[Elecraft] Directional Coupler CPI

[PE1E]

I want to check the various possible mismatches between the 
(coax)feedline and antenna feedpoint( adjustable loaded dipole ).

What is the influence - if any - of the Elecraft 30 dB directional 
coupler ( mounted in the mast right at the antenna's feedpoint ) on the 
transition from feedline to antenne feedpoint ?

Does the directional coupler cause an ( extra )  impedance jump between 
feedline and antenna feedpoint , disturbing the values of the FW and Rev 
readings ?

Thanks,
Peter, PE1E