Meaning
Attenuation of the signal power passing through the main transmission line of a passive four-port RF device is an unavoidable consequence of coupling a fraction of the energy to a sampling port. This directional coupler loss is composed of both ohmic losses within the conductor and the power diverted to the coupled port. High-frequency designs require accurate characterization of this parameter to ensure that the transmitted signal maintains sufficient power for over-the-air propagation.
Excessive loss at this stage reduces the coverage range of the wireless system and raises the temperature of the coupling structure.
Insertion Attenuation
Material dissipation in the dielectric substrate and conductive traces constitutes the bulk of the thermal losses. In microstrip configurations, directional coupler loss is highly dependent on the dissipation factor of the PCB material and the surface roughness of the copper. Substrates like FR4 introduce significant attenuation at gigahertz frequencies, whereas PTFE-based materials preserve signal integrity.
Directivity Rating
The ratio of the power received at the coupled port to that at the isolated port defines how well the device distinguishes between forward and reflected waves. When the directional coupler loss is not balanced by high isolation, the measurement of the return loss becomes inaccurate. This inaccuracy corrupts the antenna tuning algorithms and leads to sub-optimal power delivery.
Power Budget
System sensitivity depends directly on minimizing the decay of the transmitted signal before it reaches the antenna. Designers evaluate directional coupler loss during the initial link budget calculations to determine the required gain of the power amplifier. Low-loss couplers allow the use of smaller and cheaper amplifiers, which reduces the total bill of materials.
