Gas discharge tube principle

In the event of a voltage surge, a gas section between the inner and the outer conductor of the coaxial transmission line will spark over, resulting in potential equalization to ground. This system works as a voltage-dependent switch that is automatically turned on and off. This design features a special gas-filled discharge tube (GDT).

If lightning strikes into the antenna mast or the antenna of a transceiver system, a current will flow toward the transceiver. Part of the current will be directed through the down conductor of the lightning protection system (LPS) or the antenna mast to the ground. The other part will flow through the RF cable to the lightning protector installed at the entry point into the building or equipment. An interference voltage may also be induced in the RF cable by lightning striking in the proximity of the station, causing an interference current to flow toward the equipment.

The GDT incorporated in the lightning/EMP protector sparks over (thereby becoming low-ohmic), equalizing the potential between the inner conductor and the ground. The current and thereby the energy of the lightning are discharged to the ground. Care must be taken to ensure that the current will be discharged on the outside of the building or equipment, and not inside. It is therefore important to install the actual surge protective device (SPD) on the outside, the so-called unprotected zone, in order to prevent any interference voltage from being induced in the protected zone. This is also true for other protection principles.

Once the interference subsides, the gas discharge tube will revert to its original condition, i.e. it will again become high-ohmic, and the system will be able to continue operation the same way as before.

To understand the existing inter-relationships and also to compare this system to other principles, let's consider the mode of operation for gas discharge tubes:

«Load» stands for the electronic equipment that has to be protected. The surge protective device is symbolized by the discharge tube.

The gas discharge tube consists of two electrodes that are insulated by a small ceramic tube. Its static sparkover is determined by the gas properties, its pressure, and the electrode gap.

In the event of a surge, a current will flow through the cable to the equipment, represented here as a surge wave.

The voltage across the gas discharge tube then rises very rapidly. When the dynamic spark-over voltage is reached, the GDT will be ignited and become conductive. At this moment, the voltage across the GDT (called the glow-arc voltage) is between 72 and 90V. This collapses to 10 – 20V (called the arc voltage) as the current rises. The dynamic spark-over voltage of the GDT is a function of the pulse rise time.

The gas discharge tube, once it sparks over, creates a potential equalization between the inner and the outer conductor (ground) of the coaxial transmission line. The current flows along the path of lowest resistance through the GDT to the ground. Only a very small portion of the energy, the so-called residual pulse, reaches the equipment. Its magnitude is determined by the GDT data, the interference pulse rise time, and the ground conductor impedance (determined by the quality of the lightning protection system).

After the interference has subsided, the GDT is extinguished, reverting to its original high-ohmic condition.
GDT protectors can generally be used in wideband applications from DC to over 2.5 GHz. The upper limit for the operating frequency range is determined by the capacitive characteristics of the GDT.
GDT protectors allow DC to be carried and thus tower-mounted electronic equipment to be power-fed via the coax line.