Directional couplers are also the first stage of taps. Most taps consist of directional couplers to remove signal from the distribution cable, coupling it to a splitter or series of splitters that divide the signal into homes. When you worry about the isolation from one tap to another, you are worrying about the performance of the directional coupler. If it has poor isolation, you get poor isolation from one tap to another. One problem arising from this is the coupling of local oscillator signals from one TV to another.
All TVs have some local oscillator signal present at the antenna terminals.
If this local oscillator signal gets from the output of one TV to the input of another, because of poor isolation in the tap or any splitters in the network, interference results. A few years ago, the cable industry discovered hum modulation that had to do with the directional coupler in some taps. Figure 3 illustrates the problem. We show the power as an ideal trapezoidal waveform with flat tops and somewhat fast rise and fall times. The problem was first noted when a lot of standby power supplies were used.