Resonance effects can also boost or reduce certain frequencies but not others. The audio characteristics of an STL should be tested during installation as they are somewhat unpredictable. Using transformers to lower the cable's beginning and ending impedances to 60 - 150 ohms greatly extends the distance signals can travel before they need equalization. If the program audio is stereo, two cables of the same length should connect the studio and transmitter. A third potential problem on long wire STLs is the program audio picking up noise or interference from external electro-magnetic fields.
Shielding can help, as can changing the cable's route if the noise is caused by a locatable source.
Another risk is the wire being cut or accidentally damaged. The chance for this to happen grows with the length, and obviously depends on the activities occurring nearby. All we can say is install the cable where you think it will be safe. Despite these warnings, a wire STL poses no insurmountable technical problems, even when it is 10 - 25 kilometers long. For the best results, the wire should have no splices.