Mine is going to be left on all year round(but then i plan to attach and antenna to it for a radio, i do amateur radio) and will be used all year round.
I did some experiments with this last year, trying out APRS. The bike and rack don't make a particularly good ground plane, and unless you have a very long antenna (making you look a bit silly) then your antenna doesn't have clearance above you which affects efficiency. The next thing to try here is something like an FBK attached to a fibreglass whip, but the main reason for me stopping the experiment is that I found that even with a good antenna system on my car 5W wasn't really enough to give consistent tracking in this area. 25W seems about optimal, but then I'm into the extra size and weight of a mobile radio and battery rather than a hand held and tracker stuffed into a side pocket of the saddle bag.
My idea was to either have a L shaped mount on the back, or have a metal plate ontop of the rack with a body mount on that and a 1/4 wave of 2m ontop of that.
I dont want a 1/2wave on, i really would look like a right twat with that on.
But, the places where i ride i should be able to hit the local repeater with the handheld(ill be using the Jingtong) and be ok. I did have it mounted on a plate which was then attached to the seat rails but i just couldnt get the SWR down enough. Hoping the rack will make abit of difference.
Got an idea of going into Derbyshire up a hill and seeing what contacts i can get.