Mobile swr went from 2.0 to 1.0 with ice on car

[M0GVZ]

-small sedan
-trimag mount
-tram bigcat antenna
-lmr400 straight down through roof to radio
-best swr i could get out of 3 antennas was 2.0
-2 inches of snow/ice on roof and swr is now flat.
This goes against what i am researching, as snow, ice, salt, etc usually raise swr. several of us are stumped as to exactly why, or what i need to do to achieve that 1.0swr without snow and ice. the hole is drilled, the mount will be staying, no leaks whatsoever. if reflectivity is a problem, would rubber undercoating paint on the roof help? there is no regard for the looks of the vehicle compared to the desired low swr. many thanks for ANY responses

Actually it doesn't surprise me. Snow/Ice doesn't raise SWR, what it does is alter tuning. As RF flows over the surface of a conductor (both radiating element and counterpoise whether wire or vehicle body) then anything on that surface affects the RF typically making that conductor, altering the velocity factor of the conductor making it appear electrically longer.

So if your antenna was a bit shorter than it needed to be for the frequency you're testing, say you're testing on Ch1 and you set the lowest SWR to be on Ch40, then when it or the roof got wet/covered in snow/ice then as far as RF is concerned it would look electrically longer which would lower the SWR on Ch1 but then it would raise it on higher frequencies.

I see the same happen on my ham radio base antennas, both the vertical Hustler 5BTV and wire dipoles. What I didn't see it on was my Imax 2000 because the wire conductor is in a fibreglass tube so doesn't get wet or covered in snow/ice and therefore isn't affected.