why are you spending more on coax than you are on an antenna?
why dont you use a base antenna rather than a mobile on a plate?
a VENTENNA would do you just as well and be hidden a WeatherVane antenna
would do the same and have better gain than the dualband mobile
either of these antennas can be found online using a google search and
are both stealthy the neighboors would never know what you have up
if that isnt the issue i would suggest a Comet GP6 or GP9 both are great
antennas
and the performance would be a lot better than the mobile whip

ml wrote:

I was planning on using a mobile 2/440 antenna either w/a mag mount or
thru hole on some sheet metal , on my roof .


naturally unlike in a car the coax run will be rather longer than is
typically used in a car , i presume therefore this might change to the
point where using this set up might be 'useless'??


are they particularly sensitive to say impedance changes or such?


100ft run of lmr400 or 1/2'' heliax will be used

thanks