RF Switching

The primary reasons for using vacuum relays in RF applications are their exceptional insulating qualities and their low RF contact resistance-as low as .03 ohms at 30 MHz. This low RF resistance remains stable throughout the service life of the relay because of the advantages provided by the vacuum environment.

Switching RF equipment "hot" can cause damage to various circuit elements, so RF circuits generally are switched "cold".

Vacuum relays used in RF applications have a frequency dependent current and voltage limitation. As frequency increases, the conduction path through the contacts decreases, causing contact surface heating and limiting the maximum RF voltage which can be withstood.

When a vacuum relay is open, in the insulating mode, RF voltage is seen across the open contacts or the contacts to ground. The relay in effect behaves as a high voltage capacitor measuring 1-2 pF. Current leakage through the insulator causes heating which further establishes limits to maximum current ratings. Figure 10 shows a typical transmitter application for an RB 3 Vacuum Relay. This relay has a set of low voltage sequence contacts as well as high voltage contacts. The high voltage contacts complete the circuit to the antenna before the transmitter is turned on, and delay switching the antenna until power is turned off. This assures the transmitter is properly loaded whenever power is applied.

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