Ensuring RF applications are functioning correctly requires proper monitoring and controlling of RF power levels. Such applications also benefit from wide frequency ranges and methods to monitor standing wave ratio while the instrument is operating. However, this is difficult to achieve without incurring losses in transmission power.
In especially demanding RF environments, such as communications or electronic warfare, high losses in transmission lines and loads cannot be tolerated. Therefore, we need some device that will provide accurate and efficient measurements with minimal loss and other interference.
Directional couplers are RF components that are employed in transmission lines to both monitor RF power levels and direct a portion of power to other parts of the system; the incoming power to the coupler can be divided and part of it used for another application.
The standard four-port directional coupler consists of two coupled transmission lines, as shown in the figure below.
The coupler is also symmetrical, or bi-directional, meaning power can be input through port 2 to output through port 1. In this case, the coupled port would be port 4 and the isolated port is port 3.
Directional couplers are used extensively in electronic warfare, electronic countermeasures, electronic counter-countermeasures (ECCM), and instrumentation.
Manufacturers in RF and microwave components typically offer directional couplers as a product. When browsing for couplers, make sure to check that they are compliant with US specifications and international standards. In particular, attention should be given to frequency range, VSWR, Insertion Loss, and especially directivity as they directly reflect the accuracy of any measuring instrument.