/antenna combination that could interfere with it. More so with radios that have plastic cases and minimal shielding often only being the heat shield fins. You risk damaging the radio by using them too close to the antenna or another radio and its antenna. As for cable, thirty feet is not much, and as long as it is shielded with low reflected power, checked with a meter, that power is going out the antenna....you do not want your radio next to that.