I've got an issue with my VHF. I replaced the aerial over winter after discovering that the previous owner had a piece of coax run to the top of the mast which did not connect to the aerial! So, a brand new aerial, cable and deck plug were procured and installed. However, I was told the other day by a marina that my transmission was garbled and unreadable - I was within 500m of the receiving station. I was transmitting on ch 80 @ 1W. I later tried a radio check with the coast guard (I think approx 5Nm from their closes aerial) on Ch16 @ 25W - weak but readable was the response.... Reception is great, I can pick up Ch 16 transmissions from a long way from aerials and AIS gets pings from boats 20 or 30 miles away. The aerial goes through a deck plug, into the AIS (Raymarine one with built in splitter) then into the VHF. All plugs are proper marine grade purchased from a marine electronics shop and the cable is all marine VHF cable.
I'm thinking that the problem may be that it was used for a while without a proper aerial (when the coax was not connected to the aerial at the top of the mast) and this has damaged the VHF (which is only 18 months old) - is this likely??