Terminated vs. Unterminated Coaxial Switches

The primary distinction between terminated and unterminated coaxial switches lies in how the switch treats the inactive or unused RF port.

In an unterminated switch, such as the Keysight N1810UL or the Keysight 8765B, the inactive channel is left open-circuited. The path is simply disconnected, leaving the center conductor suspended within the connector cavity.

In a terminated switch, such as the Keysight N1810TL or the Keysight 8762A, the internal mechanical actuator automatically connects the center conductor of the unused port to a built-in 50 Ω resistive load. This ensures that the unused port maintains its characteristic impedance even when it is not actively routing signals.

When an active RF signal travels down a transmission line and encounters an unterminated (open-circuit) port, it experiences a massive impedance mismatch. Because there is no load to absorb the energy, the RF signal is reflected entirely back toward the source. This generates high voltage standing wave ratios (VSWR) on the transmission line.

These signal reflections can lead to voltage constructive interference, creating standing waves that risk damaging sensitive transmitters or source instrumentation. Conversely, terminated switches keep the inactive line matched. For example, the Keysight 8762A maintains a Voltage Standing Wave Ratio (SWR) of less than 1.1 from DC to 2 GHz and less than 1.2 from 2 to 4 GHz on its terminated paths, absorbing the unused energy and eliminating reflections.

Unterminated ports can act as antennas, radiating RF energy into the switch cavity and causing signal leakage into neighboring channels. Terminating the unused port significantly improves channel-to-channel isolation.

In systems where isolation must remain high across a broad frequency spectrum, terminated configurations are often preferred to prevent crosstalk. With unterminated switches, isolation naturally degrades as frequencies increase. For example, the unterminated Keysight 8765B provides 90 dB of isolation from DC to 4 GHz, but this value falls to 60 dB at 18 to 20 GHz.

While terminated switches offer excellent RF performance, they introduce physical power limits. The internal 50 Ω resistors are small and have limited heat dissipation capabilities.

For instance, the Keysight 8762A can handle up to 24 W of average RF power through its active, non-terminated transmission path. However, its internal 50 Ω termination has an average power rating of only 1 W (with a peak power rating of 100 W for a 10 µs pulse width at a 1% duty cycle). Similarly, the Keysight N1810TL has a maximum continuous RF power rating of 1 W CW for its internal termination. Exceeding these thermal limits will permanently damage the termination resistor, destroying the switch's impedance-matching properties.