Physical considerations such as internal load on the isolated port will limit port operation. The coupled output from
the directional coupler can be used to obtain the information (i.e., frequency and power level) on the signal without
interrupting the main power flow in the system (except for a power reduction - see Figure 2). When the power coupled out
to port three is half the input power (i.e. 3 dB below the input power level), the power on the main transmission line is also
3 dB below the input power and equals the coupled power. Such a coupler is referred to as a 90 degree hybrid, hybrid, or
3 dB coupler. The frequency range for coaxial couplers specified by manufacturers is that of the coupling arm. The main
arm response is much wider (i.e. if the spec is 2-4 GHz, the main arm could operate at 1 or 5 GHz - see Figure 3). However
it should be recognized that the coupled response is periodic with frequency. For example, a 8/4 coupled line coupler will
have responses at n8/4 where n is an odd integer.