Quarter-wave stub principle

This technology is based on a quarter-wave transformation. The coaxial shorting stub applied for this purpose is short-circuited at its end, and the length is matched to the mid-band frequency of the operation band. It thereby forms a bandpass filter. Its bandwidth can be adjusted within the range of 2 to 50% of the center band.

Since lightning interferences have a low frequency spectrum as described above, the shorting stub acts as a short circuit, conducting the current to the ground.

The basic principle of RF signal transmission through a quarter-wave lightning protector is described in the following:

In regular operation, the RF signal reaches the entry of the shorting stub (shown here as point 1). It then runs along the shorting stub up to the short (point 2). This corresponds to a 90° phase shift. At the short, the signal is reflected (point 2') – a sudden phase shift of 180° is created – and flows back to the start of the shorting stub (point 1'), where it arrives after another 90° phase shift. As a result, the reflected signal is again in phase with the arriving signal. Therefore, the RF signal does not «detect» the short.

Quarter-wave lightning/EMP protectors with their maximum bandwidth of up to 50% (± 25%) – relative to the mid-frequency – are relatively narrowband compared with gas lightning protectors, but offer considerably lower residual pulses and a high current handling capability. This is maintained even under multiple loading.

The operating principle of quarter-wave lightning/EMP protectors allows them to be manufactured for operating frequencies ranging from some MHz to more than 20 GHz (basically up to the frequency limit of the coaxial interface of the protector). The lower end of the availability range is determined by the increasing geometric length of the quarter-wave shorting stub.

They can be designed to show very low intermodulation values. The fact that they are maintenance-free is an important advantage for their use in the field. The residual pulse of the quarter-wave lightning/EMP protector has a considerably lower voltage amplitude (and thereby also energy) than that of the gas lightning/EMP protector. Unlike the GDT protector, it is not possible to carry any DC here, since the inner conductor is connected directly to the ground.