High current differentials and subsequent magnetic fields on coax cable can cause premature failures. A discussion of cumulative damage to coax in line components.

Not all duplexers have shunt feeds, but those that do can handle some amount of the lower frequency lightning surge current if properly grounded. It depends on the length of the cavity (frequency band), the size of the shunt fed loop and the stiffness of the loop. (It is really best to prevent the lightning energy from ever entering the equipment building, let alone the equipment itself.) Large magnetic fields can be generated in the duplexer that can bend the loop, de-tune the cavity, and allow even stronger magnetic fields to exist in subsequent strikes.

The strike can also weld the cavity input connectors together so the coax line can not be removed. The "on frequency" antenna ringing can create large voltages inside the cavities and cause internal arcing. If the first piece of equipment seen by the incoming low frequency coax surge is an isolator, with each strike (if it survives), a gradual increase in insertion loss will occur due to the surge current’ s magnetic field re-orienting the isolator’ s magnetic field, and/or changing the magnet’ s flux density.

High VSWR or increased insertion loss through these devices is not always attributed to lightning damage, although this could be the cause. The device is simply replaced with no further thought until the next failure. A dc blocked coaxial lightning protector would stop this kind of latent damage.