Coaxial Power Splitters vs. Directional Couplers

Coaxial power splitters and directional couplers route RF signals using different mechanisms. A two-resistor splitter, such as the 11667A, utilizes internal resistive elements to split an input signal into two outputs. Because it is resistive, it operates from DC up to 18 GHz. This configuration introduces a fixed nominal insertion loss of 6 dB on each path and limits power handling, with the 11667A supporting a maximum CW input power of 0.5 W (+27 dBm).

Directional couplers rely on electromagnetic coupling rather than resistors. A dual-directional coupler like the 11692D operates over a specific frequency band, such as 2 to 18 GHz. It extracts a small portion of the main-line power to a coupled port—with a nominal coupling factor of 22 dB—while allowing the primary signal to pass through with very low insertion loss. This passive electromagnetic design enables much higher power handling, allowing the 11692D to handle a maximum average input power of 50 W.

Port isolation and directivity determine how effectively a device separates signals based on their direction of travel. In a two-resistor splitter, isolation between the two output ports is low. While suitable for matching and leveling, splitters cannot isolate reflected waves from incident waves. This makes them unsuitable for measuring return loss.

Directional couplers are designed specifically for directional selectivity. The 11692D is a dual-directional configuration that simultaneously monitors forward and reflected power. It features a minimum directivity of 30 dB from 2 to 8 GHz and 26 dB from 8 to 18 GHz (or 24 dB when configured with a Type-N connector on the test port). This high directivity ensures that reverse-traveling reflected signals are kept separate from the forward reference signal at the coupled ports.

The choice of component depends on the system application. For source leveling and wideband ratio measurements, a two-resistor power splitter is preferred. The 11667A provides excellent output SWR matching, with an equivalent output SWR of 1.10 from DC to 4 GHz and 1.15 from 4 to 8 GHz. When used in an automatic leveling control loop, this low equivalent SWR minimizes source mismatch uncertainties.

For broadband reflectometry, dual-directional couplers are required. The 11692D maintains a low primary line SWR of 1.3 from 2 to 12.4 GHz and 1.4 from 12.4 to 18 GHz. This allows engineers to monitor incoming and reflected power simultaneously without interrupting the primary transmission line, making it highly effective for swept reflection measurements.