To summarize, with the mentioned assumptions:
i) Port 4 which is named also as Isolated port, always has a zero output, without being dependent of the electrical length of the coupling region. Major cause of poor isolation in practical circuits are unequal phase velocities of even and odd mode.
ii) The input at each port is matched to the feed line characteristic impedance, Z0, irrespective of the electrical length.
iii) The total output power equals the input power.
iv) The maximum coupling to port 2 occurs at the frequency that gives a quarter wave coupling length which is l/4. This is the mid-band frequency and due to this property, this couplers are also known as quarter-wave couplers.
v) At the maximum coupling frequency, there is a 90° phase difference between the voltages at port 2 and port 3. Such a coupler can also be described as quadrature coupler.
vi)At the frequencies other than maximum coupling frequency (design frequency)
the coupling of the coupler can be found from |V3(q)|.