don't mess with the two inside screws too much... those are the data lines and they're easy to damage.
but yeah, if you're getting 17volts on the outside screws of the comm port at the outdoor panel and nothing at the antenna, then it seems like the cable is broken.
You might want to power down, disconnect the terminal block from the antenna card, power back up and measure the voltage on the antenna's two outside screws again. (it's not strictly necessary but eliminates the possibility that the antenna is causing the voltage problem... in reality, if the antenna was drawing too much power and killing the voltage, then you wouldn't be seeing 17volts at the outdoor comm port either)
I usually temporarily replace any questionable cable with a short length to see if fixes the problem... just power everything off before you mess with the comm port screws or you could damage the boards!