I have a vessel in orbit of Jool that has a 100G relay antenna and no other communication parts other than a probe core.  When I am controlling this vessel I noticed something peculiar: the ship seems to prefer a direct connection to the KSC even if it is a lower signal strength than using other relays that I have in my system.  I used the custom setting to disable the extra ground stations, so when the KSC is on the opposite side of Kerbin from where Jool is, then it is forced to use the relay