reflected power

I have an output line with a directional coupler with a good directivity and then a Schottky diode to measure the reflected power. At the output of the apparatus (a transmitter) there is a coaxial line of a certain lenght that could vary according to the configuration of the transmitter. My question is: that way i will measure different power reflected according to the lenght of the coaxial line because the measure doesnt depend only to the directivity of the directional power but even to the distance from the load to the directional coupler because the Schottky diode makes a voltage measure (squaring it of course) and so the voltage wave vary along the line. Then it doesnt matter how it is good the directional power, the measure could be wrong. Is that correct?
thank you

Unless the coaxial line has a lot of insertion loss per unit length, then the length of the coaxial line will have almost no effect on the reflected power recieved at the directional coupler. You need the high directivity in the coupler itself so that the reflected power can be differentiated from the forward power. Also, you need a good output VSWR at the transmitter output, otherwise the directivity of the reflected power measurement will be degraded.