Radioman,
If you are certain your coverage isn't being affected by interference, then most likely your antenna system has gone out of tune. I presume you are using the "antenna guy" (Carl Van Orden) coil, or one of your own making that is similar in design. This is a fairly high Q coil that resonates with the antenna capacitance, which is the most efficient approach with a 3m antenna. Snow, ice, and moisture can de-tune the coil and knock the antenna system out of resonance, causing the signal strength to drop dramatically. You didn't mention what you are using for a ground system. I have read of situations where the ground connection deteriorated, and this certainly can have a deleterious effect on your coverage.
Now, please don't even think about going up on the roof to check or re-tune your coil until winter has passed! Even if the roof appears to be free of ice and snow, it could be slippery, and you could fall and injure yourself very seriously. When warmer weather returns, you might consider taking the antenna down off the roof and mounting it at ground level where you can work on it more easily. You can then take advantage of the opportunity to install a good ground system. I would also suggest you consider fitting a waterproof cover over the coil to protect it from moisture and the accumulation of snow and ice in the winter time. I am probably going to go in the direction of using an iron-core toroid in the future, which can be enclosed in the transmitter housing where it is totally protected from the elements. This is the arrangement used in the Rangemaster transmitter, and I have not observed any changes in tuning once it is adjusted.
My range is much more affected by co-channel interference and IBOC hash from adjacent channels than anything else. I have one frequency that is pretty quiet in the daytime, but it's horrible at night (aren't they all)! At times I can hear my station for up to 4 blocks in the middle of the day, but at night the interference-free range can drop to 100' or less. I've pretty much resigned myself to the idea that night time operation is mainly useful for listening around the house.
I would seriously doubt that coax problems have anything to do with your range problem. For one thing, there are very few situations where coax would be appropriate for use in a Part 15 antenna system.
By the way, I have observed significant variations in mid-day range from one day to the next. Some of my best distance records have been set on cold, snowy days right after a fresh snowfall (especially a heavy one)! I suspect that the snow may be enhancing the ground conductivity, because I can't think of any other reason why this would be the case.
