Actually very surprised to hear that regarding the performance. As mentioned in the past, that FM4000 is the same machine as the Standard Horizon GX5500SM; the same model used in every USCG Response Boat and a model I've used on a ton of boats, including my bosses' boat. Sensitivity and true TX output are on the same level as say an Icom M602/604. Not disputing your claim, but curious what kind of antenna being used and the condition of the connectors? Also, may not seem like it makes much of a difference, but ANY splice or additional connector/adapter will cause major loss, .5db per each connector, not to mention the type of cable (go with RG8X at the MIN.). You'd be shocked how many hidden junctions we stumble, especially when running through a radar mast.. Happy Thanksgiving BTW!!
One of the boats has a 19 foot Shakespeare , the other has an 8 foot on top of the radar stand, maybe 17 feet to the top of the antenna,and there is a matched pair on that boat, the other vhf is a Standrad Horizon 2000 series, on both boats, and the smaller radios work better in comparison, on both boats. One connection on all antennas. We have 4 boats, and have had trouble with the Shakespeare antennas, so I cut one up and looked inside the extension, and the wire inside was all short pieces butt connected together. The connections were all black and full of moisture, and pulled apart easy. Complete junk in my opinion. Hamilton got them replaced, but I would assume the new ones are the same.