My new powerboat (I still have VALIS, my sailboat!) had a factory-installed Garmin
AIS receiver, that included a splitter to share the antenna with the VHF transceiver. The AIS was receiving OK, but not great. The VHF worked in a radio-check with a close-by vessel, but it was weak beyond a few miles.
I brought my SWR meter down to the boat
and started testing. The SWR into the antenna cable was 5:1 or worse -- it was huge.
I was hoping it was a connector problem, so I measured with an Ohmmeter: virtual dead short. Since the antenna is normally a dead short, this might be OK (an open circuit would be bad). The antenna had no connector, just the cable coming out the bottom, so I had to hope the problem was a shorted connector. I cut off the connector and measured it with an Ohmmeter which verified the short, cleaned it out, and re-soldered it.
What surprised me was how well the AIS and radio
worked even with the shorted connector. I guess it was just a whisker of shield that was shorting. It worked well enough to not be obviously broken, but now it works *much* better. The SWR is under 1.5:1 now.